[PATCH] Don't pass NULL pointer to error [BZ #24556]

[Florian Weimer]

* H. J. Lu:

> On Thu, May 23, 2019 at 8:20 AM Florian Weimer <fweimer@redhat.com> wrote:
>>
>> * H. J. Lu:
>>
>> > On Thu, May 23, 2019 at 12:02 AM Florian Weimer <fweimer@redhat.com> wrote:
>> >>
>> >> * H. J. Lu:
>> >>
>> >> > In function ‘error’,
>> >> >     inlined from ‘do_one_test’ at bench-strstr.c:149:7,
>> >> >     inlined from ‘do_test’ at bench-strstr.c:201:5,
>> >> >     inlined from ‘test_main’ at bench-strstr.c:220:2:
>> >> > ../include/bits/../../misc/bits/error.h:42:5: error: ‘%s’ directive argument is null [-Werror=format-overflow=]
>> >> >    42 |     __error_alias (__status, __errnum, __format, __va_arg_pack ());
>> >> >       |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>> >>
>> >> Isn't this warning wrong for glibc in general (but not for
>> >> _dl_debug_printf)?
>> >>
>> >> I think printing "(null)" for null pointers is a widely-used GNU
>> >> extension.
>> >
>> > Only for limited cases:
>> >
>> > [hjl@gnu-cfl-1 tmp]$ cat x.c
>> > #include <stdio.h>
>> >
>> > char *p;
>> >
>> > int
>> > main ()
>> > {
>> >   printf("null string:%s\n", p);
>> >   printf ("%s\n", p);
>> >   return 0;
>> > }
>> > [hjl@gnu-cfl-1 tmp]$ gcc x.c
>> > [hjl@gnu-cfl-1 tmp]$ ./a.out
>> > null string:(null)
>> > Segmentation fault
>> > [hjl@gnu-cfl-1 tmp]$
>>
>> Ah, because GCC transforms printf with "%s\n" to puts?  Hmm.
>>
>
> Yes.

We document the printf behavior:

|    If you accidentally pass a null pointer as the argument for a ‘%s’
| conversion, the GNU C Library prints it as ‘(null)’.  We think this is
| more useful than crashing.  But it’s not good practice to pass a null
| argument intentionally.

So we should perhaps fix puts to behave in the same way.  (puts isn't
even annotated with __nonnull today.)

Thanks,
Florian