Use memset to zero stack allocations containing unions #16206
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C99 6.7.8.17 says that when an undesignated initialiser is used, only the first element of a union is initialised. If the first element is not the largest within the union, how the remaining space is initialised is up to the compiler. GCC extends the initialiser to the entire union, while Clang treats the remainder as padding, and so initialises according to whatever automatic/implicit initialisation rules are currently active. When Linux is compiled with CONFIG_INIT_STACK_ALL_PATTERN, -ftrivial-auto-var-init=pattern is added to the kernel CFLAGS. This flag sets the policy for automatic/implicit initialisation of variables on the stack. Taken together, this means that when compiling under CONFIG_INIT_STACK_ALL_PATTERN on Clang, the "zero" initialiser will only zero the first element in a union, and the rest will be filled with a pattern. This is significant for aes_ctx_t, which in aes_encrypt_atomic() and aes_decrypt_atomic() is initialised to zero, but then used as a gcm_ctx_t, which is the fifth element in the union, and thus gets pattern initialisation. Later, it's assumed to be zero, resulting in a hang. As confusing and undiscoverable as it is, by the spec, we are at fault when we initialise a structure containing a union with the zero initializer. As such, this commit replaces these uses with an explicit memset(0). Sponsored-by: Klara, Inc. Sponsored-by: Wasabi Technology, Inc. Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
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Other things I looked into:
Ultimately, I settled on this, just explicitly zeroing those ones that need it. I would have liked any kind of indicator that something special was needed, like a compiler warning, or a macro initialiser that would do the right thing in all cases, but I can't see anything. If we can come up with something in the future, we can improve it. |
mcmilk
approved these changes
May 17, 2024
behlendorf
approved these changes
May 25, 2024
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Motivation and Context
Closes #16135.
Description
C99 6.7.8.17 says that when an undesignated initialiser is used, only the first element of a union is initialised. If the first element is not the largest within the union, how the remaining space is initialised is up to the compiler.
GCC extends the initialiser to the entire union, while Clang treats the remainder as padding, and so initialises according to whatever automatic/implicit initialisation rules are currently active.
When Linux is compiled with
CONFIG_INIT_STACK_ALL_PATTERN,-ftrivial-auto-var-init=patternis added to the kernel CFLAGS. This flag sets the policy for automatic/implicit initialisation of variables on the stack.Taken together, this means that when compiling under
CONFIG_INIT_STACK_ALL_PATTERNon Clang, the "zero" initialiser will only zero the first element in a union, and the rest will be filled with a pattern. This is significant foraes_ctx_t, which inaes_encrypt_atomic()andaes_decrypt_atomic()is initialised to zero, but then used as agcm_ctx_t, which is the fifth element in the union, and thus gets pattern initialisation. Later, it's assumed to be zero, resulting in a hang.As confusing and undiscoverable as it is, by the spec, we are at fault when we initialise a structure containing a union with the zero initializer. As such, this commit replaces these uses with an explicit
memset(0).How Has This Been Tested?
Full test suite run succeeded against a 6.7.12 kernel and this patch, both compiled using Clang 18.
Types of changes
Checklist:
Signed-off-by.