[RFC PATCH 1/2] elf: Port ldconfig away from stack-allocated paths

[Sergey Bugaev <bugaevc@gmail.com>]

ldconfig was allocating PATH_MAX bytes on the stack for the library file
name. The issues with PATH_MAX usage are well documented [0][1]; even if
a program does not rely on paths being limited to PATH_MAX bytes,
allocating 4096 bytes on the stack for paths that are typically rather
short (strlen ("/lib64/libc.so.6") is 16) is wasteful and dangerous.

[0]: https://insanecoding.blogspot.com/2007/11/pathmax-simply-isnt.html
[1]: https://eklitzke.org/path-max-is-tricky

Instead, make use of asprintf to dynamically allocate memory of just the
right size on the heap.

Checked on x86_64-linux-gnu and i686-gnu.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
---
 elf/ldconfig.c | 59 +++++++++++++++++---------------------------------
 1 file changed, 20 insertions(+), 39 deletions(-)

diff --git a/elf/ldconfig.c b/elf/ldconfig.c
index 2fc45ad8..7f2e4226 100644
--- a/elf/ldconfig.c
+++ b/elf/ldconfig.c
@@ -677,28 +677,18 @@ search_dir (const struct dir_entry *entry)
 
   char *dir_name;
   char *real_file_name;
-  size_t real_file_name_len;
-  size_t file_name_len = PATH_MAX;
-  char *file_name = alloca (file_name_len);
+  char *file_name;
   if (opt_chroot != NULL)
-    {
-      dir_name = chroot_canon (opt_chroot, entry->path);
-      real_file_name_len = PATH_MAX;
-      real_file_name = alloca (real_file_name_len);
-    }
+    dir_name = chroot_canon (opt_chroot, entry->path);
   else
-    {
-      dir_name = entry->path;
-      real_file_name_len = 0;
-      real_file_name = file_name;
-    }
+    dir_name = entry->path;
 
   DIR *dir;
   if (dir_name == NULL || (dir = opendir (dir_name)) == NULL)
     {
       if (opt_verbose)
 	error (0, errno, _("Can't open directory %s"), entry->path);
-      if (opt_chroot != NULL && dir_name != NULL)
+      if (opt_chroot != NULL)
 	free (dir_name);
       return;
     }
@@ -733,25 +723,11 @@ search_dir (const struct dir_entry *entry)
 			 + 1, ".#prelink#.", sizeof (".#prelink#.") - 1) == 0)
 	    continue;
 	}
-      len += strlen (entry->path) + 2;
-      if (len > file_name_len)
-	{
-	  file_name_len = len;
-	  file_name = alloca (file_name_len);
-	  if (!opt_chroot)
-	    real_file_name = file_name;
-	}
-      sprintf (file_name, "%s/%s", entry->path, direntry->d_name);
+      asprintf (&file_name, "%s/%s", entry->path, direntry->d_name);
       if (opt_chroot != NULL)
-	{
-	  len = strlen (dir_name) + strlen (direntry->d_name) + 2;
-	  if (len > real_file_name_len)
-	    {
-	      real_file_name_len = len;
-	      real_file_name = alloca (real_file_name_len);
-	    }
-	  sprintf (real_file_name, "%s/%s", dir_name, direntry->d_name);
-	}
+	asprintf (&real_file_name, "%s/%s", dir_name, direntry->d_name);
+      else
+	real_file_name = file_name;
 
       struct stat lstat_buf;
       /* We optimize and try to do the lstat call only if needed.  */
@@ -761,7 +737,7 @@ search_dir (const struct dir_entry *entry)
 	if (__glibc_unlikely (lstat (real_file_name, &lstat_buf)))
 	  {
 	    error (0, errno, _("Cannot lstat %s"), file_name);
-	    continue;
+	    goto next;
 	  }
 
       struct stat stat_buf;
@@ -778,7 +754,7 @@ search_dir (const struct dir_entry *entry)
 		{
 		  if (strstr (file_name, ".so") == NULL)
 		    error (0, 0, _("Input file %s not found.\n"), file_name);
-		  continue;
+		  goto next;
 		}
 	    }
 	  if (__glibc_unlikely (stat (target_name, &stat_buf)))
@@ -793,7 +769,7 @@ search_dir (const struct dir_entry *entry)
 	      if (opt_chroot != NULL)
 		free (target_name);
 
-	      continue;
+	      goto next;
 	    }
 
 	  if (opt_chroot != NULL)
@@ -806,7 +782,7 @@ search_dir (const struct dir_entry *entry)
 	  lstat_buf.st_ctime = stat_buf.st_ctime;
 	}
       else if (!S_ISREG (lstat_buf.st_mode))
-	continue;
+	goto next;
 
       char *real_name;
       if (opt_chroot != NULL && is_link)
@@ -816,7 +792,7 @@ search_dir (const struct dir_entry *entry)
 	    {
 	      if (strstr (file_name, ".so") == NULL)
 		error (0, 0, _("Input file %s not found.\n"), file_name);
-	      continue;
+	      goto next;
 	    }
 	}
       else
@@ -828,7 +804,7 @@ search_dir (const struct dir_entry *entry)
 	  && __builtin_expect (lstat (real_file_name, &lstat_buf), 0))
 	{
 	  error (0, errno, _("Cannot lstat %s"), file_name);
-	  continue;
+	  goto next;
 	}
 
       /* First search whether the auxiliary cache contains this
@@ -842,7 +818,7 @@ search_dir (const struct dir_entry *entry)
 	    {
 	      if (real_name != real_file_name)
 		free (real_name);
-	      continue;
+	      goto next;
 	    }
 	  else if (opt_build_cache)
 	    add_to_aux_cache (&lstat_buf, flag, isa_level, soname);
@@ -948,6 +924,11 @@ search_dir (const struct dir_entry *entry)
 	  dlib_ptr->next = dlibs;
 	  dlibs = dlib_ptr;
 	}
+
+    next:
+      free (file_name);
+      if (opt_chroot != NULL)
+	free (real_file_name);
     }
 
   closedir (dir);
-- 
2.40.1