[PATCH 4/4] stdlib: Remove lstat usage from realpath [BZ #24970]

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

The readlink already tells whether the file is a symlink, so there is
no need to call lstat to check it.  However for '..' it requires an
extra readlink check if the previous component can be really accessed,
otherwise the next iteration will check a possible valid path and end
early.  It should performance-wise acceptable and a gain over lstat,
afaik symlink should not update any inode information.

Checked on x86_64-linux-gnu and i686-linux-gnu.
---
 stdlib/canonicalize.c | 52 ++++++++++++++++++++++---------------------
 1 file changed, 27 insertions(+), 25 deletions(-)

diff --git a/stdlib/canonicalize.c b/stdlib/canonicalize.c
index 44a25a9a59..952f4dca41 100644
--- a/stdlib/canonicalize.c
+++ b/stdlib/canonicalize.c
@@ -55,6 +55,7 @@ __realpath (const char *name, char *resolved)
   const char *start, *end, *rpath_limit;
   const size_t path_max = PATH_MAX;
   int num_links = 0;
+  char buf[PATH_MAX];
   char extra_buf[PATH_MAX];
 
   if (name == NULL)
@@ -104,12 +105,6 @@ __realpath (const char *name, char *resolved)
 
   for (end = start; *start; start = end)
     {
-#ifdef _LIBC
-      struct stat64 st;
-#else
-      struct stat st;
-#endif
-
       /* Skip sequence of multiple path-separators.  */
       while (*start == '/')
         ++start;
@@ -118,12 +113,25 @@ __realpath (const char *name, char *resolved)
       for (end = start; *end && *end != '/'; ++end)
         /* Nothing.  */;
 
-      if (end - start == 0)
-        break;
-      else if (end - start == 1 && start[0] == '.')
+      if (end - start == 1 && start[0] == '.')
         /* nothing */;
       else if (end - start == 2 && start[0] == '.' && start[1] == '.')
         {
+          ssize_t n;
+
+          if (dest[-1] != '/')
+            *dest++ = '/';
+          *dest = '\0';
+
+          n = __readlink (rpath, buf, path_max - 1);
+          if (n == -1)
+            {
+              if (errno == ENOTDIR && dest[-1] == '/')
+                dest[-1] = '\0';
+              if (errno != EINVAL)
+                goto error;
+            }
+
           /* Back up to previous component, ignore if at root already.  */
           if (dest > rpath + 1)
             for (--dest; dest > rpath && dest[-1] != '/'; --dest)
@@ -132,6 +140,7 @@ __realpath (const char *name, char *resolved)
       else
         {
           size_t new_size;
+          ssize_t n;
 
           if (dest[-1] != '/')
             *dest++ = '/';
@@ -166,25 +175,23 @@ __realpath (const char *name, char *resolved)
           dest = __mempcpy (dest, start, end - start);
           *dest = '\0';
 
-          if (__lxstat64 (_STAT_VER, rpath, &st) < 0)
-            goto error;
-
-          if (S_ISLNK (st.st_mode))
+          n = __readlink (rpath, buf, path_max - 1);
+          if (n < 0)
+            {
+              if (errno == ENOTDIR && dest[-1] == '/')
+                dest[-1] = '\0';
+              if (errno != EINVAL)
+                goto error;
+            }
+          else
             {
-              char buf[PATH_MAX];
               size_t len;
-              ssize_t n;
 
               if (++num_links > __eloop_threshold ())
                 {
                   __set_errno (ELOOP);
                   goto error;  }
 
-              n = __readlink (rpath, buf, path_max - 1);
-              if (n < 0)
-                goto error;
-              buf[n] = '\0';
-
               len = strlen (end);
               /* Check that n + len + 1 doesn't overflow and is <= path_max. */
               if (n >= SIZE_MAX - len || n + len >= path_max)
@@ -211,11 +218,6 @@ __realpath (const char *name, char *resolved)
                       continue;
                 }
             }
-          else if (!S_ISDIR (st.st_mode) && *end != '\0')
-            {
-              __set_errno (ENOTDIR);
-              goto error;
-            }
         }
     }
   if (dest > rpath + 1 && dest[-1] == '/')
-- 
2.25.1