posix: execvpe cleanup

This patch removes all the dynamic allocation on execvpe code and
instead use direct stack allocation.  This is QoI approach to make
it possible use in scenarios where memory is shared with parent
(vfork or clone with CLONE_VM).

For default process spawn (script file without a shebang), stack
allocation is bounded by NAME_MAX plus PATH_MAX plus 1.  Large
file arguments returns an error (ENAMETOOLONG).  This differs than
current GLIBC pratice in general, but it used to limit stack
allocation for large inputs.  Also, path in PATH environment variable
larger than PATH_MAX are ignored.

The shell direct execution exeception, where execve returns ENOEXEC,
might requires a large stack allocation due large input argument list.

Tested on i686, x86_64, powerpc64le, and aarch64.

	* posix/execvpe.c (__execvpe): Remove dynamic allocation.
	* posix/Makefile (tests): Add tst-execvpe{1,2,3,4,5,6}.
	* posix/tst-execvp1.c (do_test): Use a macro to call execvp.
	* posix/tst-execvp2.c (do_test): Likewise.
	* posix/tst-execvp3.c (do_test): Likewise.
	* posix/tst-execvp4.c (do_test): Likewise.
	* posix/tst-execvpe1.c: New file.
	* posix/tst-execvpe2.c: Likewise.
	* posix/tst-execvpe3.c: Likewise.
	* posix/tst-execvpe4.c: Likewise.
	* posix/tst-execvpe5.c: Likewise.
	* posix/tst-execvpe6.c: Likewise.

1 files changed, 108 insertions, 147 deletions

diff --git a/posix/execvpe.c b/posix/execvpe.c
index 61697a7..d933f9c 100644
--- a/posix/execvpe.c
+++ b/posix/execvpe.c
@@ -15,7 +15,6 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
-#include <alloca.h>
 #include <unistd.h>
 #include <stdarg.h>
 #include <stdbool.h>
@@ -23,22 +22,43 @@
 #include <string.h>
 #include <errno.h>
 #include <paths.h>
+#include <confstr.h>
+#include <sys/param.h>
 
+#ifndef PATH_MAX
+# ifdef MAXPATHLEN
+#  define PATH_MAX MAXPATHLEN
+# else
+#  define PATH_MAX 1024
+# endif
+#endif
 
 /* The file is accessible but it is not an executable file.  Invoke
    the shell to interpret it as a script.  */
 static void
-internal_function
-scripts_argv (const char *file, char *const argv[], int argc, char **new_argv)
+maybe_script_execute (const char *file, char *const argv[], char *const envp[])
 {
+  ptrdiff_t argc = 0;
+  while (argv[argc++] != NULL)
+    {
+      if (argc == INT_MAX - 1)
+	{
+	  errno = E2BIG;
+	  return;
+	}
+    }
+
   /* Construct an argument list for the shell.  */
+  char *new_argv[argc + 1];
   new_argv[0] = (char *) _PATH_BSHELL;
   new_argv[1] = (char *) file;
-  while (argc > 1)
-    {
-      new_argv[argc] = argv[argc - 1];
-      --argc;
-    }
+  if (argc > 1)
+    memcpy (new_argv + 2, argv + 1, argc * sizeof(char *));
+  else
+    new_argv[2] = NULL;
+
+  /* Execute the shell.  */
+  __execve (new_argv[0], new_argv, envp);
 }
 
 
@@ -47,170 +67,111 @@ scripts_argv (const char *file, char *const argv[], int argc, char **new_argv)
 int
 __execvpe (const char *file, char *const argv[], char *const envp[])
 {
+  /* We check the simple case first. */
   if (*file == '\0')
     {
-      /* We check the simple case first. */
       __set_errno (ENOENT);
       return -1;
     }
 
+  /* Don't search when it contains a slash.  */
   if (strchr (file, '/') != NULL)
     {
-      /* Don't search when it contains a slash.  */
       __execve (file, argv, envp);
 
       if (errno == ENOEXEC)
-	{
-	  /* Count the arguments.  */
-	  int argc = 0;
-	  while (argv[argc++])
-	    ;
-	  size_t len = (argc + 1) * sizeof (char *);
-	  char **script_argv;
-	  void *ptr = NULL;
-	  if (__libc_use_alloca (len))
-	    script_argv = alloca (len);
-	  else
-	    script_argv = ptr = malloc (len);
-
-	  if (script_argv != NULL)
-	    {
-	      scripts_argv (file, argv, argc, script_argv);
-	      __execve (script_argv[0], script_argv, envp);
-
-	      free (ptr);
-	    }
-	}
+        maybe_script_execute (file, argv, envp);
+
+      return -1;
     }
-  else
+
+  const char *path = getenv ("PATH");
+  if (!path)
+    path = CS_PATH;
+  /* Although GLIBC does not enforce NAME_MAX, we set it as the maximum
+     size to avoid unbounded stack allocation.  Same applies for
+     PATH_MAX.  */
+  size_t file_len = __strnlen (file, NAME_MAX + 1);
+  size_t path_len = __strnlen (path, PATH_MAX - 1) + 1;
+
+  if ((file_len > NAME_MAX)
+      || !__libc_alloca_cutoff (path_len + file_len + 1))
     {
-      size_t pathlen;
-      size_t alloclen = 0;
-      char *path = getenv ("PATH");
-      if (path == NULL)
-	{
-	  pathlen = confstr (_CS_PATH, (char *) NULL, 0);
-	  alloclen = pathlen + 1;
-	}
-      else
-	pathlen = strlen (path);
+      errno = ENAMETOOLONG;
+      return -1;
+    }
 
-      size_t len = strlen (file) + 1;
-      alloclen += pathlen + len + 1;
+  const char *subp;
+  bool got_eacces = false;
+  char buffer[path_len + file_len + 1];
+  for (const char *p = path; ; p = subp)
+    {
+      subp = __strchrnul (p, ':');
 
-      char *name;
-      char *path_malloc = NULL;
-      if (__libc_use_alloca (alloclen))
-	name = alloca (alloclen);
-      else
+      /* PATH is larger than PATH_MAX and thus potentially larger than
+	 the stack allocation.  */
+      if (subp - p >= path_len)
 	{
-	  path_malloc = name = malloc (alloclen);
-	  if (name == NULL)
-	    return -1;
+          /* If there is only one path, bail out.  */
+	  if (*subp == '\0')
+	    break;
+	  /* Otherwise skip to next one.  */
+	  continue;
 	}
 
-      if (path == NULL)
-	{
-	  /* There is no `PATH' in the environment.
-	     The default search path is the current directory
-	     followed by the path `confstr' returns for `_CS_PATH'.  */
-	  path = name + pathlen + len + 1;
-	  path[0] = ':';
-	  (void) confstr (_CS_PATH, path + 1, pathlen);
-	}
+      /* Use the current path entry, plus a '/' if nonempty, plus the file to
+         execute.  */
+      char *pend = mempcpy (buffer, p, subp - p);
+      *pend = '/';
+      memcpy (pend + (p < subp), file, file_len + 1);
+
+      __execve (buffer, argv, envp);
 
-      /* Copy the file name at the top.  */
-      name = (char *) memcpy (name + pathlen + 1, file, len);
-      /* And add the slash.  */
-      *--name = '/';
+      if (errno == ENOEXEC)
+        /* This has O(P*C) behavior, where P is the length of the path and C
+           is the argument count.  A better strategy would be allocate the
+           substitute argv and reuse it each time through the loop (so it
+           behaves as O(P+C) instead.  */
+        maybe_script_execute (buffer, argv, envp);
 
-      char **script_argv = NULL;
-      void *script_argv_malloc = NULL;
-      bool got_eacces = false;
-      char *p = path;
-      do
+      switch (errno)
 	{
-	  char *startp;
-
-	  path = p;
-	  p = __strchrnul (path, ':');
-
-	  if (p == path)
-	    /* Two adjacent colons, or a colon at the beginning or the end
-	       of `PATH' means to search the current directory.  */
-	    startp = name + 1;
-	  else
-	    startp = (char *) memcpy (name - (p - path), path, p - path);
-
-	  /* Try to execute this name.  If it works, execve will not return. */
-	  __execve (startp, argv, envp);
-
-	  if (errno == ENOEXEC)
-	    {
-	      if (script_argv == NULL)
-		{
-		  /* Count the arguments.  */
-		  int argc = 0;
-		  while (argv[argc++])
-		    ;
-		  size_t arglen = (argc + 1) * sizeof (char *);
-		  if (__libc_use_alloca (alloclen + arglen))
-		    script_argv = alloca (arglen);
-		  else
-		    script_argv = script_argv_malloc = malloc (arglen);
-		  if (script_argv == NULL)
-		    {
-		      /* A possible EACCES error is not as important as
-			 the ENOMEM.  */
-		      got_eacces = false;
-		      break;
-		    }
-		  scripts_argv (startp, argv, argc, script_argv);
-		}
-
-	      __execve (script_argv[0], script_argv, envp);
-	    }
-
-	  switch (errno)
-	    {
-	    case EACCES:
-	      /* Record the we got a `Permission denied' error.  If we end
-		 up finding no executable we can use, we want to diagnose
-		 that we did find one but were denied access.  */
-	      got_eacces = true;
-	    case ENOENT:
-	    case ESTALE:
-	    case ENOTDIR:
-	      /* Those errors indicate the file is missing or not executable
-		 by us, in which case we want to just try the next path
-		 directory.  */
-	    case ENODEV:
-	    case ETIMEDOUT:
-	      /* Some strange filesystems like AFS return even
-		 stranger error numbers.  They cannot reasonably mean
-		 anything else so ignore those, too.  */
-	      break;
-
-	    default:
-	      /* Some other error means we found an executable file, but
-		 something went wrong executing it; return the error to our
-		 caller.  */
-	      return -1;
-	    }
+	  case EACCES:
+	  /* Record that we got a 'Permission denied' error.  If we end
+	     up finding no executable we can use, we want to diagnose
+	     that we did find one but were denied access.  */
+	    got_eacces = true;
+	  case ENOENT:
+	  case ESTALE:
+	  case ENOTDIR:
+	  /* Those errors indicate the file is missing or not executable
+	     by us, in which case we want to just try the next path
+	     directory.  */
+	  case ENODEV:
+	  case ETIMEDOUT:
+	  /* Some strange filesystems like AFS return even
+	     stranger error numbers.  They cannot reasonably mean
+	     anything else so ignore those, too.  */
+	    break;
+
+          default:
+	  /* Some other error means we found an executable file, but
+	     something went wrong executing it; return the error to our
+	     caller.  */
+	    return -1;
 	}
-      while (*p++ != '\0');
 
-      /* We tried every element and none of them worked.  */
-      if (got_eacces)
-	/* At least one failure was due to permissions, so report that
-	   error.  */
-	__set_errno (EACCES);
-
-      free (script_argv_malloc);
-      free (path_malloc);
+      if (*subp++ == '\0')
+	break;
     }
 
-  /* Return the error from the last attempt (probably ENOENT).  */
+  /* We tried every element and none of them worked.  */
+  if (got_eacces)
+    /* At least one failure was due to permissions, so report that
+       error.  */
+    __set_errno (EACCES);
+
   return -1;
 }
+
 weak_alias (__execvpe, execvpe)