From: Kees Cook kees@kernel.org
[ Upstream commit e136a4062174a9a8d1c1447ca040ea81accfa6a8 ]
When building the randomized replacement tree of struct members, the randstruct GCC plugin would insert, as the first member, a 0-sized void member. This appears as though it was done to catch non-designated ("unnamed") static initializers, which wouldn't be stable since they depend on the original struct layout order.
This was accomplished by having the side-effect of the "void member" tripping an assert in GCC internals (count_type_elements) if the member list ever needed to be counted (e.g. for figuring out the order of members during a non-designated initialization), which would catch impossible type (void) in the struct:
security/landlock/fs.c: In function ‘hook_file_ioctl_common’: security/landlock/fs.c:1745:61: internal compiler error: in count_type_elements, at expr.cc:7075 1745 | .u.op = &(struct lsm_ioctlop_audit) { | ^
static HOST_WIDE_INT count_type_elements (const_tree type, bool for_ctor_p) { switch (TREE_CODE (type)) ... case VOID_TYPE: default: gcc_unreachable (); } }
However this is a redundant safety measure since randstruct uses the __designated_initializer attribute both internally and within the __randomized_layout attribute macro so that this would be enforced by the compiler directly even when randstruct was not enabled (via -Wdesignated-init).
A recent change in Landlock ended up tripping the same member counting routine when using a full-struct copy initializer as part of an anonymous initializer. This, however, is a false positive as the initializer is copying between identical structs (and hence identical layouts). The "path" member is "struct path", a randomized struct, and is being copied to from another "struct path", the "f_path" member:
landlock_log_denial(landlock_cred(file->f_cred), &(struct landlock_request) { .type = LANDLOCK_REQUEST_FS_ACCESS, .audit = { .type = LSM_AUDIT_DATA_IOCTL_OP, .u.op = &(struct lsm_ioctlop_audit) { .path = file->f_path, .cmd = cmd, }, }, ...
As can be seen with the coming randstruct KUnit test, there appears to be no behavioral problems with this kind of initialization when the void member is removed from the randstruct GCC plugin, so remove it.
Reported-by: "Dr. David Alan Gilbert" linux@treblig.org Closes: https://lore.kernel.org/lkml/Z_PRaKx7q70MKgCA@gallifrey/ Reported-by: Mark Brown broonie@kernel.org Closes: https://lore.kernel.org/lkml/20250407-kbuild-disable-gcc-plugins-v1-1-5d46ae... Reported-by: WangYuli wangyuli@uniontech.com Closes: https://lore.kernel.org/lkml/337D5D4887277B27+3c677db3-a8b9-47f0-93a4-780935... Fixes: 313dd1b62921 ("gcc-plugins: Add the randstruct plugin") Signed-off-by: Kees Cook kees@kernel.org Stable-dep-of: f39f18f3c353 ("randstruct: gcc-plugin: Fix attribute addition") Signed-off-by: Sasha Levin sashal@kernel.org --- scripts/gcc-plugins/randomize_layout_plugin.c | 18 +----------------- 1 file changed, 1 insertion(+), 17 deletions(-)
diff --git a/scripts/gcc-plugins/randomize_layout_plugin.c b/scripts/gcc-plugins/randomize_layout_plugin.c index c7ff92b4189cb..a5aea51ecca99 100644 --- a/scripts/gcc-plugins/randomize_layout_plugin.c +++ b/scripts/gcc-plugins/randomize_layout_plugin.c @@ -377,29 +377,13 @@ static int relayout_struct(tree type)
shuffle(type, (tree *)newtree, shuffle_length);
- /* - * set up a bogus anonymous struct field designed to error out on unnamed struct initializers - * as gcc provides no other way to detect such code - */ - list = make_node(FIELD_DECL); - TREE_CHAIN(list) = newtree[0]; - TREE_TYPE(list) = void_type_node; - DECL_SIZE(list) = bitsize_zero_node; - DECL_NONADDRESSABLE_P(list) = 1; - DECL_FIELD_BIT_OFFSET(list) = bitsize_zero_node; - DECL_SIZE_UNIT(list) = size_zero_node; - DECL_FIELD_OFFSET(list) = size_zero_node; - DECL_CONTEXT(list) = type; - // to satisfy the constify plugin - TREE_READONLY(list) = 1; - for (i = 0; i < num_fields - 1; i++) TREE_CHAIN(newtree[i]) = newtree[i+1]; TREE_CHAIN(newtree[num_fields - 1]) = NULL_TREE;
main_variant = TYPE_MAIN_VARIANT(type); for (variant = main_variant; variant; variant = TYPE_NEXT_VARIANT(variant)) { - TYPE_FIELDS(variant) = list; + TYPE_FIELDS(variant) = newtree[0]; TYPE_ATTRIBUTES(variant) = copy_list(TYPE_ATTRIBUTES(variant)); TYPE_ATTRIBUTES(variant) = tree_cons(get_identifier("randomize_performed"), NULL_TREE, TYPE_ATTRIBUTES(variant)); TYPE_ATTRIBUTES(variant) = tree_cons(get_identifier("designated_init"), NULL_TREE, TYPE_ATTRIBUTES(variant));