[3/3] linux: Optimize realpath stack usage

diff --git a/include/stdlib.h b/include/stdlib.h
index ffcefd7b85..dd51f66b26 100644
--- a/include/stdlib.h
+++ b/include/stdlib.h
@@ -20,6 +20,14 @@ 
 
 # include <rtld-malloc.h>
 
+# ifndef PATH_MAX
+#  ifdef MAXPATHLEN
+#   define PATH_MAX MAXPATHLEN
+#  else
+#   define PATH_MAX 1024
+#  endif
+# endif
+
 extern __typeof (strtol_l) __strtol_l;
 extern __typeof (strtoul_l) __strtoul_l;
 extern __typeof (strtoll_l) __strtoll_l;
@@ -92,6 +100,11 @@  extern int __unsetenv (const char *__name) attribute_hidden;
 extern int __clearenv (void) attribute_hidden;
 extern char *__mktemp (char *__template) __THROW __nonnull ((1));
 extern char *__canonicalize_file_name (const char *__name);
+extern _Bool __resolve_path (const char *name, char *resolved,
+			     size_t path_max)
+     attribute_hidden;
+extern char *__realpath_system (const char *name, char *resolved)
+     attribute_hidden;
 extern char *__realpath (const char *__name, char *__resolved);
 libc_hidden_proto (__realpath)
 extern int __ptsname_r (int __fd, char *__buf, size_t __buflen)
diff --git a/stdlib/Makefile b/stdlib/Makefile
index 7093b8a584..35ca04541f 100644
--- a/stdlib/Makefile
+++ b/stdlib/Makefile
@@ -53,7 +53,7 @@  routines	:=							      \
 	strtof strtod strtold						      \
 	strtof_l strtod_l strtold_l					      \
 	strtof_nan strtod_nan strtold_nan				      \
-	system canonicalize						      \
+	system realpath canonicalize canonicalize-internal		      \
 	a64l l64a							      \
 	rpmatch strfmon strfmon_l getsubopt xpg_basename fmtmsg		      \
 	strtoimax strtoumax wcstoimax wcstoumax				      \
diff --git a/stdlib/canonicalize-internal.c b/stdlib/canonicalize-internal.c
new file mode 100644
index 0000000000..1b5f73a1cc
--- /dev/null
+++ b/stdlib/canonicalize-internal.c
@@ -0,0 +1,136 @@ 
+/* Internal function for canonicalize absolute name of a given file.
+   Copyright (C) 1996-2020 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/>.  */
+
+#include <stdbool.h>
+#include <string.h>
+#include <stdlib.h>
+#include <errno.h>
+#include <unistd.h>
+#include <eloop-threshold.h>
+
+_Bool
+__resolve_path (const char *name, char *resolved, size_t path_max)
+{
+  const char *start, *end;
+  char *rpath = resolved;
+  char *rpath_limit = rpath + path_max;
+  char *dest = resolved;
+  char extra_buf[PATH_MAX];
+  int num_links = 0;
+
+  if (name[0] != '/')
+    {
+      if (__getcwd (rpath, path_max) == NULL)
+	{
+	  rpath[0] = '\0';
+	  return false;
+	}
+      dest = __rawmemchr (rpath, '\0');
+    }
+  else
+    {
+      rpath[0] = '/';
+      dest = rpath + 1;
+    }
+
+  for (start = end = name; *start; start = end)
+    {
+      /* Skip sequence of multiple path-separators.  */
+      while (*start == '/')
+	++start;
+
+      /* Find end of path component.  */
+      for (end = start; *end && *end != '/'; ++end)
+	/* Nothing.  */;
+
+      if (end - start == 0)
+	break;
+      else if (end - start == 1 && start[0] == '.')
+	/* nothing */;
+      else if (end - start == 2 && start[0] == '.' && start[1] == '.')
+	{
+	  /* Back up to previous component, ignore if at root already.  */
+	  if (dest > rpath + 1)
+	    while ((--dest)[-1] != '/');
+	}
+      else
+	{
+	  struct stat64 st;
+
+	  if (dest[-1] != '/')
+	    *dest++ = '/';
+	  if (dest + (end - start) >= rpath_limit)
+	    {
+	      __set_errno (ENAMETOOLONG);
+	      if (dest > rpath + 1)
+		dest--;
+	      *dest = '\0';
+	      return false;
+	    }
+
+	  dest = __mempcpy (dest, start, end - start);
+	  *dest = '\0';
+
+	  if (__lstat64 (rpath, &st) < 0)
+	    return false;
+
+	  if (S_ISLNK (st.st_mode))
+	    {
+	      if (++num_links > __eloop_threshold ())
+		{
+		  __set_errno (ELOOP);
+		  return false;
+		}
+
+	      char buf[PATH_MAX];
+	      ssize_t n = __readlink (rpath, buf, sizeof (buf) - 1);
+	      if (n < 0)
+		return false;
+	      buf[n] = '\0';
+
+	      size_t len = strlen (end);
+	      if (path_max - n <= len)
+		{
+		  __set_errno (ENAMETOOLONG);
+		  return false;
+		}
+
+	      memmove (&extra_buf[n], end, len + 1);
+	      name = end = memcpy (extra_buf, buf, n);
+
+	      if (buf[0] == '/')
+		dest = rpath + 1;	/* It's an absolute symlink */
+	      else
+		/* Back up to previous component, ignore if at root already: */
+		if (dest > rpath + 1)
+		  while ((--dest)[-1] != '/');
+	    }
+	  else if (!S_ISDIR (st.st_mode) && *end != '\0')
+	    {
+	      __set_errno (ENOTDIR);
+	      return false;
+	    }
+	}
+    }
+
+  if (dest > rpath + 1 && dest[-1] == '/')
+    --dest;
+  *dest = '\0';
+
+  return true;
+}
diff --git a/stdlib/canonicalize.c b/stdlib/canonicalize.c
index 91c30e38be..f4ab528a15 100644
--- a/stdlib/canonicalize.c
+++ b/stdlib/canonicalize.c
@@ -16,26 +16,11 @@ 
    License along with the GNU C Library; if not, see
    <https://www.gnu.org/licenses/>.  */
 
-#include <assert.h>
 #include <stdlib.h>
-#include <string.h>
-#include <unistd.h>
-#include <limits.h>
-#include <sys/stat.h>
 #include <errno.h>
-#include <stddef.h>
 
-#include <eloop-threshold.h>
 #include <shlib-compat.h>
 
-#ifndef PATH_MAX
-# ifdef MAXPATHLEN
-#  define PATH_MAX MAXPATHLEN
-# else
-#  define PATH_MAX 1024
-# endif
-#endif
-
 /* Return the canonical absolute name of file NAME.  A canonical name
    does not contain any `.', `..' components nor any repeated path
    separators ('/') or symlinks.  All path components must exist.  If
@@ -50,10 +35,6 @@ 
 char *
 __realpath (const char *name, char *resolved)
 {
-  char *rpath, *dest, extra_buf[PATH_MAX];
-  const char *start, *end, *rpath_limit;
-  int num_links = 0;
-
   if (name == NULL)
     {
       /* As per Single Unix Specification V2 we must return an error if
@@ -72,127 +53,7 @@  __realpath (const char *name, char *resolved)
       return NULL;
     }
 
-  if (resolved == NULL)
-    {
-      rpath = malloc (PATH_MAX);
-      if (rpath == NULL)
-	return NULL;
-    }
-  else
-    rpath = resolved;
-  rpath_limit = rpath + PATH_MAX;
-
-  if (name[0] != '/')
-    {
-      if (!__getcwd (rpath, PATH_MAX))
-	{
-	  rpath[0] = '\0';
-	  goto error;
-	}
-      dest = __rawmemchr (rpath, '\0');
-    }
-  else
-    {
-      rpath[0] = '/';
-      dest = rpath + 1;
-    }
-
-  for (start = end = name; *start; start = end)
-    {
-      struct stat64 st;
-      int n;
-
-      /* Skip sequence of multiple path-separators.  */
-      while (*start == '/')
-	++start;
-
-      /* Find end of path component.  */
-      for (end = start; *end && *end != '/'; ++end)
-	/* Nothing.  */;
-
-      if (end - start == 0)
-	break;
-      else if (end - start == 1 && start[0] == '.')
-	/* nothing */;
-      else if (end - start == 2 && start[0] == '.' && start[1] == '.')
-	{
-	  /* Back up to previous component, ignore if at root already.  */
-	  if (dest > rpath + 1)
-	    while ((--dest)[-1] != '/');
-	}
-      else
-	{
-	  if (dest[-1] != '/')
-	    *dest++ = '/';
-
-	  if (dest + (end - start) >= rpath_limit)
-	    {
-	      __set_errno (ENAMETOOLONG);
-	      if (dest > rpath + 1)
-		dest--;
-	      *dest = '\0';
-	      goto error;
-	    }
-
-	  dest = __mempcpy (dest, start, end - start);
-	  *dest = '\0';
-
-	  if (__lxstat64 (_STAT_VER, rpath, &st) < 0)
-	    goto error;
-
-	  if (S_ISLNK (st.st_mode))
-	    {
-	      char buf[PATH_MAX];
-	      size_t len;
-
-	      if (++num_links > __eloop_threshold ())
-		{
-		  __set_errno (ELOOP);
-		  goto error;
-		}
-
-	      n = __readlink (rpath, buf, sizeof (buf) - 1);
-	      if (n < 0)
-		goto error;
-	      buf[n] = '\0';
-
-	      len = strlen (end);
-	      if (PATH_MAX - n <= len)
-		{
-		  __set_errno (ENAMETOOLONG);
-		  goto error;
-		}
-
-	      /* Careful here, end may be a pointer into extra_buf... */
-	      memmove (&extra_buf[n], end, len + 1);
-	      name = end = memcpy (extra_buf, buf, n);
-
-	      if (buf[0] == '/')
-		dest = rpath + 1;	/* It's an absolute symlink */
-	      else
-		/* Back up to previous component, ignore if at root already: */
-		if (dest > rpath + 1)
-		  while ((--dest)[-1] != '/');
-	    }
-	  else if (!S_ISDIR (st.st_mode) && *end != '\0')
-	    {
-	      __set_errno (ENOTDIR);
-	      goto error;
-	    }
-	}
-    }
-  if (dest > rpath + 1 && dest[-1] == '/')
-    --dest;
-  *dest = '\0';
-
-  assert (resolved == NULL || resolved == rpath);
-  return rpath;
-
-error:
-  assert (resolved == NULL || resolved == rpath);
-  if (resolved == NULL)
-    free (rpath);
-  return NULL;
+  return __realpath_system (name, resolved);
 }
 libc_hidden_def (__realpath)
 versioned_symbol (libc, __realpath, realpath, GLIBC_2_3);
diff --git a/stdlib/realpath.c b/stdlib/realpath.c
new file mode 100644
index 0000000000..1a70c658b7
--- /dev/null
+++ b/stdlib/realpath.c
@@ -0,0 +1,42 @@ 
+/* Return the canonical absolute name of a given file.
+   Copyright (C) 1996-2020 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/>.  */
+
+#include <stdlib.h>
+#include <errno.h>
+#include <shlib-compat.h>
+
+char *
+__realpath_system (const char *name, char *resolved)
+{
+  bool resolved_malloc = false;
+  if (resolved == NULL)
+    {
+      resolved = malloc (PATH_MAX);
+      if (resolved == NULL)
+	return NULL;
+      resolved_malloc = true;
+    }
+
+  if (! __resolve_path (name, resolved, PATH_MAX))
+    {
+      if (resolved_malloc)
+      	free (resolved);
+      return NULL;
+    }
+  return resolved;
+}
diff --git a/sysdeps/unix/sysv/linux/realpath.c b/sysdeps/unix/sysv/linux/realpath.c
new file mode 100644
index 0000000000..4c69011f92
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/realpath.c
@@ -0,0 +1,65 @@ 
+/* Return the canonical absolute name of a given file.  Linux version.
+   Copyright (C) 2020 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/>.  */
+
+#include <stdlib.h>
+#include <errno.h>
+#include <not-cancel.h>
+#include <shlib-compat.h>
+
+char *
+__realpath_system (const char *name, char *resolved)
+{
+  int fd = __open64_nocancel (name, O_PATH | O_NONBLOCK | O_CLOEXEC);
+  if (fd == -1)
+    {
+      /* If the call fails with either EACCES or ENOENT and resolved_path is
+	 not NULL, then the prefix of path that is not readable or does not
+	 exist is returned in resolved_path.  This is a GNU extension.  */
+      if (resolved != NULL)
+	__resolve_path (name, resolved, PATH_MAX);
+      return NULL;
+    }
+
+  char procname[sizeof ("/proc/self/fd/") + 3 * sizeof (int)];
+  *_fitoa_word (fd, __stpcpy (procname, "/proc/self/fd/"), 10, 0) = '\0';
+
+  char path[PATH_MAX];
+  ssize_t len = __readlink (procname, path, sizeof (path) - 1);
+  if (len < 0)
+    {
+      __close_nocancel_nostatus (fd);
+      return NULL;
+    }
+  path[len] = '\0';
+
+  struct stat64 st;
+  fstat64 (fd, &st);
+  dev_t st_dev = st.st_dev;
+  ino_t st_ino = st.st_ino;
+  int r = stat64 (path, &st);
+  if (r == -1 || st.st_dev != st_dev || st.st_ino != st_ino)
+    {
+      if (r == 0)
+	__set_errno (ELOOP);
+      __close_nocancel_nostatus (fd);
+      return NULL;
+    }
+
+  __close_nocancel_nostatus (fd);
+  return resolved != NULL ? strcpy (resolved, path) : __strdup (path);
+}