[glibc] stdlib: Simplify buffer management in canonicalize

[glibc] stdlib: Simplify buffer management in canonicalize

[Florian Weimer]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=ef0700004bf0dccf493a5e8e21f71d9e7972ea9f

commit ef0700004bf0dccf493a5e8e21f71d9e7972ea9f
Author: Florian Weimer <fweimer@redhat.com>
Date:   Tue Jul 5 11:04:45 2022 +0200

    stdlib: Simplify buffer management in canonicalize
    
    Move the buffer management from realpath_stk to __realpath.  This
    allows returning directly after allocation errors.
    
    Always make a copy of the result buffer using strdup even if it is
    already heap-allocated.  (Heap-allocated buffers are somewhat rare.)
    This avoids GCC warnings at certain optimization levels.
    
    Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>

Diff:
---
 stdlib/canonicalize.c | 115 ++++++++++++++++++++++----------------------------
 1 file changed, 51 insertions(+), 64 deletions(-)

diff --git a/stdlib/canonicalize.c b/stdlib/canonicalize.c
index e6566bd7d9..54dfac454c 100644
--- a/stdlib/canonicalize.c
+++ b/stdlib/canonicalize.c
@@ -49,6 +49,7 @@
 #else
 # define __canonicalize_file_name canonicalize_file_name
 # define __realpath realpath
+# define __strdup strdup
 # include "pathmax.h"
 # define __faccessat faccessat
 # if defined _WIN32 && !defined __CYGWIN__
@@ -176,27 +177,16 @@ get_path_max (void)
   return path_max < 0 ? 1024 : path_max <= IDX_MAX ? path_max : IDX_MAX;
 }
 
-/* Act like __realpath (see below), with an additional argument
-   rname_buf that can be used as temporary storage.
-
-   If GCC_LINT is defined, do not inline this function with GCC 10.1
-   and later, to avoid creating a pointer to the stack that GCC
-   -Wreturn-local-addr incorrectly complains about.  See:
-   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93644
-   Although the noinline attribute can hurt performance a bit, no better way
-   to pacify GCC is known; even an explicit #pragma does not pacify GCC.
-   When the GCC bug is fixed this workaround should be limited to the
-   broken GCC versions.  */
-#if __GNUC_PREREQ (10, 1)
-# if defined GCC_LINT || defined lint
-__attribute__ ((__noinline__))
-# elif __OPTIMIZE__ && !__NO_INLINE__
-#  define GCC_BOGUS_WRETURN_LOCAL_ADDR
-# endif
-#endif
+/* Scratch buffers used by realpath_stk and managed by __realpath.  */
+struct realpath_bufs
+{
+  struct scratch_buffer rname;
+  struct scratch_buffer extra;
+  struct scratch_buffer link;
+};
+
 static char *
-realpath_stk (const char *name, char *resolved,
-              struct scratch_buffer *rname_buf)
+realpath_stk (const char *name, char *resolved, struct realpath_bufs *bufs)
 {
   char *dest;
   char const *start;
@@ -221,12 +211,7 @@ realpath_stk (const char *name, char *resolved,
       return NULL;
     }
 
-  struct scratch_buffer extra_buffer, link_buffer;
-  scratch_buffer_init (&extra_buffer);
-  scratch_buffer_init (&link_buffer);
-  scratch_buffer_init (rname_buf);
-  char *rname_on_stack = rname_buf->data;
-  char *rname = rname_on_stack;
+  char *rname = bufs->rname.data;
   bool end_in_extra_buffer = false;
   bool failed = true;
 
@@ -236,16 +221,16 @@ realpath_stk (const char *name, char *resolved,
 
   if (!IS_ABSOLUTE_FILE_NAME (name))
     {
-      while (!__getcwd (rname, rname_buf->length))
+      while (!__getcwd (bufs->rname.data, bufs->rname.length))
         {
           if (errno != ERANGE)
             {
               dest = rname;
               goto error;
             }
-          if (!scratch_buffer_grow (rname_buf))
-            goto error_nomem;
-          rname = rname_buf->data;
+          if (!scratch_buffer_grow (&bufs->rname))
+	    return NULL;
+          rname = bufs->rname.data;
         }
       dest = __rawmemchr (rname, '\0');
       start = name;
@@ -299,13 +284,13 @@ realpath_stk (const char *name, char *resolved,
           if (!ISSLASH (dest[-1]))
             *dest++ = '/';
 
-          while (rname + rname_buf->length - dest
+          while (rname + bufs->rname.length - dest
                  < startlen + sizeof dir_suffix)
             {
               idx_t dest_offset = dest - rname;
-              if (!scratch_buffer_grow_preserve (rname_buf))
-                goto error_nomem;
-              rname = rname_buf->data;
+              if (!scratch_buffer_grow_preserve (&bufs->rname))
+                return NULL;
+              rname = bufs->rname.data;
               dest = rname + dest_offset;
             }
 
@@ -316,13 +301,13 @@ realpath_stk (const char *name, char *resolved,
           ssize_t n;
           while (true)
             {
-              buf = link_buffer.data;
-              idx_t bufsize = link_buffer.length;
+              buf = bufs->link.data;
+              idx_t bufsize = bufs->link.length;
               n = __readlink (rname, buf, bufsize - 1);
               if (n < bufsize - 1)
                 break;
-              if (!scratch_buffer_grow (&link_buffer))
-                goto error_nomem;
+              if (!scratch_buffer_grow (&bufs->link))
+		return NULL;
             }
           if (0 <= n)
             {
@@ -334,7 +319,7 @@ realpath_stk (const char *name, char *resolved,
 
               buf[n] = '\0';
 
-              char *extra_buf = extra_buffer.data;
+              char *extra_buf = bufs->extra.data;
               idx_t end_idx IF_LINT (= 0);
               if (end_in_extra_buffer)
                 end_idx = end - extra_buf;
@@ -342,13 +327,13 @@ realpath_stk (const char *name, char *resolved,
               if (INT_ADD_OVERFLOW (len, n))
                 {
                   __set_errno (ENOMEM);
-                  goto error_nomem;
+                  return NULL;
                 }
-              while (extra_buffer.length <= len + n)
+              while (bufs->extra.length <= len + n)
                 {
-                  if (!scratch_buffer_grow_preserve (&extra_buffer))
-                    goto error_nomem;
-                  extra_buf = extra_buffer.data;
+                  if (!scratch_buffer_grow_preserve (&bufs->extra))
+		    return NULL;
+                  extra_buf = bufs->extra.data;
                 }
               if (end_in_extra_buffer)
                 end = extra_buf + end_idx;
@@ -406,25 +391,24 @@ error:
 	 to the path not existing or not being accessible.  */
       if ((!failed || errno == ENOENT || errno == EACCES)
 	  && dest - rname <= get_path_max ())
-	rname = strcpy (resolved, rname);
-      else if (!failed)
 	{
-	  failed = true;
-	  __set_errno (ENAMETOOLONG);
+	  strcpy (resolved, rname);
+	  if (failed)
+	    return NULL;
+	  else
+	    return resolved;
 	}
+      if (!failed)
+	__set_errno (ENAMETOOLONG);
+      return NULL;
     }
-
-error_nomem:
-  scratch_buffer_free (&extra_buffer);
-  scratch_buffer_free (&link_buffer);
-
-  if (failed || rname == resolved)
+  else
     {
-      scratch_buffer_free (rname_buf);
-      return failed ? NULL : resolved;
+      if (failed)
+	return NULL;
+      else
+	return __strdup (bufs->rname.data);
     }
-
-  return scratch_buffer_dupfree (rname_buf, dest - rname);
 }
 
 /* Return the canonical absolute name of file NAME.  A canonical name
@@ -441,12 +425,15 @@ error_nomem:
 char *
 __realpath (const char *name, char *resolved)
 {
-  #ifdef GCC_BOGUS_WRETURN_LOCAL_ADDR
-   #warning "GCC might issue a bogus -Wreturn-local-addr warning here."
-   #warning "See <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93644>."
-  #endif
-  struct scratch_buffer rname_buffer;
-  return realpath_stk (name, resolved, &rname_buffer);
+  struct realpath_bufs bufs;
+  scratch_buffer_init (&bufs.rname);
+  scratch_buffer_init (&bufs.extra);
+  scratch_buffer_init (&bufs.link);
+  char *result = realpath_stk (name, resolved, &bufs);
+  scratch_buffer_free (&bufs.link);
+  scratch_buffer_free (&bufs.extra);
+  scratch_buffer_free (&bufs.rname);
+  return result;
 }
 libc_hidden_def (__realpath)
 versioned_symbol (libc, __realpath, realpath, GLIBC_2_3);