RE: [PATCH] crypto/caam: Avoid GCC constprop bug warning

[David Laight <David.Laight@ACULAB.COM>]

From: Anders Roxell
> Sent: 02 December 2022 00:58
> 
> On 2022-10-28 14:05, Kees Cook wrote:
> > GCC 12 appears to perform constant propagation incompletely(?) and can
> > no longer notice that "len" is always 0 when "data" is NULL. Expand the
> > check to avoid warnings about memcpy() having a NULL argument:
> >
> >    ...
> >                     from drivers/crypto/caam/key_gen.c:8:
> >    drivers/crypto/caam/desc_constr.h: In function 'append_data.constprop':
> >    include/linux/fortify-string.h:48:33: warning: argument 2 null where non-null expected [-
> Wnonnull]
> >       48 | #define __underlying_memcpy     __builtin_memcpy
> >          |                                 ^
> >    include/linux/fortify-string.h:438:9: note: in expansion of macro '__underlying_memcpy'
> >      438 |         __underlying_##op(p, q, __fortify_size);                        \
> >          |         ^~~~~~~~~~~~~
...

Is this really a bug in the fortify-string wrappers?
IIRC the call is memcpy(NULL, ptr, 0) (or maybe memcpy(ptr, NULL, 0).
In either case call can be removed at compile time.

I'd bet that the constant propagation of 'len' fails because
of all the intermediate variables that get used in order to
avoid multiple evaluation.

The some 'tricks' that are used in min() (see minmax.h) to
generate a constant output for constant input could be
use to detect a compile-time zero length.

Something like:
#define memcpy(dst, src, len) \
	(__is_constzero(len) ? (dst) : memcpy_check(dst, src, len))

With:
#define __is_constzero(x) sizeof(*(1 ? (void *)(x) : (int *)0) != 1)
Which could go into const.h and used in the definition of __is_constexpr().

	David

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