commit-reach: fix sorting commits by generation
PATCH 1/1 commit-reach: fix sorting commits by generation
compare_commit_by_gen is used to sort a list of pointers to 'struct commit'. The comparison function for qsort is called with pointers to the objects it needs to compare, so when sorting a list of 'struct commit *', the arguments are of type 'struct commit **'. However, currently the comparison function casts it's arguments to 'struct commit *' and uses those, leading to out of bounds memory access and potentially to wrong results. Fix that.
---
I noticed this by running the test suite through valgrind. I'm not familiar with this code, so I'm not sure why this didn't cause any issues or how they would manifest, but this seems like the right fix for this function either way.
commit-reach.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
commit-reach.c +2 -2
@@ -516,8 +516,8 @@ int commit_contains(struct ref_filter *filter, struct commit *commit,static int compare_commits_by_gen (const void *_a,const void *_b) { -const struct commit *a = (const struct commit *)_a; -const struct commit *b = (const struct commit *)_b; +const struct commit *a = *(const struct commit **)_a; +const struct commit *b = *(const struct commit **)_b;if (a->generation < b->generation)return -1 ;
Signature · 1 line
2.19.1.759.g500967bb5e
Am 22.10.2018 um 23:10 schrieb Thomas Gummerer:
Quoted 8 lines
compare_commit_by_genis used to sort a list of pointers to 'struct commit'. The comparison function for qsort is called with pointers to the objects it needs to compare, so when sorting a list of 'struct commit *', the arguments are of type 'struct commit **'. However, currently the comparison function casts it's arguments to 'struct commit *' and uses those, leading to out of bounds memory access and potentially to wrong results. Fix that.
Quoted 7 lines
Signed-off-by Thomas Gummerer <[email protected]>---
I noticed this by running the test suite through valgrind. I'm not familiar with this code, so I'm not sure why this didn't cause any issues or how they would manifest, but this seems like the right fix for this function either way.
Right; I sent a similar patch a while ago, but it seems to have fallen through the cracks:
https://public-inbox.org/git/[email protected]/
Anyway, your implied question was discussed back then. Derrick wrote:
The reason to sort is to hopefully minimize the amount we walk by exploring the "lower" commits first. This is a performance-only thing, not a correctness issue (which is why the bug exists). Even then, it is just a heuristic.
Does b6723e4671 in pu (commit-reach: fix first-parent heuristic) change that picture? Did a quick test and found no performance difference with and without the fix on top, i.e. proper sorting didn't seem to matter.
Quoted 9 lines
commit-reach.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)commit-reach.c +0
-0
commit-reach.c int commit_contains(struct ref_filter *filter, struct commit *commit, · line 523
Quoted 10 lines
commit-reach.c +2
-2@@ -516,8 +516,8 @@ int commit_contains(struct ref_filter *filter, struct commit *commit,static int compare_commits_by_gen (const void *_a,const void *_b) { -const struct commit *a = (const struct commit *)_a; -const struct commit *b = (const struct commit *)_b; +const struct commit *a = *(const struct commit **)_a; +const struct commit *b = *(const struct commit **)_b;if (a->generation < b->generation)return -1 ;
Looks good to me.
René
Quoted 17 lines · René Scharfe
Am 22.10.2018 um 23:10 schrieb Thomas Gummerer:
Quoted 8 lines
compare_commit_by_genis used to sort a list of pointers to 'struct commit'. The comparison function for qsort is called with pointers to the objects it needs to compare, so when sorting a list of 'struct commit *', the arguments are of type 'struct commit **'. However, currently the comparison function casts it's arguments to 'struct commit *' and uses those, leading to out of bounds memory access and potentially to wrong results. Fix that.Quoted 7 lines
Signed-off-by Thomas Gummerer <[email protected]>---
I noticed this by running the test suite through valgrind. I'm not familiar with this code, so I'm not sure why this didn't cause any issues or how they would manifest, but this seems like the right fix for this function either way.
Quoted 4 lines
Right; I sent a similar patch a while ago, but it seems to have fallen through the cracks:
Whoops I didn't notice that, I only checked whether the problem still exists in pu. I'd be more than happy to go with your patch instead.
Quoted 6 lines
Anyway, your implied question was discussed back then. Derrick wrote:
The reason to sort is to hopefully minimize the amount we walk by exploring the "lower" commits first. This is a performance-only thing, not a correctness issue (which is why the bug exists). Even then, it is just a heuristic.
Thanks for pointing that out!
Quoted 3 lines
Does
b6723e4671in pu (commit-reach: fix first-parent heuristic) change that picture? Did a quick test and found no performance difference with and without the fix on top, i.e. proper sorting didn't seem to matter.
I just gave 'test-tool reach can_all_from_reach' a try and got the same results, with or without the fix the times are very similar. I haven't had time to follow the commit-graph series though, so I'm not sure I used it correctly. I tried it on the linux repository with the following input:
X:v4.10
X:v4.9
X:v4.8
X:v4.7
X:v4.6
X:v4.5
X:v4.4
X:v4.3
X:v4.2
X:v4.1
Y:v3.10
Y:v3.9
Y:v3.8
Y:v3.7
Y:v3.6
Y:v3.5
Y:v3.4
Y:v3.3
Y:v3.2
Y:v3.1
Quoted 23 lines
Quoted 9 lines
commit-reach.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)commit-reach.c +0
-0commit-reach.c int commit_contains(struct ref_filter *filter, struct commit *commit, · line 523
Quoted 10 lines
commit-reach.c +2
-2@@ -516,8 +516,8 @@ int commit_contains(struct ref_filter *filter, struct commit *commit,static int compare_commits_by_gen (const void *_a,const void *_b) { -const struct commit *a = (const struct commit *)_a; -const struct commit *b = (const struct commit *)_b; +const struct commit *a = *(const struct commit **)_a; +const struct commit *b = *(const struct commit **)_b;if (a->generation < b->generation)return -1 ;Looks good to me.
René
Quoted 11 lines · Thomas Gummerer
Quoted 8 lines · René Scharfe
Am 22.10.2018 um 23:10 schrieb Thomas Gummerer:
Anyway, your implied question was discussed back then. Derrick wrote:
The reason to sort is to hopefully minimize the amount we walk by exploring the "lower" commits first. This is a performance-only thing, not a correctness issue (which is why the bug exists). Even then, it is just a heuristic.
Thanks for pointing that out!
Quoted 7 lines
Quoted 3 lines
Does
b6723e4671in pu (commit-reach: fix first-parent heuristic) change that picture? Did a quick test and found no performance difference with and without the fix on top, i.e. proper sorting didn't seem to matter.I just gave 'test-tool reach
can_all_from_reach' a try and got the same results, with or without the fix the times are very similar. I haven't had time to follow the commit-graph series though, so I'm not sure I used it correctly.
Thanks for your attention here. I've been thinking a lot about this heuristic and have concluded the following two things are true:
(1) When we return 1, the order that we explore the 'from' commits does not change the explored set of commits.
(2) When we return 0, the order that we explore the 'to' commits will change the explored set, but it is difficult to say that the heuristic helps more than it hurts.
Item (1) is contrary to what I had thought when I first created the heuristic.
The details are tricky, but essentially each DFS starting at a 'from' commit may be short-circuited due to a prior walk, but swapping the order of those two 'from' commits would lead to the same set of commits to be explored (with the short-circuit happening in the other commit). The only change is that we can terminate early if we fully explore a 'from' commit and do not find a commit marked with 'with_flag'. In this sense, it would be best to explore the commits that are "closest" to the generation number cutoff first, as we can maybe find a negative answer earlier in the search.
In this sense, we could remove the sort entirely and probably not have much performance hit. But since the set of 'from' commits is probably much smaller than the set of commits to explore, the sort is likely inexpensive.
In conclusion: I cannot say that this sort is super-important. As for the potential benefits in (2), I'll leave that to people who run git as a server who may have telemetry around fetch negotiation. How often do we actually say we need more rounds of negotiation? What kinds of data shapes matter there?
Thanks,
-Stolee