[PATCH] posix: Remove dynamic memory allocation from execl{e,p}

[Adhemerval Zanella <adhemerval dot zanella at linaro dot org>]

GLIBC execl{e,p} implementation might use malloc if the total number of
arguments exceeds initial assumption size (1024).  This might lead to
issues in two situations:

1. execl/execle is stated to be async-signal-safe by POSIX [1].  However
   if it is used in a signal handler with a large argument set (that
   may call malloc internally) and the resulting call fails, it might
   lead malloc in the program in a bad state.

2. If the functions are used in a vfork/clone(VFORK) situation it also
   might break internal malloc state from parent.

This patch fixes it by using stack allocation instead.  It fixes
BZ#19534.

One caveat is current stack allocation allocation limit the patch is using
is based on previous posix_spawn{p}/execvpe comments to use internal
__MAX_ALLOCA_CUTOFF definition.  It is an arbitrary value that limits
total argument handlings to 8192 for 32-bits and 4096 for 64-bits.
I think it would be better to increase this value for higher threshold,
either by increasing the __MAX_ALLOCA_CUTOFF value or using a different
strategy to limit stack allocation on exec functions.

Tested on x86_64.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html

	[BZ #19534]
	* posix/execl.c (execl): Remove dynamic memory allocation.
	* posix/execle.c (execle): Likewise.
	* posix/execlp.c (execlp): Likewise.
---
 ChangeLog      |  5 +++++
 posix/execl.c  | 70 ++++++++++++++++++++-------------------------------------
 posix/execle.c | 71 +++++++++++++++++++++-------------------------------------
 posix/execlp.c | 69 ++++++++++++++++++++------------------------------------
 4 files changed, 78 insertions(+), 137 deletions(-)

diff --git a/posix/execl.c b/posix/execl.c
index 102d19d..26c28e1 100644
--- a/posix/execl.c
+++ b/posix/execl.c
@@ -16,58 +16,36 @@
    <http://www.gnu.org/licenses/>.  */
 
 #include <unistd.h>
+#include <errno.h>
 #include <stdarg.h>
-#include <stddef.h>
-#include <stdlib.h>
-#include <string.h>
-
-#include <stackinfo.h>
-
+#include <sys/param.h>
 
 /* Execute PATH with all arguments after PATH until
    a NULL pointer and environment from `environ'.  */
 int
 execl (const char *path, const char *arg, ...)
 {
-#define INITIAL_ARGV_MAX 1024
-  size_t argv_max = INITIAL_ARGV_MAX;
-  const char *initial_argv[INITIAL_ARGV_MAX];
-  const char **argv = initial_argv;
-  va_list args;
-
-  argv[0] = arg;
-
-  va_start (args, arg);
-  unsigned int i = 0;
-  while (argv[i++] != NULL)
-    {
-      if (i == argv_max)
-	{
-	  argv_max *= 2;
-	  const char **nptr = realloc (argv == initial_argv ? NULL : argv,
-				       argv_max * sizeof (const char *));
-	  if (nptr == NULL)
-	    {
-	      if (argv != initial_argv)
-		free (argv);
-	      va_end (args);
-	      return -1;
-	    }
-	  if (argv == initial_argv)
-	    /* We have to copy the already filled-in data ourselves.  */
-	    memcpy (nptr, argv, i * sizeof (const char *));
-
-	  argv = nptr;
-	}
-
-      argv[i] = va_arg (args, const char *);
-    }
-  va_end (args);
-
-  int ret = __execve (path, (char *const *) argv, __environ);
-  if (argv != initial_argv)
-    free (argv);
-
-  return ret;
+  int argc;
+  int limit = MIN (NCARGS, __MAX_ALLOCA_CUTOFF / sizeof (char *));
+  va_list ap;
+  va_start (ap, arg);
+  for (argc = 1; va_arg (ap, const char *); argc++)
+    if (argc+1 >= limit)
+      {
+	errno = E2BIG;
+	return -1;
+      }
+  va_end (ap);
+
+  int i;
+  char *argv[argc+1];
+  va_start (ap, arg);
+  argv[0] = (char*) arg;
+  for (i = 1; i < argc; i++)
+     argv[i] = va_arg (ap, char *);
+  argv[i] = NULL;
+  va_end (ap);
+
+  return __execve (path, argv, __environ);
 }
 libc_hidden_def (execl)
diff --git a/posix/execle.c b/posix/execle.c
index 8edc03a..79c13e3 100644
--- a/posix/execle.c
+++ b/posix/execle.c
@@ -17,57 +17,36 @@
 
 #include <unistd.h>
 #include <stdarg.h>
-#include <stddef.h>
-#include <stdlib.h>
-#include <string.h>
-
-#include <stackinfo.h>
+#include <errno.h>
+#include <sys/param.h>
 
 /* Execute PATH with all arguments after PATH until a NULL pointer,
    and the argument after that for environment.  */
 int
 execle (const char *path, const char *arg, ...)
 {
-#define INITIAL_ARGV_MAX 1024
-  size_t argv_max = INITIAL_ARGV_MAX;
-  const char *initial_argv[INITIAL_ARGV_MAX];
-  const char **argv = initial_argv;
-  va_list args;
-  argv[0] = arg;
-
-  va_start (args, arg);
-  unsigned int i = 0;
-  while (argv[i++] != NULL)
-    {
-      if (i == argv_max)
-	{
-	  argv_max *= 2;
-	  const char **nptr = realloc (argv == initial_argv ? NULL : argv,
-				       argv_max * sizeof (const char *));
-	  if (nptr == NULL)
-	    {
-	      if (argv != initial_argv)
-		free (argv);
-	      va_end (args);
-	      return -1;
-	    }
-	  if (argv == initial_argv)
-	    /* We have to copy the already filled-in data ourselves.  */
-	    memcpy (nptr, argv, i * sizeof (const char *));
-
-	  argv = nptr;
-	}
-
-      argv[i] = va_arg (args, const char *);
-    }
-
-  const char *const *envp = va_arg (args, const char *const *);
-  va_end (args);
-
-  int ret = __execve (path, (char *const *) argv, (char *const *) envp);
-  if (argv != initial_argv)
-    free (argv);
-
-  return ret;
+  int argc;
+  int limit = MIN (NCARGS, __MAX_ALLOCA_CUTOFF / sizeof (char *));
+  va_list ap;
+  va_start (ap, arg);
+  for (argc = 1; va_arg (ap, const char *); argc++)
+    if (argc+1 >= limit)
+      {
+	errno = E2BIG;
+	return -1;
+      }
+  va_end (ap);
+
+  int i;
+  char *argv[argc+1];
+  char **envp;
+  va_start (ap, arg);
+  argv[0] = (char*) arg;
+  for (i = 1; i < argc; i++)
+     argv[i] = va_arg (ap, char *);
+  envp = va_arg (ap, char **);
+  va_end (ap);
+
+  return __execve (path, argv, envp);
 }
 libc_hidden_def (execle)
diff --git a/posix/execlp.c b/posix/execlp.c
index 6700994..a4b603c 100644
--- a/posix/execlp.c
+++ b/posix/execlp.c
@@ -17,11 +17,8 @@
 
 #include <unistd.h>
 #include <stdarg.h>
-#include <stddef.h>
-#include <stdlib.h>
-#include <string.h>
-
-#include <stackinfo.h>
+#include <errno.h>
+#include <sys/param.h>
 
 /* Execute FILE, searching in the `PATH' environment variable if
    it contains no slashes, with all arguments after FILE until a
@@ -29,45 +26,27 @@
 int
 execlp (const char *file, const char *arg, ...)
 {
-#define INITIAL_ARGV_MAX 1024
-  size_t argv_max = INITIAL_ARGV_MAX;
-  const char *initial_argv[INITIAL_ARGV_MAX];
-  const char **argv = initial_argv;
-  va_list args;
-
-  argv[0] = arg;
-
-  va_start (args, arg);
-  unsigned int i = 0;
-  while (argv[i++] != NULL)
-    {
-      if (i == argv_max)
-	{
-	  argv_max *= 2;
-	  const char **nptr = realloc (argv == initial_argv ? NULL : argv,
-				       argv_max * sizeof (const char *));
-	  if (nptr == NULL)
-	    {
-	      if (argv != initial_argv)
-		free (argv);
-	      va_end (args);
-	      return -1;
-	    }
-	  if (argv == initial_argv)
-	    /* We have to copy the already filled-in data ourselves.  */
-	    memcpy (nptr, argv, i * sizeof (const char *));
-
-	  argv = nptr;
-	}
-
-      argv[i] = va_arg (args, const char *);
-    }
-  va_end (args);
-
-  int ret = execvp (file, (char *const *) argv);
-  if (argv != initial_argv)
-    free (argv);
-
-  return ret;
+  int argc;
+  int limit = MIN (NCARGS, __MAX_ALLOCA_CUTOFF / sizeof (char *));
+  va_list ap;
+  va_start (ap, arg);
+  for (argc = 1; va_arg (ap, const char *); argc++)
+    if (argc+1 >= limit)
+      {
+	errno = E2BIG;
+	return -1;
+      }
+  va_end (ap);
+
+  int i;
+  char *argv[argc+1];
+  va_start (ap, arg);
+  argv[0] = (char*) arg;
+  for (i = 1; i < argc; i++)
+     argv[i] = va_arg (ap, char *);
+  argv[i] = NULL;
+  va_end (ap);
+
+  return __execvpe (file, argv, __environ);
 }
 libc_hidden_def (execlp)
-- 
1.9.1