Re: [PATCH] KEYS: trusted: fix -Wvarags warning

On Mon, Oct 15, 2018 at 2:26 AM David Laight [off-list ref] wrote:

From: ndesaulniers@google.com

quoted

Sent: 11 October 2018 21:31

...

quoted

by swapping h2 and h3.

security/keys/trusted.c:146:17: warning: passing an object that
undergoes default
      argument promotion to 'va_start' has undefined behavior [-Wvarargs]
        va_start(argp, h3);
                       ^
security/keys/trusted.c:126:37: note: parameter of type 'unsigned
char' is declared here
unsigned char *h2, unsigned char h3, ...)
                               ^
Specifically, it seems that both the C90 (4.8.1.1) and C11 (7.16.1.4)
standards explicitly call this out as undefined behavior:

I guess that problems arise when all the arguments are stacked
and va_start/va_arg use naive pointer manipulation.
In that case &h3 might be 4n+3 aligned so va_arg() will access
misaligned stack locations.

I doubt any modern compilers (where va_start and va_arg are builtins)
will get this 'wrong' even when all arguments are stacked.

Seems clang is being over cautious.

Yes; did you have feedback on the Denis' proposed fix, or another?

        David

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-- 
Thanks,
~Nick Desaulniers