Fix Hurd getcwd build with GCC >= 13

[Samuel Thibault]

Joseph Myers, le mer. 26 avril 2023 17:14:18 +0000, a ecrit:
> The build of glibc for i686-gnu has been failing for a while with GCC
> mainline / GCC 13:
> 
> ../sysdeps/mach/hurd/getcwd.c: In function '__hurd_canonicalize_directory_name_internal':
> ../sysdeps/mach/hurd/getcwd.c:242:48: error: pointer 'file_name' may be used after 'realloc' [-Werror=use-after-free]
>   242 |                   file_namep = &buf[file_namep - file_name + size / 2];
>       |                                     ~~~~~~~~~~~^~~~~~~~~~~
> ../sysdeps/mach/hurd/getcwd.c:236:25: note: call to 'realloc' here
>   236 |                   buf = realloc (file_name, size);
>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
> 
> This appears to be a genuine bug; fix by doing the subtraction before
> the reallocation makes the pointer invalid for arithmetic.

Well, it's actually not a genuine bug: the values of the file_namep
and file_name pointers are still coherent, so the subtraction is still
valid. But better please gcc 13 indeed, I have pushed it, thanks!

> Tested with build-many-glibcs.py for i686-gnu.
> 
> diff --git a/sysdeps/mach/hurd/getcwd.c b/sysdeps/mach/hurd/getcwd.c
> index f24b35b380..cd3aedd9cd 100644
> --- a/sysdeps/mach/hurd/getcwd.c
> +++ b/sysdeps/mach/hurd/getcwd.c
> @@ -222,8 +222,9 @@ __hurd_canonicalize_directory_name_internal (file_t thisdir,
>        found:
>  	{
>  	  /* Prepend the directory name just discovered.  */
> +	  size_t offset = file_namep - file_name;
>  
> -	  if (file_namep - file_name < d->d_namlen + 1)
> +	  if (offset < d->d_namlen + 1)
>  	    {
>  	      if (orig_size > 0)
>  		{
> @@ -239,7 +240,7 @@ __hurd_canonicalize_directory_name_internal (file_t thisdir,
>  		      free (file_name);
>  		      return NULL;
>  		    }
> -		  file_namep = &buf[file_namep - file_name + size / 2];
> +		  file_namep = &buf[offset + size / 2];
>  		  file_name = buf;
>  		  /* Move current contents up to the end of the buffer.
>  		     This is guaranteed to be non-overlapping.  */
> 
> -- 
> Joseph S. Myers
> joseph@codesourcery.com