gcc-plugins: randstruct: Only warn about true flexible arrays

PATCH Applied Kees Cook 3 participants 4 messages Nov 4, 2023 – Nov 8, 2023 Gcc plugins

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Kees Cook

author

2023-11-04 20:43 UTC #0 lore raw

PATCH 1/1 gcc-plugins: randstruct: Only warn about true flexible arrays

The randstruct GCC plugin tried to discover "fake" flexible arrays to issue warnings about them in randomized structs. In the future LSM overhead reduction series, it would be legal to have a randomized struct with a 1-element array, and this should _not_ be treated as a flexible array, especially since commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"). Disable the 0-sized and 1-element array discovery logic in the plugin, but keep the "true" flexible array check.

Cc KP Singh <[email protected]>

Cc [email protected]

Reported-by kernel test robot <[email protected]>

Closes https://lore.kernel.org/oe-kbuild-all/[email protected]/ open here

Fixes df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")

Signed-off-by Kees Cook <[email protected]>

---
 scripts/gcc-plugins/randomize_layout_plugin.c | 10 ----------
 1 file changed, 10 deletions(-)

scripts/gcc-plugins/randomize_layout_plugin.c +0 ~~-10~~

index 366395cab490..910bd21d08f4 100644
@@ -278,8 +278,6 @@ static bool is_flexible_array(const_tree field)
 {
 	const_tree fieldtype;
 	const_tree typesize;
-	const_tree elemtype;
-	const_tree elemsize;
 
 	fieldtype = TREE_TYPE(field);
 	typesize = TYPE_SIZE(fieldtype);

@@ -287,20 +285,12 @@ static bool is_flexible_array(const_tree field)
 	if (TREE_CODE(fieldtype) != ARRAY_TYPE)
 		return false;
 
-	elemtype = TREE_TYPE(fieldtype);
-	elemsize = TYPE_SIZE(elemtype);
-
 	/* size of type is represented in bits */
 
 	if (typesize == NULL_TREE && TYPE_DOMAIN(fieldtype) != NULL_TREE &&
 	    TYPE_MAX_VALUE(TYPE_DOMAIN(fieldtype)) == NULL_TREE)
 		return true;
 
-	if (typesize != NULL_TREE &&
-	    (TREE_CONSTANT(typesize) && (!tree_to_uhwi(typesize) ||
-	     tree_to_uhwi(typesize) == tree_to_uhwi(elemsize))))
-		return true;
-
 	return false;
 }

Signature · 1 line

2.34.1

Bill Wendling

Reviewed-by on v1 1/1

2023-11-06 08:47 UTC #1 lore raw

Quoted 9 lines · Kees Cook

The randstruct GCC plugin tried to discover "fake" flexible arrays to issue warnings about them in randomized structs. In the future LSM overhead reduction series, it would be legal to have a randomized struct with a 1-element array, and this should _not_ be treated as a flexible array, especially since commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"). Disable the 0-sized and 1-element array discovery logic in the plugin, but keep the "true" flexible array check.

Quoted 6 lines

Cc KP Singh <[email protected]>
Cc [email protected]
Reported-by kernel test robot <[email protected]>
Closes https://lore.kernel.org/oe-kbuild-all/[email protected]/ open here
Fixes df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
Signed-off-by Kees Cook <[email protected]>

Reviewed-by Bill Wendling <[email protected]>

Quoted 42 lines

---
 scripts/gcc-plugins/randomize_layout_plugin.c | 10 ----------
 1 file changed, 10 deletions(-)

scripts/gcc-plugins/randomize_layout_plugin.c +0 ~~-11~~

index 366395cab490..910bd21d08f4 100644
@@ -278,8 +278,6 @@ static bool is_flexible_array(const_tree field)
 {
        const_tree fieldtype;
        const_tree typesize;
-       const_tree elemtype;
-       const_tree elemsize;
 
        fieldtype = TREE_TYPE(field);
        typesize = TYPE_SIZE(fieldtype);

@@ -287,20 +285,12 @@ static bool is_flexible_array(const_tree field)
        if (TREE_CODE(fieldtype) != ARRAY_TYPE)
                return false;
 
-       elemtype = TREE_TYPE(fieldtype);
-       elemsize = TYPE_SIZE(elemtype);
-
        /* size of type is represented in bits */
 
        if (typesize == NULL_TREE && TYPE_DOMAIN(fieldtype) != NULL_TREE &&
            TYPE_MAX_VALUE(TYPE_DOMAIN(fieldtype)) == NULL_TREE)
                return true;
 
-       if (typesize != NULL_TREE &&
-           (TREE_CONSTANT(typesize) && (!tree_to_uhwi(typesize) ||
-            tree_to_uhwi(typesize) == tree_to_uhwi(elemsize))))
-               return true;
-
        return false;
 }
 
--

2.34.1

Gustavo A. R. Silva

Acked-by on v1 1/1

2023-11-06 15:53 UTC #2 lore raw

In reply to Kees Cook

Quoted 8 lines · Kees Cook

The randstruct GCC plugin tried to discover "fake" flexible arrays to issue warnings about them in randomized structs. In the future LSM overhead reduction series, it would be legal to have a randomized struct with a 1-element array, and this should _not_ be treated as a flexible array, especially since commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"). Disable the 0-sized and 1-element array discovery logic in the plugin, but keep the "true" flexible array check.

Quoted 6 lines

Cc KP Singh <[email protected]>
Cc [email protected]
Reported-by kernel test robot <[email protected]>
Closes https://lore.kernel.org/oe-kbuild-all/[email protected]/ open here
Fixes df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
Signed-off-by Kees Cook <[email protected]>

Acked-by Gustavo A. R. Silva <[email protected]>

Thanks!
--
Gustavo

Quoted 38 lines

---
  scripts/gcc-plugins/randomize_layout_plugin.c | 10 ----------
  1 file changed, 10 deletions(-)

scripts/gcc-plugins/randomize_layout_plugin.c +0 ~~-10~~

index 366395cab490..910bd21d08f4 100644
@@ -278,8 +278,6 @@ static bool is_flexible_array(const_tree field)
  {
  	const_tree fieldtype;
  	const_tree typesize;
-	const_tree elemtype;
-	const_tree elemsize;
 
  	fieldtype = TREE_TYPE(field);
  	typesize = TYPE_SIZE(fieldtype);

@@ -287,20 +285,12 @@ static bool is_flexible_array(const_tree field)
  	if (TREE_CODE(fieldtype) != ARRAY_TYPE)
  		return false;
 
-	elemtype = TREE_TYPE(fieldtype);
-	elemsize = TYPE_SIZE(elemtype);
-
  	/* size of type is represented in bits */
 
  	if (typesize == NULL_TREE && TYPE_DOMAIN(fieldtype) != NULL_TREE &&
  	    TYPE_MAX_VALUE(TYPE_DOMAIN(fieldtype)) == NULL_TREE)
  		return true;
 
-	if (typesize != NULL_TREE &&
-	    (TREE_CONSTANT(typesize) && (!tree_to_uhwi(typesize) ||
-	     tree_to_uhwi(typesize) == tree_to_uhwi(elemsize))))
-		return true;
-
  	return false;
  }

Kees Cook

author Applied

2023-11-08 22:20 UTC #3 lore raw

In reply to Kees Cook

Quoted 9 lines · Kees Cook

The randstruct GCC plugin tried to discover "fake" flexible arrays to issue warnings about them in randomized structs. In the future LSM overhead reduction series, it would be legal to have a randomized struct with a 1-element array, and this should _not_ be treated as a flexible array, especially since commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3"). Disable the 0-sized and 1-element array discovery logic in the plugin, but keep the "true" flexible array check.
[...]

Applied to for-next/hardening, thanks!

[1/1] gcc-plugins: randstruct: Only warn about true flexible arrays

      https://git.kernel.org/kees/c/1ee60356c2dc

Take care,

Signature · 1 line

Kees Cook