Use memset to zero stack allocations containing unions

[robn]

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=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).

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

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[robn]

Other things I looked into:

• reordering the unions to have the largest one at the front. This just makes dmu_replay_record_t harder to read, and it's difficult to keep sensibly updated
• detecting and disabling the -ftrivial-auto-var-init switch. Decided against because it's a statement of intent and we should honour it
• adding __attribute__(uninitialized) to these variables, which disables all automatic/implicit initialisation. I was going to try to add a cstyle lint to detect use of {0} without this attribute, but it's difficult to parse and doesn't work for static, not to mention zero initialisers being perfectly fine in most cases
• writing macro initialisers eg AES_CTX_INITITALIZER with a working designated initialiser under it, but I couldn't make it work

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.