stdlib: Support malloc-managed environ arrays for compatibility

Some applications set environ to a heap-allocated pointer, call
setenv (expecting it to call realloc), free environ, and then
restore the original environ pointer.  This breaks after
commit 7a61e7f557a97ab597d6fca5e2d1f13f65685c61 ("stdlib: Make
getenv thread-safe in more cases") because after the setenv call,
the environ pointer does not point to the start of a heap allocation.
Instead, setenv creates a separate allocation and changes environ
to point into that.  This means that the free call in the application
results in heap corruption.

The interim approach was more compatible with other libcs because
it does not assume that the incoming environ pointer is allocated
as if by malloc (if it was written by the application).  However,
it seems to be more important to stay compatible with previous
glibc version: assume the incoming pointer is heap allocated,
and preserve this property after setenv calls.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

csu/init-first.c

@@ -61,6 +61,7 @@ _init_first (int argc, char **argv, char **envp)
   __libc_argc = argc;
   __libc_argv = argv;
   __environ = envp;
+  __environ_startup = envp;
 
 #ifndef SHARED
   /* First the initialization which normally would be done by the

csu/libc-start.c

@@ -244,6 +244,7 @@ LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
   char **ev = &argv[argc + 1];
 
   __environ = ev;
+  __environ_startup = ev;
 
   /* Store the lowest stack address.  This is done in ld.so if this is
      the code for the DSO.  */

include/unistd.h

@@ -203,6 +203,9 @@ libc_hidden_proto (__tcsetpgrp)
 extern int __libc_enable_secure attribute_relro;
 rtld_hidden_proto (__libc_enable_secure)
 
+/* Original value of __environ.  Initialized by _init_first (dynamic)
+   or __libc_start_main (static).  */
+extern char **__environ_startup attribute_hidden;
 
 /* Various internal function.  */
 extern void __libc_check_standard_fds (void) attribute_hidden;

posix/environ.c

@@ -10,3 +10,5 @@ weak_alias (__environ, environ)
 /* The SVR4 ABI says `_environ' will be the name to use
    in case the user overrides the weak alias `environ'.  */
 weak_alias (__environ, _environ)
+
+char **__environ_startup;

stdlib/Makefile

@@ -316,6 +316,7 @@ tests := \
   tst-setcontext9 \
   tst-setcontext10 \
   tst-setcontext11 \
+  tst-setenv-malloc \
   tst-stdbit-Wconversion \
   tst-stdbit-builtins \
   tst-stdc_bit_ceil \

stdlib/setenv.c

@@ -191,52 +191,52 @@ __add_to_environ (const char *name, const char *value, const char *combined,
 	ep[1] = NULL;
       else
 	{
-	  /* We cannot use __environ as is and need to copy over the
-	     __environ contents into an array managed via
-	     __environ_array_list.  */
+	  /* We cannot use __environ as is and need a larger allocation.  */
 
-	  struct environ_array *target_array;
-	  if (__environ_array_list != NULL
-	      && required_size <= __environ_array_list->allocated)
-	    /* Existing array has enough room.  Contents is copied below.  */
-	    target_array = __environ_array_list;
+	  if (start_environ == __environ_startup
+	      || __environ_is_from_array_list (start_environ))
+	    {
+	      /* Allocate a new array, managed in the list.  */
+	      struct environ_array *target_array
+		= __environ_new_array (required_size);
+	      if (target_array == NULL)
+		{
+		  UNLOCK;
+		  return -1;
+		}
+	      result_environ = &target_array->array[0];
+
+	      /* Copy over the __environ array contents.  This code
+		 handles the case start_environ == ep == NULL, too.  */
+	      size_t i;
+	      for (i = 0; start_environ + i < ep; ++i)
+		/* Regular store because unless there has been direct
+		   manipulation of the environment, target_array is still
+		   a private copy.  */
+		result_environ[i] = atomic_load_relaxed (start_environ + i);
+	    }
 	  else
 	    {
-	      /* Allocate a new array.  */
-	      target_array = __environ_new_array (required_size);
-	      if (target_array == NULL)
+	      /* Otherwise the application installed its own pointer.
+		 Historically, this pointer was managed using realloc.
+		 Continue doing so.  This disables multi-threading
+		 support.  */
+	      result_environ = __libc_reallocarray (start_environ,
+						    required_size,
+						    sizeof (*result_environ));
+	      if (result_environ == NULL)
 		{
 		  UNLOCK;
 		  return -1;
 		}
 	    }
 
-	  /* Copy over the __environ array contents.  This forward
-	     copy slides backwards part of the array if __environ
-	     points into target_array->array.  This happens if an
-	     application makes an assignment like:
-
-	       environ = &environ[1];
-
-	     The forward copy avoids clobbering values that still
-	     needing copying.  This code handles the case
-	     start_environ == ep == NULL, too.  */
-	  size_t i;
-	  for (i = 0; start_environ + i < ep; ++i)
-	    /* Regular store because unless there has been direct
-	       manipulation of the environment, target_array is still
-	       a private copy.  */
-	    target_array->array[i] = atomic_load_relaxed (start_environ + i);
-
 	  /* This is the new place where we should add the element.  */
-	  ep = target_array->array + i;
+	  ep = result_environ + (required_size - 2);
 
 	  /* Add the null terminator in case there was a pointer there
 	     previously.  */
 	  ep[1] = NULL;
-
-	  /* And __environ should be repointed to our array.  */
-	  result_environ = &target_array->array[0];
 	}
     }
 

stdlib/tst-setenv-malloc.c

@@ -0,0 +1,64 @@
+/* Test using setenv with a malloc-allocated environ variable.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+/* This test is not in the scope for POSIX or any other standard, but
+   some applications assume that environ is a heap-allocated pointer
+   after a call to setenv on an empty environment.  */
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <support/check.h>
+#include <support/support.h>
+
+static const char *original_path;
+static char **save_environ;
+
+static void
+rewrite_environ (void)
+{
+  save_environ = environ;
+  environ = xmalloc (sizeof (*environ));
+  *environ = NULL;
+  TEST_COMPARE (setenv ("A", "1", 1), 0);
+  TEST_COMPARE (setenv ("B", "2", 1), 0);
+  TEST_VERIFY (environ != save_environ);
+  TEST_COMPARE_STRING (environ[0], "A=1");
+  TEST_COMPARE_STRING (environ[1], "B=2");
+  TEST_COMPARE_STRING (environ[2], NULL);
+  TEST_COMPARE_STRING (getenv ("PATH"), NULL);
+  free (environ);
+  environ = save_environ;
+  TEST_COMPARE_STRING (getenv ("PATH"), original_path);
+}
+
+static int
+do_test (void)
+{
+  original_path = getenv ("PATH");
+  rewrite_environ ();
+
+  /* Test again after reallocated the environment due to an initial
+     setenv call.  */
+  TEST_COMPARE (setenv ("TST_SETENV_MALLOC", "1", 1), 0);
+  TEST_VERIFY (environ != save_environ);
+  rewrite_environ ();
+
+  return 0;
+}
+
+#include <support/test-driver.c>