Re: [PATCH v2] [RFC] security: allow using Clang's zero initialization for stack variables

In addition to -ftrivial-auto-var-init=pattern (used by
CONFIG_INIT_STACK_ALL now) Clang also supports zero initialization for
locals enabled by -ftrivial-auto-var-init=zero.
The future of this flag is still being debated, see
https://bugs.llvm.org/show_bug.cgi?id=45497
Right now it is guarded by another flag,
-enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang,
which means it may not be supported by future Clang releases.
Another possible resolution is that -ftrivial-auto-var-init=zero will
persist (as certain users have already started depending on it), but the
name of the guard flag will change.

In the meantime, zero initialization has proven itself as a good
production mitigation measure against uninitialized locals. Unlike
pattern initialization, which has a higher chance of triggering existing
bugs, zero initialization provides safe defaults for strings, pointers,
indexes, and sizes. On the other hand, pattern initialization remains
safer for return values.
Performance-wise, the difference between pattern and zero initialization
is usually negligible, although the generated code for zero
initialization is more compact.

This patch renames CONFIG_INIT_STACK_ALL to
CONFIG_INIT_STACK_ALL_PATTERN and introduces another config option,
CONFIG_INIT_STACK_ALL_ZERO, that enables zero initialization for locals
if the corresponding flags are supported by Clang.

I'm a great fan of zero initialization as opposed to pattern.
I don't understand why clang is refusing to make this a supported option.

Anyway:

Reviewed-by: Maciej Żenczykowski <redacted>