Invalid -Wstringop-overread warning for valid POSIX constructs

[Florian Weimer <fweimer@redhat.com>]

This code:

#include <pthread.h>
#include <sys/mman.h>

void
f (pthread_key_t key)
{
  pthread_setspecific (key, MAP_FAILED);
}

Results in a warning:

t.c: In function ‘f’:
t.c:7:3: warning: ‘pthread_setspecific’ expecting 1 byte in a region of size 0 [-Wstringop-overread]
    7 |   pthread_setspecific (key, MAP_FAILED);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from t.c:1:
/usr/include/pthread.h:1308:12: note: in a call to function ‘pthread_setspecific’ declared with attribute ‘access (none, 2)’
 1308 | extern int pthread_setspecific (pthread_key_t __key,
      |            ^~~~~~~~~~~~~~~~~~~


This also results in the same warning, for different reasons:

#include <pthread.h>

extern int x[1];

void
f (pthread_key_t key)
{
  pthread_setspecific (key, &x[1]);
}

t.c: In function ‘f’:
t.c:8:3: warning: ‘pthread_setspecific’ expecting 1 byte in a region of size 0 [-Wstringop-overread]
    8 |   pthread_setspecific (key, &x[1]);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
t.c:3:12: note: at offset 4 into source object ‘x’ of size 4
    3 | extern int x[1];
      |            ^
In file included from t.c:1:
/usr/include/pthread.h:1308:12: note: in a call to function ‘pthread_setspecific’ declared with attribute ‘access (none, 2)’
 1308 | extern int pthread_setspecific (pthread_key_t __key,
      |            ^~~~~~~~~~~~~~~~~~~

The original argument justifying this warning was that passing
non-pointer constants is invalid.  But MAP_FAILED is a valid POSIX
pointer constant, so it is allowed here as well.  And the second example
shows that the warning also fires for completely valid pointers.  So the
none access attribute is clearly not correct here.  (“none” requires
that the pointer is valid, there just aren't any accesses to the object
it points to, but the object must exist.  Apparently, this is what the
kernel expects for its use of the annotation.)

The root of the problem is the const void * pointer argument.  Without
the access attribute, we warn for other examples:

typedef unsigned int pthread_key_t;
int pthread_setspecific (pthread_key_t __key, const void *);

void
f (pthread_key_t key)
{
  int x;
  pthread_setspecific (key, &x);
}

t.c: In function ‘f’:
t.c:10:3: warning: ‘x’ may be used uninitialized [-Wmaybe-uninitialized]
   10 |   pthread_setspecific (key, &x);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
t.c:4:5: note: by argument 2 of type ‘const void *’ to ‘pthread_setspecific’ declared here
    4 | int pthread_setspecific (pthread_key_t __key, const void *);
      |     ^~~~~~~~~~~~~~~~~~~
t.c:9:7: note: ‘x’ declared here
    9 |   int x;
      |       ^

This is why we added the none access attribute, but this leads to the
other problem.

We could change glibc to use a different attribute (preferable one that
we can probe using __has_attribute) if one were to become available, and
backport that.  But so far, I see nothing on the GCC side, and
GCC PR 102329 seems to have stalled.

Thanks,
Florian