Re: [PATCH v2 4/5] avoid -Wuse-after-free [BZ #26779]

[Martin Sebor <msebor@gmail.com>]

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On 1/24/22 17:52, Martin Sebor wrote:
> This is a repost of the original patch but broken down by source
> file and with some suppression done by #pragma GCC diagnostic
> instead of conversion to intptr_t.  It also adds fixes for
> the same problem in the test suite that I overlooked before.

The attached patch suppresses the -Wuse-after-free instance in
stdlib/setenv.c.

> 
> On 1/15/22 17:21, Martin Sebor wrote:
>> GCC 12 features a couple of new warnings designed to detect uses
>> of pointers made invalid by the pointees lifetimes having ended.
>> Building Glibc with the enhanced GCC exposes a few such uses,
>> mostly after successful calls to realloc.  The attached patch
>> avoids the new warnings by converting the pointers to uintptr_t
>> first and using the converted integers instead.
>>
>> The patch suppresses all instances of the warning at the strictest
>> setting (-Wuse-after-free=3), which includes even uses in equality
>> expressions.  The default setting approved for GCC 12 is
>> -Wuse-after-free=2, which doesn't warn on such uses to accommodate
>> the pointer-adjustment-after-realloc idiom.  At the default setting,
>> the changes to ldconfig.c and setenv are not necessary.
>>
>> Martin
> 

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diff --git a/stdlib/setenv.c b/stdlib/setenv.c
index c3d2cee7b6..2176cbac31 100644
--- a/stdlib/setenv.c
+++ b/stdlib/setenv.c
@@ -150,7 +150,9 @@ __add_to_environ (const char *name, const char *value, const char *combined,
     {
       char **new_environ;
 
-      /* We allocated this space; we can extend it.  */
+      /* We allocated this space; we can extend it.  Avoid using the raw
+	 reallocated pointer to avoid GCC -Wuse-after-free.  */
+      uintptr_t ip_last_environ = (uintptr_t)last_environ;
       new_environ = (char **) realloc (last_environ,
 				       (size + 2) * sizeof (char *));
       if (new_environ == NULL)
@@ -159,7 +161,7 @@ __add_to_environ (const char *name, const char *value, const char *combined,
 	  return -1;
 	}
 
-      if (__environ != last_environ)
+      if ((uintptr_t)__environ != ip_last_environ)
 	memcpy ((char *) new_environ, (char *) __environ,
 		size * sizeof (char *));