[glibc] Fix Hurd getcwd build with GCC >= 13

[glibc] Fix Hurd getcwd build with GCC >= 13

[Samuel Thibault]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=af16a59ee1f72392b88d439d8f802c9844f86f4f

commit af16a59ee1f72392b88d439d8f802c9844f86f4f
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Apr 26 17:14:18 2023 +0000

    Fix Hurd getcwd build with GCC >= 13
    
    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);
          |                         ^~~~~~~~~~~~~~~~~~~~~~~~~
    
    Fix by doing the subtraction before the reallocation.
    
    Tested with build-many-glibcs.py for i686-gnu.
    
    [samuel.thibault@ens-lyon.rg: Removed mention of this being a bug]
    
    Message-Id: <18587337-7815-4056-ebd0-724df262d591@codesourcery.com>

Diff:
---
 sysdeps/mach/hurd/getcwd.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

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.  */