range-diff: add configurable memory limit for cost matrix

From: pcasaretto <[email protected]>

When comparing large commit ranges (e.g., 250,000+ commits), range-diff attempts to allocate an n×n cost matrix that can exhaust available memory. For example, with 256,784 commits (n = 513,568), the matrix would require approximately 256GB of memory (513,568² × 4 bytes), causing either immediate segmentation faults due to integer overflow or system hangs.

Add a memory limit check in get_correspondences() before allocating the cost matrix. This check uses the total size in bytes (n² × sizeof(int)) and compares it against a configurable maximum, preventing both excessive memory usage and integer overflow issues.

The limit is configurable via a new --max-memory option that accepts human-readable sizes (e.g., "1G", "500M"). The default is 4GB for 64 bit systems and 2GB for 32 bit systems. This allows comparing ranges of approximately 32,000 (16,000) commits - generous for real-world use cases while preventing impractical operations.

When the limit is exceeded, range-diff now displays a clear error message showing both the requested memory size and the maximum allowed, formatted in human-readable units for better user experience.

Example usage:
git range-diff --max-memory=1G branch1...branch2 git range-diff --max-memory=500M base..topic1 base..topic2

This approach was chosen over alternatives:
- Pre-counting commits: Would require spawning additional git processes and reading all commits twice
- Limiting by commit count: Less precise than actual memory usage
- Streaming approach: Would require significant refactoring of the current algorithm

This issue was previously discussed in:
https://lore.kernel.org/git/[email protected]/

---
range-diff: add configurable memory limit for cost matrix

Problem Description
===================

When git range-diff is given extremely large ranges, it can result in either:

1. Segmentation fault due to integer overflow in array index calculations
2. Excessive memory consumption leading to system hangs or OOM kills
3. Poor user experience with the command appearing to hang for minutes

Reproduction Case
=================

In a Shopify's large monorepo a range-diff command like this crashes after several minutes with a SIGBUS error

$ git range-diff 4430f36511..316c1276c6 cb5240b6a8..2bbd292091

Range statistics:

* First range: 256,783 commits
* Second range: 1 commit
* Total: 256,784 commits
* Memory required for cost matrix: n² × 4 bytes = ~260GB

Stack Trace (Segmentation Fault)
================================

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x6e000ae3b8)
* frame #0: 0x000000010029a284 git`get_correspondences(a=0x000000016fde6188, b=0x000000016fde6160, creation_factor=60) at range-diff.c:356:20
frame #1: 0x0000000100299310 git`show_range_diff(range1="4430f36511cbacf5c517c6851e2b8508a72dfd30..316c1276c63f55ad9413fa18bf3b6483564a9cf4", range2="cb5240b6a8ba59b4a1f282559ee0742721b0cafc..2bbd292091e376d177ce62e264dae8872ca6be5a", range_diff_opts=0x000000016fde6308) at range-diff.c:593:3 frame #2: 0x00000001000c719c git`cmd_range_diff(argc=2, argv=0x0000600000bcd8c0, prefix="areas/core/shopify/", repo=0x0000000100468b20) at range-diff.c:167:8
frame #3: 0x000000010000277c git`run_builtin(p=0x00000001004408d8, argc=3, argv=0x0000600000bcd8c0, repo=0x0000000100468b20) at git.c:480:11
frame #4: 0x0000000100001020 git`handle_builtin(args=0x000000016fde6c70) at git.c:746:9
frame #5: 0x0000000100002074 git`run_argv(args=0x000000016fde6c70) at git.c:813:4
frame #6: 0x0000000100000d3c git`cmd_main(argc=3, argv=0x000000016fde7350) at git.c:953:19
frame #7: 0x000000010012750c git`main(argc=4, argv=0x000000016fde7348) at common-main.c:9:11
frame #8: 0x000000018573ab98 dyld`start + 6076

Root Cause Analysis
===================

The crash occurs in get_correspondences() at line 356:

static void get_correspondences(struct string_list *a, struct string_list *b, ...) {
int n = a->nr + b->nr; // Integer overflow: 256,784 fits in int
...
ALLOC_ARRAY(cost, st_mult(n, n)); // Would allocate ~260GB
...
cost[i + n * j] = c; // Line 356: Invalid memory access
}

Problems:

1. Integer overflow: While n=256,784 fits in an int, n*n overflows
2. Memory allocation: Even with proper types, allocating 260GB is impractical

Solution
========

Add a memory limit check in get_correspondences() before allocating the cost matrix. This check uses the total size in bytes (n² × sizeof(int)) and compares it against a configurable maximum, preventing both excessive memory usage and integer overflow issues.

The limit is configurable via a new --max-memory option that accepts human-readable sizes (e.g., "1G", "500M"). The default is 4GB for 64 bit systems and 2GB for 32 bit systems. This allows comparing ranges of approximately 32,000 (16,000) commits - generous for real-world use cases while preventing impractical operations.

When the limit is exceeded, range-diff now displays a clear error message showing both the requested memory size and the maximum allowed, formatted in human-readable units for better user experience.

Example usage: git range-diff --max-memory=1G branch1...branch2 git range-diff --max-memory=500M base..topic1 base..topic2

This approach was chosen over alternatives:

* Pre-counting commits: Would require spawning additional git processes and reading all commits twice
* Limiting by commit count: Less precise than actual memory usage
* Streaming approach: Would require significant refactoring of the current algorithm

This issue was previously discussed in:
https://lore.kernel.org/git/[email protected]/

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1958%2Fpcasaretto%2Frange-diff-size-limit-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1958/pcasaretto/range-diff-size-limit-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1958

 builtin/log.c        |  1 +
 builtin/range-diff.c | 30 ++++++++++++++++++++++++++----
 log-tree.c           |  1 +
 range-diff.c         | 35 +++++++++++++++++++++++++----------
 range-diff.h         |  5 +++++
 5 files changed, 58 insertions(+), 14 deletions(-)

index c2f8bbf8630..5f552d14c0f 100644
@@ -1404,6 +1404,7 @@ static void make_cover_letter(struct rev_info *rev, int use_separate_file,
 		struct range_diff_options range_diff_opts = {
 			.creation_factor = rev->creation_factor,
 			.dual_color = 1,
+			.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 			.diffopt = &opts,
 			.other_arg = &other_arg
 		};

index a563abff5fe..10fa82fd285 100644
@@ -6,6 +6,7 @@
 #include "parse-options.h"
 #include "range-diff.h"
 #include "config.h"
+#include "parse.h"
 
 
 static const char * const builtin_range_diff_usage[] = {

@@ -15,6 +16,22 @@ N_("git range-diff [<options>] <base> <old-tip> <new-tip>"),
 NULL
 };
 
+static int parse_max_memory(const struct option *opt, const char *arg, int unset)
+{
+	size_t *max_memory = opt->value;
+	uintmax_t val;
+
+	if (unset) {
+		return 0;
+	}
+
+	if (!git_parse_unsigned(arg, &val, SIZE_MAX))
+		return error(_("invalid max-memory value: %s"), arg);
+
+	*max_memory = (size_t)val;
+	return 0;
+}
+
 int cmd_range_diff(int argc,
 		   const char **argv,
 		   const char *prefix,

@@ -25,6 +42,7 @@ int cmd_range_diff(int argc,
 	struct strvec diff_merges_arg = STRVEC_INIT;
 	struct range_diff_options range_diff_opts = {
 		.creation_factor = RANGE_DIFF_CREATION_FACTOR_DEFAULT,
+		.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 		.diffopt = &diffopt,
 		.other_arg = &other_arg
 	};

@@ -33,17 +51,21 @@ int cmd_range_diff(int argc,
 		OPT_INTEGER(0, "creation-factor",
 			    &range_diff_opts.creation_factor,
 			    N_("percentage by which creation is weighted")),
+		OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
+				  N_("style"), N_("passed to 'git log'"), 0),
+		OPT_BOOL(0, "left-only", &left_only,
+			 N_("only emit output related to the first range")),
+		OPT_CALLBACK(0, "max-memory", &range_diff_opts.max_memory,
+			     N_("size"),
+			     N_("maximum memory for cost matrix (default 4G)"),
+			     parse_max_memory),
 		OPT_BOOL(0, "no-dual-color", &simple_color,
 			    N_("use simple diff colors")),
 		OPT_PASSTHRU_ARGV(0, "notes", &other_arg,
 				  N_("notes"), N_("passed to 'git log'"),
 				  PARSE_OPT_OPTARG),
-		OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
-				  N_("style"), N_("passed to 'git log'"), 0),
 		OPT_PASSTHRU_ARGV(0, "remerge-diff", &diff_merges_arg, NULL,
 				  N_("passed to 'git log'"), PARSE_OPT_NOARG),
-		OPT_BOOL(0, "left-only", &left_only,
-			 N_("only emit output related to the first range")),
 		OPT_BOOL(0, "right-only", &right_only,
 			 N_("only emit output related to the second range")),
 		OPT_END()

index 233bf9f227c..73d21f71764 100644
@@ -717,6 +717,7 @@ static void show_diff_of_diff(struct rev_info *opt)
 		struct range_diff_options range_diff_opts = {
 			.creation_factor = opt->creation_factor,
 			.dual_color = 1,
+			.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 			.diffopt = &opts
 		};
 

index 8a2dcbee322..6e9b6b115e5 100644
@@ -21,6 +21,7 @@
 #include "apply.h"
 #include "revision.h"
 
+
 struct patch_util {
 	/* For the search for an exact match */
 	struct hashmap_entry e;

@@ -287,8 +288,8 @@ static void find_exact_matches(struct string_list *a, struct string_list *b)
 }
 
 static int diffsize_consume(void *data,
-			     char *line UNUSED,
-			     unsigned long len UNUSED)
+			    char *line UNUSED,
+			    unsigned long len UNUSED)
 {
 	(*(int *)data)++;
 	return 0;

@@ -325,13 +326,24 @@ static int diffsize(const char *a, const char *b)
 }
 
 static void get_correspondences(struct string_list *a, struct string_list *b,
-				int creation_factor)
+				int creation_factor, size_t max_memory)
 {
 	int n = a->nr + b->nr;
 	int *cost, c, *a2b, *b2a;
 	int i, j;
-
-	ALLOC_ARRAY(cost, st_mult(n, n));
+	size_t cost_size = st_mult(n, n);
+	size_t cost_bytes = st_mult(sizeof(int), cost_size);
+	if (cost_bytes >= max_memory) {
+		struct strbuf cost_str = STRBUF_INIT;
+		struct strbuf max_str = STRBUF_INIT;
+		strbuf_humanise_bytes(&cost_str, cost_bytes);
+		strbuf_humanise_bytes(&max_str, max_memory);
+		die(_("range-diff: unable to compute the range-diff, since it "
+		      "exceeds the maximum memory for the cost matrix: %s "
+		      "(%"PRIuMAX" bytes) needed, %s (%"PRIuMAX" bytes) available"),
+		    cost_str.buf, (uintmax_t)cost_bytes, max_str.buf, (uintmax_t)max_memory);
+	}
+	ALLOC_ARRAY(cost, cost_size);
 	ALLOC_ARRAY(a2b, n);
 	ALLOC_ARRAY(b2a, n);
 

@@ -351,7 +363,8 @@ static void get_correspondences(struct string_list *a, struct string_list *b,
 		}
 
 		c = a_util->matching < 0 ?
-			a_util->diffsize * creation_factor / 100 : COST_MAX;
+			    a_util->diffsize * creation_factor / 100 :
+			    COST_MAX;
 		for (j = b->nr; j < n; j++)
 			cost[i + n * j] = c;
 	}

@@ -360,7 +373,8 @@ static void get_correspondences(struct string_list *a, struct string_list *b,
 		struct patch_util *util = b->items[j].util;
 
 		c = util->matching < 0 ?
-			util->diffsize * creation_factor / 100 : COST_MAX;
+			    util->diffsize * creation_factor / 100 :
+			    COST_MAX;
 		for (i = a->nr; i < n; i++)
 			cost[i + n * j] = c;
 	}

@@ -539,7 +553,7 @@ static void output(struct string_list *a, struct string_list *b,
 		if (i < a->nr && a_util->matching < 0) {
 			if (!range_diff_opts->right_only)
 				output_pair_header(&opts, patch_no_width,
-					   &buf, &dashes, a_util, NULL);
+						   &buf, &dashes, a_util, NULL);
 			i++;
 			continue;
 		}

@@ -548,7 +562,7 @@ static void output(struct string_list *a, struct string_list *b,
 		while (j < b->nr && b_util->matching < 0) {
 			if (!range_diff_opts->left_only)
 				output_pair_header(&opts, patch_no_width,
-					   &buf, &dashes, NULL, b_util);
+						   &buf, &dashes, NULL, b_util);
 			b_util = ++j < b->nr ? b->items[j].util : NULL;
 		}
 

@@ -591,7 +605,8 @@ int show_range_diff(const char *range1, const char *range2,
 	if (!res) {
 		find_exact_matches(&branch1, &branch2);
 		get_correspondences(&branch1, &branch2,
-				    range_diff_opts->creation_factor);
+				    range_diff_opts->creation_factor,
+				    range_diff_opts->max_memory);
 		output(&branch1, &branch2, range_diff_opts);
 	}
 

index cd85000b5a0..9d39818e349 100644
@@ -5,6 +5,10 @@
 #include "strvec.h"
 
 #define RANGE_DIFF_CREATION_FACTOR_DEFAULT 60
+#define RANGE_DIFF_MAX_MEMORY_DEFAULT \
+	(sizeof(void*) >= 8 ? \
+		((size_t)(1024L * 1024L) * (size_t)(4L * 1024L)) : /* 4GB on 64-bit */ \
+		((size_t)(1024L * 1024L) * (size_t)(2L * 1024L)))   /* 2GB on 32-bit */
 
 /*
  * A much higher value than the default, when we KNOW we are comparing

@@ -17,6 +21,7 @@ struct range_diff_options {
 	unsigned dual_color:1;
 	unsigned left_only:1, right_only:1;
 	unsigned include_merges:1;
+	size_t max_memory;
 	const struct diff_options *diffopt; /* may be NULL */
 	const struct strvec *other_arg; /* may be NULL */
 };

base-commit: 954d33a9757fcfab723a824116902f1eb16e05f7

gitgitgadget

<administrivia>

It is usual to see a less human readable name embedded in the commit object than the mail header when a mail comes from GGG.

Just in case you want to be known to this community as "Paulo Casaretto", not "pcasaretto", I thought I'd point it out that you may want to redo the commit. I do not mind what name you like to use, as long as it is identifiable, and From: identity matches the identity you add your Signed-off-by: with.

</administrivia>

It is unusual to lack <> around e-mail address here.

		c = a_util->matching < 0
		  ? a_util->diffsize * creation_factor / 100
		  : COST_MAX;

Problem Description
===================

When git range-diff is given extremely large ranges, it can result in either:

1. Segmentation fault due to integer overflow in array index calculations
2. Excessive memory consumption leading to system hangs or OOM kills
3. Poor user experience with the command appearing to hang for minutes

Reproduction Case
=================

In a Shopify's large monorepo a range-diff command like this crashes after several minutes with a SIGBUS error

$ git range-diff 4430f36511..316c1276c6 cb5240b6a8..2bbd292091

Range statistics:

* First range: 256,783 commits
* Second range: 1 commit
* Total: 256,784 commits
* Memory required for cost matrix: n² × 4 bytes = ~260GB

Stack Trace (Segmentation Fault)
================================

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x6e000ae3b8)
* frame #0: 0x000000010029a284 git`get_correspondences(a=0x000000016fde6188, b=0x000000016fde6160, creation_factor=60) at range-diff.c:356:20

    frame #1: 0x0000000100299310 git`show_range_diff(range1="4430f36511cbacf5c517c6851e2b8508a72dfd30..316c1276c63f55ad9413fa18bf3b6483564a9cf4", range2="cb5240b6a8ba59b4a1f282559ee0742721b0cafc..2bbd292091e376d177ce62e264dae8872ca6be5a", range_diff_opts=0x000000016fde6308) at range-diff.c:593:3
    frame #2: 0x00000001000c719c git`cmd_range_diff(argc=2, argv=0x0000600000bcd8c0, prefix="areas/core/shopify/", repo=0x0000000100468b20) at range-diff.c:167:8
    frame #3: 0x000000010000277c git`run_builtin(p=0x00000001004408d8, argc=3, argv=0x0000600000bcd8c0, repo=0x0000000100468b20) at git.c:480:11
    frame #4: 0x0000000100001020 git`handle_builtin(args=0x000000016fde6c70) at git.c:746:9
    frame #5: 0x0000000100002074 git`run_argv(args=0x000000016fde6c70) at git.c:813:4
    frame #6: 0x0000000100000d3c git`cmd_main(argc=3, argv=0x000000016fde7350) at git.c:953:19
    frame #7: 0x000000010012750c git`main(argc=4, argv=0x000000016fde7348) at common-main.c:9:11
    frame #8: 0x000000018573ab98 dyld`start + 6076

Root Cause Analysis
===================

The crash occurs in get_correspondences() at line 356:

static void get_correspondences(struct string_list *a, struct string_list *b, ...)

{
    int n = a->nr + b->nr;  // Integer overflow: 256,784 fits in int
    ...
    ALLOC_ARRAY(cost, st_mult(n, n));  // Would allocate ~260GB
    ...
    cost[i + n * j] = c;  // Line 356: Invalid memory access
}

Problems:

1. Integer overflow: While n=256,784 fits in an int, n*n overflows
2. Memory allocation: Even with proper types, allocating 260GB is impractical

Solution
========

Add a memory limit check in get_correspondences() before allocating the cost matrix. This check uses the total size in bytes (n² × sizeof(int)) and compares it against a configurable maximum, preventing both excessive memory usage and integer overflow issues.

The limit is configurable via a new --max-memory option that accepts human-readable sizes (e.g., "1G", "500M"). The default is 4GB for 64 bit systems and 2GB for 32 bit systems. This allows comparing ranges of approximately 32,000 (16,000) commits - generous for real-world use cases while preventing impractical operations.

When the limit is exceeded, range-diff now displays a clear error message showing both the requested memory size and the maximum allowed, formatted in human-readable units for better user experience.

Example usage: git range-diff --max-memory=1G branch1...branch2 git range-diff --max-memory=500M base..topic1 base..topic2

This approach was chosen over alternatives:

* Pre-counting commits: Would require spawning additional git processes and reading all commits twice
* Limiting by commit count: Less precise than actual memory usage
* Streaming approach: Would require significant refactoring of the current algorithm

This issue was previously discussed in:
https://lore.kernel.org/git/[email protected]/

pcasaretto (2):
range-diff: reorder options lexicographically
range-diff: add configurable memory limit for cost matrix

 builtin/log.c        |  1 +
 builtin/range-diff.c | 29 +++++++++++++++++++++++++----
 log-tree.c           |  1 +
 range-diff.c         | 20 ++++++++++++++++----
 range-diff.h         |  5 +++++
 5 files changed, 48 insertions(+), 8 deletions(-)

base-commit: 954d33a9757fcfab723a824116902f1eb16e05f7
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1958%2Fpcasaretto%2Frange-diff-size-limit-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1958/pcasaretto/range-diff-size-limit-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1958

Range-diff vs v1:

 -:  ----------- > 1:  ec5dcdf9d00 range-diff: reorder options lexicographically
 1:  5cf3e8921a7 ! 2:  c81f920fee0 range-diff: add configurable memory limit for cost matrix

@@ Commit message
This issue was previously discussed in:
https://lore.kernel.org/git/[email protected]/

- Acked-by: Johannes Schindelin [email protected]
- Signed-off-by: pcasaretto <[email protected]>
+ Acked-by: Johannes Schindelin <[email protected]>
+ Signed-off-by: Paulo Casaretto <[email protected]>

## builtin/log.c ##
@@ builtin/log.c: static void make_cover_letter(struct rev_info *rev, int use_separate_file,
@@ builtin/range-diff.c: N_("git range-diff [<options>] <base> <old-tip> <new-tip>"
+ size_t *max_memory = opt->value;
+ uintmax_t val;
+
-+ if (unset) {
++ if (unset)
+ return 0;
-+ }
+
+ if (!git_parse_unsigned(arg, &val, SIZE_MAX))
+ return error(_("invalid max-memory value: %s"), arg);
@@ builtin/range-diff.c: int cmd_range_diff(int argc,
.other_arg = &other_arg
};
@@ builtin/range-diff.c: int cmd_range_diff(int argc,
- OPT_INTEGER(0, "creation-factor",
- &range_diff_opts.creation_factor,
- N_("percentage by which creation is weighted")),
-+ OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
-+ N_("style"), N_("passed to 'git log'"), 0),
-+ OPT_BOOL(0, "left-only", &left_only,
-+ N_("only emit output related to the first range")),
+ N_("style"), N_("passed to 'git log'"), 0),
+ OPT_BOOL(0, "left-only", &left_only,
+ N_("only emit output related to the first range")),
+ OPT_CALLBACK(0, "max-memory", &range_diff_opts.max_memory,
+ N_("size"),
+ N_("maximum memory for cost matrix (default 4G)"),
@@ builtin/range-diff.c: int cmd_range_diff(int argc,
OPT_BOOL(0, "no-dual-color", &simple_color,
N_("use simple diff colors")),
OPT_PASSTHRU_ARGV(0, "notes", &other_arg,
- N_("notes"), N_("passed to 'git log'"),
- PARSE_OPT_OPTARG),
-- OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
-- N_("style"), N_("passed to 'git log'"), 0),
- OPT_PASSTHRU_ARGV(0, "remerge-diff", &diff_merges_arg, NULL,
- N_("passed to 'git log'"), PARSE_OPT_NOARG),
-- OPT_BOOL(0, "left-only", &left_only,
-- N_("only emit output related to the first range")),
- OPT_BOOL(0, "right-only", &right_only,
- N_("only emit output related to the second range")),
- OPT_END()

## log-tree.c ##
@@ log-tree.c: static void show_diff_of_diff(struct rev_info *opt)
@@ log-tree.c: static void show_diff_of_diff(struct rev_info *opt)

## range-diff.c ##
-@@
- #include "apply.h"
- #include "revision.h"
-
-+
- struct patch_util {
- /* For the search for an exact match */
- struct hashmap_entry e;
-@@ range-diff.c: static void find_exact_matches(struct string_list *a, struct string_list *b)
- }
-
- static int diffsize_consume(void *data,
-- char *line UNUSED,
-- unsigned long len UNUSED)
-+ char *line UNUSED,
-+ unsigned long len UNUSED)
- {
- (*(int *)data)++;
- return 0;
@@ range-diff.c: static int diffsize(const char *a, const char *b)
}

@@ range-diff.c: static int diffsize(const char *a, const char *b)
ALLOC_ARRAY(a2b, n);
ALLOC_ARRAY(b2a, n);

-@@ range-diff.c: static void get_correspondences(struct string_list *a, struct string_list *b,
- }
-
- c = a_util->matching < 0 ?
-- a_util->diffsize * creation_factor / 100 : COST_MAX;
-+ a_util->diffsize * creation_factor / 100 :
-+ COST_MAX;
- for (j = b->nr; j < n; j++)
- cost[i + n * j] = c;
- }
-@@ range-diff.c: static void get_correspondences(struct string_list *a, struct string_list *b,
- struct patch_util *util = b->items[j].util;
-
- c = util->matching < 0 ?
-- util->diffsize * creation_factor / 100 : COST_MAX;
-+ util->diffsize * creation_factor / 100 :
-+ COST_MAX;
- for (i = a->nr; i < n; i++)
- cost[i + n * j] = c;
- }
-@@ range-diff.c: static void output(struct string_list *a, struct string_list *b,
- if (i < a->nr && a_util->matching < 0) {
- if (!range_diff_opts->right_only)
- output_pair_header(&opts, patch_no_width,
-- &buf, &dashes, a_util, NULL);
-+ &buf, &dashes, a_util, NULL);
- i++;
- continue;
- }
-@@ range-diff.c: static void output(struct string_list *a, struct string_list *b,
- while (j < b->nr && b_util->matching < 0) {
- if (!range_diff_opts->left_only)
- output_pair_header(&opts, patch_no_width,
-- &buf, &dashes, NULL, b_util);
-+ &buf, &dashes, NULL, b_util);
- b_util = ++j < b->nr ? b->items[j].util : NULL;
- }
-
@@ range-diff.c: int show_range_diff(const char *range1, const char *range2,
if (!res) {
find_exact_matches(&branch1, &branch2);

gitgitgadget

From: pcasaretto <[email protected]>

Reorder the command-line options in builtin/range-diff.c to be in lexicographic order for better organization and readability. This is a preparatory cleanup with no functional changes.

---
 builtin/range-diff.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

index a563abff5fee..283583a80d0b 100644
@@ -33,17 +33,17 @@ int cmd_range_diff(int argc,
 		OPT_INTEGER(0, "creation-factor",
 			    &range_diff_opts.creation_factor,
 			    N_("percentage by which creation is weighted")),
+		OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
+				  N_("style"), N_("passed to 'git log'"), 0),
+		OPT_BOOL(0, "left-only", &left_only,
+			 N_("only emit output related to the first range")),
 		OPT_BOOL(0, "no-dual-color", &simple_color,
 			    N_("use simple diff colors")),
 		OPT_PASSTHRU_ARGV(0, "notes", &other_arg,
 				  N_("notes"), N_("passed to 'git log'"),
 				  PARSE_OPT_OPTARG),
-		OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
-				  N_("style"), N_("passed to 'git log'"), 0),
 		OPT_PASSTHRU_ARGV(0, "remerge-diff", &diff_merges_arg, NULL,
 				  N_("passed to 'git log'"), PARSE_OPT_NOARG),
-		OPT_BOOL(0, "left-only", &left_only,
-			 N_("only emit output related to the first range")),
 		OPT_BOOL(0, "right-only", &right_only,
 			 N_("only emit output related to the second range")),
 		OPT_END()

gitgitgadget

Thanks for splitting this out into its own commit.

I am not sure if "lexicographic order" fits well in the context of "git cmd -h" that spews out many many options, shown with related options together in groups. I find it aggressively annoying to show left/right-only far apart. A user unfamiliar with the command would look at the list, find "left-only" sitting in the list alone, and waste time and break concentration wondering what in the first range is so special to deserve such an option, until they see "right-only" further down to realize that they are symmetric.

I'd rather not to see this "lexicographic" change done, but others may have better justification (note: "for better organization and readability" I just disagreed is a good justification) that may make me change my mind.

What I would change, if there is something suboptimal in the current output from "git range-diff -h" that deserves improvement, is the lack of the grouping header before the options for range-diff operation (i.e. creation-factor to left/right-only, before the next "diff output" group begins).

Thanks.

Same issue with name here.

I do like lexicographic ordering for unrelated options, but I prefer options to be grouped by intent/use first, then by lexicographic ordering. And here, not only are--left-only & --right-only related as Junio points out, to me --diff-merges and --remerge-diff are a similar grouping that belong together. So, my $0.02 is that I'd lean towards calling both changes in the patch a reduction in organization rather than an improvement.

Yes, I concur. I noticed these were "out of order" when I added the new flag but now it's obvious that there was order. I'll remove this commit.
Regarding the name problem, I've checked and I do have "Paulo Casaretto" set as my name in my Github public profile.
I fixed my local git config and apparently that fixed it.

Paulo L F Casaretto

Yeah, these in-body From: lines GigGitGadget adds come from the authorship of the commits you are sending (in other words, what you see in "git cat-file commit <commit>" for these commits), and your GitHub profile would not affect it (and you do not want your GitHub profile name be used---otherwise you cannot send a series that contains a change written by somebody else without overtaking the authorship of their commits).

I see v3 posted there; thanks.

From: pcasaretto <[email protected]>

When comparing large commit ranges (e.g., 250,000+ commits), range-diff attempts to allocate an n×n cost matrix that can exhaust available memory. For example, with 256,784 commits (n = 513,568), the matrix would require approximately 256GB of memory (513,568² × 4 bytes), causing either immediate segmentation faults due to integer overflow or system hangs.

Add a memory limit check in get_correspondences() before allocating the cost matrix. This check uses the total size in bytes (n² × sizeof(int)) and compares it against a configurable maximum, preventing both excessive memory usage and integer overflow issues.

The limit is configurable via a new --max-memory option that accepts human-readable sizes (e.g., "1G", "500M"). The default is 4GB for 64 bit systems and 2GB for 32 bit systems. This allows comparing ranges of approximately 32,000 (16,000) commits - generous for real-world use cases while preventing impractical operations.

When the limit is exceeded, range-diff now displays a clear error message showing both the requested memory size and the maximum allowed, formatted in human-readable units for better user experience.

Example usage:
git range-diff --max-memory=1G branch1...branch2 git range-diff --max-memory=500M base..topic1 base..topic2

This approach was chosen over alternatives:
- Pre-counting commits: Would require spawning additional git processes and reading all commits twice
- Limiting by commit count: Less precise than actual memory usage
- Streaming approach: Would require significant refactoring of the current algorithm

This issue was previously discussed in:
https://lore.kernel.org/git/[email protected]/

---
 builtin/log.c        |  1 +
 builtin/range-diff.c | 21 +++++++++++++++++++++
 log-tree.c           |  1 +
 range-diff.c         | 20 ++++++++++++++++----
 range-diff.h         |  5 +++++
 5 files changed, 44 insertions(+), 4 deletions(-)

index c2f8bbf86301..5f552d14c0fe 100644
@@ -1404,6 +1404,7 @@ static void make_cover_letter(struct rev_info *rev, int use_separate_file,
 		struct range_diff_options range_diff_opts = {
 			.creation_factor = rev->creation_factor,
 			.dual_color = 1,
+			.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 			.diffopt = &opts,
 			.other_arg = &other_arg
 		};

index 283583a80d0b..79956d83e400 100644
@@ -6,6 +6,7 @@
 #include "parse-options.h"
 #include "range-diff.h"
 #include "config.h"
+#include "parse.h"
 
 
 static const char * const builtin_range_diff_usage[] = {

@@ -15,6 +16,21 @@ N_("git range-diff [<options>] <base> <old-tip> <new-tip>"),
 NULL
 };
 
+static int parse_max_memory(const struct option *opt, const char *arg, int unset)
+{
+	size_t *max_memory = opt->value;
+	uintmax_t val;
+
+	if (unset)
+		return 0;
+
+	if (!git_parse_unsigned(arg, &val, SIZE_MAX))
+		return error(_("invalid max-memory value: %s"), arg);
+
+	*max_memory = (size_t)val;
+	return 0;
+}
+
 int cmd_range_diff(int argc,
 		   const char **argv,
 		   const char *prefix,

@@ -25,6 +41,7 @@ int cmd_range_diff(int argc,
 	struct strvec diff_merges_arg = STRVEC_INIT;
 	struct range_diff_options range_diff_opts = {
 		.creation_factor = RANGE_DIFF_CREATION_FACTOR_DEFAULT,
+		.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 		.diffopt = &diffopt,
 		.other_arg = &other_arg
 	};

@@ -37,6 +54,10 @@ int cmd_range_diff(int argc,
 				  N_("style"), N_("passed to 'git log'"), 0),
 		OPT_BOOL(0, "left-only", &left_only,
 			 N_("only emit output related to the first range")),
+		OPT_CALLBACK(0, "max-memory", &range_diff_opts.max_memory,
+			     N_("size"),
+			     N_("maximum memory for cost matrix (default 4G)"),
+			     parse_max_memory),
 		OPT_BOOL(0, "no-dual-color", &simple_color,
 			    N_("use simple diff colors")),
 		OPT_PASSTHRU_ARGV(0, "notes", &other_arg,

index 233bf9f227c6..73d21f71764e 100644
@@ -717,6 +717,7 @@ static void show_diff_of_diff(struct rev_info *opt)
 		struct range_diff_options range_diff_opts = {
 			.creation_factor = opt->creation_factor,
 			.dual_color = 1,
+			.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 			.diffopt = &opts
 		};
 

index 8a2dcbee322e..e31f71c73d20 100644
@@ -325,13 +325,24 @@ static int diffsize(const char *a, const char *b)
 }
 
 static void get_correspondences(struct string_list *a, struct string_list *b,
-				int creation_factor)
+				int creation_factor, size_t max_memory)
 {
 	int n = a->nr + b->nr;
 	int *cost, c, *a2b, *b2a;
 	int i, j;
-
-	ALLOC_ARRAY(cost, st_mult(n, n));
+	size_t cost_size = st_mult(n, n);
+	size_t cost_bytes = st_mult(sizeof(int), cost_size);
+	if (cost_bytes >= max_memory) {
+		struct strbuf cost_str = STRBUF_INIT;
+		struct strbuf max_str = STRBUF_INIT;
+		strbuf_humanise_bytes(&cost_str, cost_bytes);
+		strbuf_humanise_bytes(&max_str, max_memory);
+		die(_("range-diff: unable to compute the range-diff, since it "
+		      "exceeds the maximum memory for the cost matrix: %s "
+		      "(%"PRIuMAX" bytes) needed, %s (%"PRIuMAX" bytes) available"),
+		    cost_str.buf, (uintmax_t)cost_bytes, max_str.buf, (uintmax_t)max_memory);
+	}
+	ALLOC_ARRAY(cost, cost_size);
 	ALLOC_ARRAY(a2b, n);
 	ALLOC_ARRAY(b2a, n);
 

@@ -591,7 +602,8 @@ int show_range_diff(const char *range1, const char *range2,
 	if (!res) {
 		find_exact_matches(&branch1, &branch2);
 		get_correspondences(&branch1, &branch2,
-				    range_diff_opts->creation_factor);
+				    range_diff_opts->creation_factor,
+				    range_diff_opts->max_memory);
 		output(&branch1, &branch2, range_diff_opts);
 	}
 

index cd85000b5a0d..9d39818e349c 100644
@@ -5,6 +5,10 @@
 #include "strvec.h"
 
 #define RANGE_DIFF_CREATION_FACTOR_DEFAULT 60
+#define RANGE_DIFF_MAX_MEMORY_DEFAULT \
+	(sizeof(void*) >= 8 ? \
+		((size_t)(1024L * 1024L) * (size_t)(4L * 1024L)) : /* 4GB on 64-bit */ \
+		((size_t)(1024L * 1024L) * (size_t)(2L * 1024L)))   /* 2GB on 32-bit */
 
 /*
  * A much higher value than the default, when we KNOW we are comparing

@@ -17,6 +21,7 @@ struct range_diff_options {
 	unsigned dual_color:1;
 	unsigned left_only:1, right_only:1;
 	unsigned include_merges:1;
+	size_t max_memory;
 	const struct diff_options *diffopt; /* may be NULL */
 	const struct strvec *other_arg; /* may be NULL */
 };

gitgitgadget

The names (and emails) in these should match; I believe the name in the From field is set by Gitgitgadget based on your profile settings; see https://github.com/settings/profile and set your name there.

Sounds like a good idea.

I am not a huge fan of configuration variables that do not have a command line option. Assuming that it is not like you'd be doing overly huge range-diff that would not fit your memory every day, shouldn't we start this with a command line option without a configuration variable to gauge how useful it would be for users with such a need, and then after it proves useful and we identify a workflow where a user would be passing this option all the time, add a configuration to allow it always be in effect (with command line override still available)?

Isn't that what Paulo's patch does? Maybe I'm just blind, but I've looked over the patch a couple times and don't see where he's reading from a configuration variable; am I just missing it?

Ah, I just blindly trusted that the "configurable memory limit" on the subject line is talking about configuring memory limit with some mechanism. Thanks for correcting me.

From: Paulo Casaretto <[email protected]>

When comparing large commit ranges (e.g., 250,000+ commits), range-diff attempts to allocate an n×n cost matrix that can exhaust available memory. For example, with 256,784 commits (n = 513,568), the matrix would require approximately 256GB of memory (513,568² × 4 bytes), causing either immediate segmentation faults due to integer overflow or system hangs.

Add a memory limit check in get_correspondences() before allocating the cost matrix. This check uses the total size in bytes (n² × sizeof(int)) and compares it against a configurable maximum, preventing both excessive memory usage and integer overflow issues.

The limit is configurable via a new --max-memory option that accepts human-readable sizes (e.g., "1G", "500M"). The default is 4GB for 64 bit systems and 2GB for 32 bit systems. This allows comparing ranges of approximately 32,000 (16,000) commits - generous for real-world use cases while preventing impractical operations.

When the limit is exceeded, range-diff now displays a clear error message showing both the requested memory size and the maximum allowed, formatted in human-readable units for better user experience.

Example usage:
git range-diff --max-memory=1G branch1...branch2 git range-diff --max-memory=500M base..topic1 base..topic2

This approach was chosen over alternatives:
- Pre-counting commits: Would require spawning additional git processes and reading all commits twice
- Limiting by commit count: Less precise than actual memory usage
- Streaming approach: Would require significant refactoring of the current algorithm

This issue was previously discussed in:
https://lore.kernel.org/git/[email protected]/

---
range-diff: add configurable memory limit for cost matrix

Problem Description
===================

When git range-diff is given extremely large ranges, it can result in either:

1. Segmentation fault due to integer overflow in array index calculations
2. Excessive memory consumption leading to system hangs or OOM kills
3. Poor user experience with the command appearing to hang for minutes

Reproduction Case
=================

In a Shopify's large monorepo a range-diff command like this crashes after several minutes with a SIGBUS error

$ git range-diff 4430f36511..316c1276c6 cb5240b6a8..2bbd292091

Range statistics:

* First range: 256,783 commits
* Second range: 1 commit
* Total: 256,784 commits
* Memory required for cost matrix: n² × 4 bytes = ~260GB

Stack Trace (Segmentation Fault)
================================

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x6e000ae3b8)
* frame #0: 0x000000010029a284 git`get_correspondences(a=0x000000016fde6188, b=0x000000016fde6160, creation_factor=60) at range-diff.c:356:20
frame #1: 0x0000000100299310 git`show_range_diff(range1="4430f36511cbacf5c517c6851e2b8508a72dfd30..316c1276c63f55ad9413fa18bf3b6483564a9cf4", range2="cb5240b6a8ba59b4a1f282559ee0742721b0cafc..2bbd292091e376d177ce62e264dae8872ca6be5a", range_diff_opts=0x000000016fde6308) at range-diff.c:593:3 frame #2: 0x00000001000c719c git`cmd_range_diff(argc=2, argv=0x0000600000bcd8c0, prefix="areas/core/shopify/", repo=0x0000000100468b20) at range-diff.c:167:8
frame #3: 0x000000010000277c git`run_builtin(p=0x00000001004408d8, argc=3, argv=0x0000600000bcd8c0, repo=0x0000000100468b20) at git.c:480:11
frame #4: 0x0000000100001020 git`handle_builtin(args=0x000000016fde6c70) at git.c:746:9
frame #5: 0x0000000100002074 git`run_argv(args=0x000000016fde6c70) at git.c:813:4
frame #6: 0x0000000100000d3c git`cmd_main(argc=3, argv=0x000000016fde7350) at git.c:953:19
frame #7: 0x000000010012750c git`main(argc=4, argv=0x000000016fde7348) at common-main.c:9:11
frame #8: 0x000000018573ab98 dyld`start + 6076

Root Cause Analysis
===================

The crash occurs in get_correspondences() at line 356:

static void get_correspondences(struct string_list *a, struct string_list *b, ...) {
int n = a->nr + b->nr; // Integer overflow: 256,784 fits in int
...
ALLOC_ARRAY(cost, st_mult(n, n)); // Would allocate ~260GB
...
cost[i + n * j] = c; // Line 356: Invalid memory access
}

Problems:

1. Integer overflow: While n=256,784 fits in an int, n*n overflows
2. Memory allocation: Even with proper types, allocating 260GB is impractical

Solution
========

Add a memory limit check in get_correspondences() before allocating the cost matrix. This check uses the total size in bytes (n² × sizeof(int)) and compares it against a configurable maximum, preventing both excessive memory usage and integer overflow issues.

The limit is configurable via a new --max-memory option that accepts human-readable sizes (e.g., "1G", "500M"). The default is 4GB for 64 bit systems and 2GB for 32 bit systems. This allows comparing ranges of approximately 32,000 (16,000) commits - generous for real-world use cases while preventing impractical operations.

When the limit is exceeded, range-diff now displays a clear error message showing both the requested memory size and the maximum allowed, formatted in human-readable units for better user experience.

Example usage: git range-diff --max-memory=1G branch1...branch2 git range-diff --max-memory=500M base..topic1 base..topic2

This approach was chosen over alternatives:

* Pre-counting commits: Would require spawning additional git processes and reading all commits twice
* Limiting by commit count: Less precise than actual memory usage
* Streaming approach: Would require significant refactoring of the current algorithm

This issue was previously discussed in:
https://lore.kernel.org/git/[email protected]/

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1958%2Fpcasaretto%2Frange-diff-size-limit-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1958/pcasaretto/range-diff-size-limit-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1958

Range-diff vs v2:

 1:  ec5dcdf9d0 < -:  ---------- range-diff: reorder options lexicographically
 2:  c81f920fee ! 1:  6d7ff43e56 range-diff: add configurable memory limit for cost matrix

@@
## Metadata ##
-Author: pcasaretto <[email protected]> +Author: Paulo Casaretto <[email protected]>

## Commit message ##
range-diff: add configurable memory limit for cost matrix
@@ Commit message
https://lore.kernel.org/git/[email protected]/

- Signed-off-by: Paulo Casaretto <[email protected]>
+ Signed-off-by: Paulo Casaretto <[email protected]>

## builtin/log.c ##
@@ builtin/log.c: static void make_cover_letter(struct rev_info *rev, int use_separate_file,
@@ builtin/range-diff.c: int cmd_range_diff(int argc,
.other_arg = &other_arg
};
@@ builtin/range-diff.c: int cmd_range_diff(int argc,
+ PARSE_OPT_OPTARG),
+ OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
N_("style"), N_("passed to 'git log'"), 0),
- OPT_BOOL(0, "left-only", &left_only,
- N_("only emit output related to the first range")),
+ OPT_CALLBACK(0, "max-memory", &range_diff_opts.max_memory,
+ N_("size"),
+ N_("maximum memory for cost matrix (default 4G)"),
+ parse_max_memory),
- OPT_BOOL(0, "no-dual-color", &simple_color,
- N_("use simple diff colors")),
- OPT_PASSTHRU_ARGV(0, "notes", &other_arg,
+ OPT_PASSTHRU_ARGV(0, "remerge-diff", &diff_merges_arg, NULL,
+ N_("passed to 'git log'"), PARSE_OPT_NOARG),
+ OPT_BOOL(0, "left-only", &left_only,

## log-tree.c ##
@@ log-tree.c: static void show_diff_of_diff(struct rev_info *opt)

 builtin/log.c        |  1 +
 builtin/range-diff.c | 21 +++++++++++++++++++++
 log-tree.c           |  1 +
 range-diff.c         | 20 ++++++++++++++++----
 range-diff.h         |  5 +++++
 5 files changed, 44 insertions(+), 4 deletions(-)

index c2f8bbf863..5f552d14c0 100644
@@ -1404,6 +1404,7 @@ static void make_cover_letter(struct rev_info *rev, int use_separate_file,
 		struct range_diff_options range_diff_opts = {
 			.creation_factor = rev->creation_factor,
 			.dual_color = 1,
+			.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 			.diffopt = &opts,
 			.other_arg = &other_arg
 		};

index a563abff5f..aafcc99b96 100644
@@ -6,6 +6,7 @@
 #include "parse-options.h"
 #include "range-diff.h"
 #include "config.h"
+#include "parse.h"
 
 
 static const char * const builtin_range_diff_usage[] = {

@@ -15,6 +16,21 @@ N_("git range-diff [<options>] <base> <old-tip> <new-tip>"),
 NULL
 };
 
+static int parse_max_memory(const struct option *opt, const char *arg, int unset)
+{
+	size_t *max_memory = opt->value;
+	uintmax_t val;
+
+	if (unset)
+		return 0;
+
+	if (!git_parse_unsigned(arg, &val, SIZE_MAX))
+		return error(_("invalid max-memory value: %s"), arg);
+
+	*max_memory = (size_t)val;
+	return 0;
+}
+
 int cmd_range_diff(int argc,
 		   const char **argv,
 		   const char *prefix,

@@ -25,6 +41,7 @@ int cmd_range_diff(int argc,
 	struct strvec diff_merges_arg = STRVEC_INIT;
 	struct range_diff_options range_diff_opts = {
 		.creation_factor = RANGE_DIFF_CREATION_FACTOR_DEFAULT,
+		.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 		.diffopt = &diffopt,
 		.other_arg = &other_arg
 	};

@@ -40,6 +57,10 @@ int cmd_range_diff(int argc,
 				  PARSE_OPT_OPTARG),
 		OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
 				  N_("style"), N_("passed to 'git log'"), 0),
+		OPT_CALLBACK(0, "max-memory", &range_diff_opts.max_memory,
+			     N_("size"),
+			     N_("maximum memory for cost matrix (default 4G)"),
+			     parse_max_memory),
 		OPT_PASSTHRU_ARGV(0, "remerge-diff", &diff_merges_arg, NULL,
 				  N_("passed to 'git log'"), PARSE_OPT_NOARG),
 		OPT_BOOL(0, "left-only", &left_only,

index 233bf9f227..73d21f7176 100644
@@ -717,6 +717,7 @@ static void show_diff_of_diff(struct rev_info *opt)
 		struct range_diff_options range_diff_opts = {
 			.creation_factor = opt->creation_factor,
 			.dual_color = 1,
+			.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 			.diffopt = &opts
 		};
 

index 8a2dcbee32..e31f71c73d 100644
@@ -325,13 +325,24 @@ static int diffsize(const char *a, const char *b)
 }
 
 static void get_correspondences(struct string_list *a, struct string_list *b,
-				int creation_factor)
+				int creation_factor, size_t max_memory)
 {
 	int n = a->nr + b->nr;
 	int *cost, c, *a2b, *b2a;
 	int i, j;
-
-	ALLOC_ARRAY(cost, st_mult(n, n));
+	size_t cost_size = st_mult(n, n);
+	size_t cost_bytes = st_mult(sizeof(int), cost_size);
+	if (cost_bytes >= max_memory) {
+		struct strbuf cost_str = STRBUF_INIT;
+		struct strbuf max_str = STRBUF_INIT;
+		strbuf_humanise_bytes(&cost_str, cost_bytes);
+		strbuf_humanise_bytes(&max_str, max_memory);
+		die(_("range-diff: unable to compute the range-diff, since it "
+		      "exceeds the maximum memory for the cost matrix: %s "
+		      "(%"PRIuMAX" bytes) needed, %s (%"PRIuMAX" bytes) available"),
+		    cost_str.buf, (uintmax_t)cost_bytes, max_str.buf, (uintmax_t)max_memory);
+	}
+	ALLOC_ARRAY(cost, cost_size);
 	ALLOC_ARRAY(a2b, n);
 	ALLOC_ARRAY(b2a, n);
 

@@ -591,7 +602,8 @@ int show_range_diff(const char *range1, const char *range2,
 	if (!res) {
 		find_exact_matches(&branch1, &branch2);
 		get_correspondences(&branch1, &branch2,
-				    range_diff_opts->creation_factor);
+				    range_diff_opts->creation_factor,
+				    range_diff_opts->max_memory);
 		output(&branch1, &branch2, range_diff_opts);
 	}
 

index cd85000b5a..9d39818e34 100644
@@ -5,6 +5,10 @@
 #include "strvec.h"
 
 #define RANGE_DIFF_CREATION_FACTOR_DEFAULT 60
+#define RANGE_DIFF_MAX_MEMORY_DEFAULT \
+	(sizeof(void*) >= 8 ? \
+		((size_t)(1024L * 1024L) * (size_t)(4L * 1024L)) : /* 4GB on 64-bit */ \
+		((size_t)(1024L * 1024L) * (size_t)(2L * 1024L)))   /* 2GB on 32-bit */
 
 /*
  * A much higher value than the default, when we KNOW we are comparing

@@ -17,6 +21,7 @@ struct range_diff_options {
 	unsigned dual_color:1;
 	unsigned left_only:1, right_only:1;
 	unsigned include_merges:1;
+	size_t max_memory;
 	const struct diff_options *diffopt; /* may be NULL */
 	const struct strvec *other_arg; /* may be NULL */
 };

base-commit: 954d33a9757fcfab723a824116902f1eb16e05f7

gitgitgadget

Right. I overlooked it, sorry.

Looks good, especially without the reordering existing entries in the options list. The authorship information above looks much better, too.

From: Paulo Casaretto <[email protected]>

When comparing large commit ranges (e.g., 250,000+ commits), range-diff attempts to allocate an n×n cost matrix that can exhaust available memory. For example, with 256,784 commits (n = 513,568), the matrix would require approximately 256GB of memory (513,568² × 4 bytes), causing either immediate segmentation faults due to integer overflow or system hangs.

Add a memory limit check in get_correspondences() before allocating the cost matrix. This check uses the total size in bytes (n² × sizeof(int)) and compares it against a configurable maximum, preventing both excessive memory usage and integer overflow issues.

The limit is configurable via a new --max-memory option that accepts human-readable sizes (e.g., "1G", "500M"). The default is 4GB for 64 bit systems and 2GB for 32 bit systems. This allows comparing ranges of approximately 32,000 (16,000) commits - generous for real-world use cases while preventing impractical operations.

When the limit is exceeded, range-diff now displays a clear error message showing both the requested memory size and the maximum allowed, formatted in human-readable units for better user experience.

Example usage:
git range-diff --max-memory=1G branch1...branch2 git range-diff --max-memory=500M base..topic1 base..topic2

This approach was chosen over alternatives:
- Pre-counting commits: Would require spawning additional git processes and reading all commits twice
- Limiting by commit count: Less precise than actual memory usage
- Streaming approach: Would require significant refactoring of the current algorithm

This issue was previously discussed in:
https://lore.kernel.org/git/[email protected]/

---
range-diff: add configurable memory limit for cost matrix

Problem Description
===================

When git range-diff is given extremely large ranges, it can result in either:

1. Segmentation fault due to integer overflow in array index calculations
2. Excessive memory consumption leading to system hangs or OOM kills
3. Poor user experience with the command appearing to hang for minutes

Reproduction Case
=================

In a Shopify's large monorepo a range-diff command like this crashes after several minutes with a SIGBUS error

$ git range-diff 4430f36511..316c1276c6 cb5240b6a8..2bbd292091

Range statistics:

* First range: 256,783 commits
* Second range: 1 commit
* Total: 256,784 commits
* Memory required for cost matrix: n² × 4 bytes = ~260GB

Stack Trace (Segmentation Fault)
================================

(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x6e000ae3b8)
* frame #0: 0x000000010029a284 git`get_correspondences(a=0x000000016fde6188, b=0x000000016fde6160, creation_factor=60) at range-diff.c:356:20
frame #1: 0x0000000100299310 git`show_range_diff(range1="4430f36511cbacf5c517c6851e2b8508a72dfd30..316c1276c63f55ad9413fa18bf3b6483564a9cf4", range2="cb5240b6a8ba59b4a1f282559ee0742721b0cafc..2bbd292091e376d177ce62e264dae8872ca6be5a", range_diff_opts=0x000000016fde6308) at range-diff.c:593:3 frame #2: 0x00000001000c719c git`cmd_range_diff(argc=2, argv=0x0000600000bcd8c0, prefix="areas/core/shopify/", repo=0x0000000100468b20) at range-diff.c:167:8
frame #3: 0x000000010000277c git`run_builtin(p=0x00000001004408d8, argc=3, argv=0x0000600000bcd8c0, repo=0x0000000100468b20) at git.c:480:11
frame #4: 0x0000000100001020 git`handle_builtin(args=0x000000016fde6c70) at git.c:746:9
frame #5: 0x0000000100002074 git`run_argv(args=0x000000016fde6c70) at git.c:813:4
frame #6: 0x0000000100000d3c git`cmd_main(argc=3, argv=0x000000016fde7350) at git.c:953:19
frame #7: 0x000000010012750c git`main(argc=4, argv=0x000000016fde7348) at common-main.c:9:11
frame #8: 0x000000018573ab98 dyld`start + 6076

Root Cause Analysis
===================

The crash occurs in get_correspondences() at line 356:

static void get_correspondences(struct string_list *a, struct string_list *b, ...) {
int n = a->nr + b->nr; // Integer overflow: 256,784 fits in int
...
ALLOC_ARRAY(cost, st_mult(n, n)); // Would allocate ~260GB
...
cost[i + n * j] = c; // Line 356: Invalid memory access
}

Problems:

1. Integer overflow: While n=256,784 fits in an int, n*n overflows
2. Memory allocation: Even with proper types, allocating 260GB is impractical

Solution
========

Add a memory limit check in get_correspondences() before allocating the cost matrix. This check uses the total size in bytes (n² × sizeof(int)) and compares it against a configurable maximum, preventing both excessive memory usage and integer overflow issues.

The limit is configurable via a new --max-memory option that accepts human-readable sizes (e.g., "1G", "500M"). The default is 4GB for 64 bit systems and 2GB for 32 bit systems. This allows comparing ranges of approximately 32,000 (16,000) commits - generous for real-world use cases while preventing impractical operations.

When the limit is exceeded, range-diff now displays a clear error message showing both the requested memory size and the maximum allowed, formatted in human-readable units for better user experience.

Example usage: git range-diff --max-memory=1G branch1...branch2 git range-diff --max-memory=500M base..topic1 base..topic2

This approach was chosen over alternatives:

* Pre-counting commits: Would require spawning additional git processes and reading all commits twice
* Limiting by commit count: Less precise than actual memory usage
* Streaming approach: Would require significant refactoring of the current algorithm

This issue was previously discussed in:
https://lore.kernel.org/git/[email protected]/

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1958%2Fpcasaretto%2Frange-diff-size-limit-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1958/pcasaretto/range-diff-size-limit-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/1958

Range-diff vs v3:

1: 6d7ff43e56 ! 1: 203113ea2a range-diff: add configurable memory limit for cost matrix

     @@ range-diff.c: static int diffsize(const char *a, const char *b)
      +		strbuf_humanise_bytes(&max_str, max_memory);
      +		die(_("range-diff: unable to compute the range-diff, since it "
      +		      "exceeds the maximum memory for the cost matrix: %s "
     -+		      "(%"PRIuMAX" bytes) needed, %s (%"PRIuMAX" bytes) available"),
     ++		      "(%"PRIuMAX" bytes) needed, limited to %s (%"PRIuMAX" bytes)"),
      +		    cost_str.buf, (uintmax_t)cost_bytes, max_str.buf, (uintmax_t)max_memory);
      +	}
      +	ALLOC_ARRAY(cost, cost_size);

 builtin/log.c        |  1 +
 builtin/range-diff.c | 21 +++++++++++++++++++++
 log-tree.c           |  1 +
 range-diff.c         | 20 ++++++++++++++++----
 range-diff.h         |  5 +++++
 5 files changed, 44 insertions(+), 4 deletions(-)

index c2f8bbf863..5f552d14c0 100644
@@ -1404,6 +1404,7 @@ static void make_cover_letter(struct rev_info *rev, int use_separate_file,
 		struct range_diff_options range_diff_opts = {
 			.creation_factor = rev->creation_factor,
 			.dual_color = 1,
+			.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 			.diffopt = &opts,
 			.other_arg = &other_arg
 		};

index a563abff5f..aafcc99b96 100644
@@ -6,6 +6,7 @@
 #include "parse-options.h"
 #include "range-diff.h"
 #include "config.h"
+#include "parse.h"
 
 
 static const char * const builtin_range_diff_usage[] = {

@@ -15,6 +16,21 @@ N_("git range-diff [<options>] <base> <old-tip> <new-tip>"),
 NULL
 };
 
+static int parse_max_memory(const struct option *opt, const char *arg, int unset)
+{
+	size_t *max_memory = opt->value;
+	uintmax_t val;
+
+	if (unset)
+		return 0;
+
+	if (!git_parse_unsigned(arg, &val, SIZE_MAX))
+		return error(_("invalid max-memory value: %s"), arg);
+
+	*max_memory = (size_t)val;
+	return 0;
+}
+
 int cmd_range_diff(int argc,
 		   const char **argv,
 		   const char *prefix,

@@ -25,6 +41,7 @@ int cmd_range_diff(int argc,
 	struct strvec diff_merges_arg = STRVEC_INIT;
 	struct range_diff_options range_diff_opts = {
 		.creation_factor = RANGE_DIFF_CREATION_FACTOR_DEFAULT,
+		.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 		.diffopt = &diffopt,
 		.other_arg = &other_arg
 	};

@@ -40,6 +57,10 @@ int cmd_range_diff(int argc,
 				  PARSE_OPT_OPTARG),
 		OPT_PASSTHRU_ARGV(0, "diff-merges", &diff_merges_arg,
 				  N_("style"), N_("passed to 'git log'"), 0),
+		OPT_CALLBACK(0, "max-memory", &range_diff_opts.max_memory,
+			     N_("size"),
+			     N_("maximum memory for cost matrix (default 4G)"),
+			     parse_max_memory),
 		OPT_PASSTHRU_ARGV(0, "remerge-diff", &diff_merges_arg, NULL,
 				  N_("passed to 'git log'"), PARSE_OPT_NOARG),
 		OPT_BOOL(0, "left-only", &left_only,

index 233bf9f227..73d21f7176 100644
@@ -717,6 +717,7 @@ static void show_diff_of_diff(struct rev_info *opt)
 		struct range_diff_options range_diff_opts = {
 			.creation_factor = opt->creation_factor,
 			.dual_color = 1,
+			.max_memory = RANGE_DIFF_MAX_MEMORY_DEFAULT,
 			.diffopt = &opts
 		};
 

index 8a2dcbee32..ca449a0769 100644
@@ -325,13 +325,24 @@ static int diffsize(const char *a, const char *b)
 }
 
 static void get_correspondences(struct string_list *a, struct string_list *b,
-				int creation_factor)
+				int creation_factor, size_t max_memory)
 {
 	int n = a->nr + b->nr;
 	int *cost, c, *a2b, *b2a;
 	int i, j;
-
-	ALLOC_ARRAY(cost, st_mult(n, n));
+	size_t cost_size = st_mult(n, n);
+	size_t cost_bytes = st_mult(sizeof(int), cost_size);
+	if (cost_bytes >= max_memory) {
+		struct strbuf cost_str = STRBUF_INIT;
+		struct strbuf max_str = STRBUF_INIT;
+		strbuf_humanise_bytes(&cost_str, cost_bytes);
+		strbuf_humanise_bytes(&max_str, max_memory);
+		die(_("range-diff: unable to compute the range-diff, since it "
+		      "exceeds the maximum memory for the cost matrix: %s "
+		      "(%"PRIuMAX" bytes) needed, limited to %s (%"PRIuMAX" bytes)"),
+		    cost_str.buf, (uintmax_t)cost_bytes, max_str.buf, (uintmax_t)max_memory);
+	}
+	ALLOC_ARRAY(cost, cost_size);
 	ALLOC_ARRAY(a2b, n);
 	ALLOC_ARRAY(b2a, n);
 

@@ -591,7 +602,8 @@ int show_range_diff(const char *range1, const char *range2,
 	if (!res) {
 		find_exact_matches(&branch1, &branch2);
 		get_correspondences(&branch1, &branch2,
-				    range_diff_opts->creation_factor);
+				    range_diff_opts->creation_factor,
+				    range_diff_opts->max_memory);
 		output(&branch1, &branch2, range_diff_opts);
 	}
 

index cd85000b5a..9d39818e34 100644
@@ -5,6 +5,10 @@
 #include "strvec.h"
 
 #define RANGE_DIFF_CREATION_FACTOR_DEFAULT 60
+#define RANGE_DIFF_MAX_MEMORY_DEFAULT \
+	(sizeof(void*) >= 8 ? \
+		((size_t)(1024L * 1024L) * (size_t)(4L * 1024L)) : /* 4GB on 64-bit */ \
+		((size_t)(1024L * 1024L) * (size_t)(2L * 1024L)))   /* 2GB on 32-bit */
 
 /*
  * A much higher value than the default, when we KNOW we are comparing

@@ -17,6 +21,7 @@ struct range_diff_options {
 	unsigned dual_color:1;
 	unsigned left_only:1, right_only:1;
 	unsigned include_merges:1;
+	size_t max_memory;
 	const struct diff_options *diffopt; /* may be NULL */
 	const struct strvec *other_arg; /* may be NULL */
 };

base-commit: 954d33a9757fcfab723a824116902f1eb16e05f7

gitgitgadget