mkfs.ubifs: rewrite path checking

We use the 'in_path()' function to check whether the output image is
withing the mkfs.ubifs root directory or not. However, this function
is not correct and it fails for the following situation, as
Marcus Prebble <marcus.prebble@axis.com> reports:

1. We have our root file-system mounted at / and want to build an image
   out of it.
2. We have tmpfs mounted at /tmp
3. We mount the root file-system under /tmp/newroot
4. We run mkfs.ubifs with -r /tmp/newroot -o /tmp/image

And this fails. It fails because 'in_path()' misses this use-case.

This patch re-implements the check completely. Now we use 'realpath()'
to find canonical paths and just check that the output file is not
under the root mkfs.ubifs directory.

Reported-by: Marcus Prebble <marcus.prebble@axis.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Tested-by: Marcus Prebble <marcus.prebble@axis.com>

1 files changed, 37 insertions, 79 deletions

diff --git a/mkfs.ubifs/mkfs.ubifs.c b/mkfs.ubifs/mkfs.ubifs.c
index 46f12ad..5b433bd 100644
--- a/mkfs.ubifs/mkfs.ubifs.c
+++ b/mkfs.ubifs/mkfs.ubifs.c
@@ -20,6 +20,7 @@
  *          Zoltan Sogor
  */
 
+#define _XOPEN_SOURCE 500 /* For realpath() */
 #define PROGRAM_NAME "mkfs.ubifs"
 
 #include "mkfs.ubifs.h"
@@ -235,93 +236,50 @@ static char *make_path(const char *dir, const char *name)
 }
 
 /**
- * same_dir - determine if two file descriptors refer to the same directory.
- * @fd1: file descriptor 1
- * @fd2: file descriptor 2
- */
-static int same_dir(int fd1, int fd2)
-{
-	struct stat stat1, stat2;
-
-	if (fstat(fd1, &stat1) == -1)
-		return -1;
-	if (fstat(fd2, &stat2) == -1)
-		return -1;
-	return stat1.st_dev == stat2.st_dev && stat1.st_ino == stat2.st_ino;
-}
-
-/**
- * do_openat - open a file in a directory.
- * @fd: file descriptor of open directory
- * @path: path relative to directory
- * @flags: open flags
+ * is_contained - determine if a file is beneath a directory.
+ * @file: file path name
+ * @dir: directory path name
  *
- * This function is provided because the library function openat is sometimes
- * not available.
+ * This function returns %1 if @file is accessible from the @dir directory and
+ * %0 otherwise. In case of error, returns %-1.
  */
-static int do_openat(int fd, const char *path, int flags)
+static int is_contained(const char *file, const char *dir)
 {
-	int ret;
-	char *cwd;
+	char *file_base, *copy, *real_file, *real_dir, *p;
 
-	cwd = getcwd(NULL, 0);
-	if (!cwd)
+	/* Make a copy of the file path because 'dirname()' can modify it */
+	copy = strdup(file);
+	if (!copy)
 		return -1;
-	ret = fchdir(fd);
-	if (ret != -1)
-		ret = open(path, flags);
-	if (chdir(cwd) && !ret)
-		ret = -1;
-	free(cwd);
-	return ret;
-}
+	file_base = dirname(copy);
 
-/**
- * in_path - determine if a file is beneath a directory.
- * @dir_name: directory path name
- * @file_name: file path name
- */
-static int in_path(const char *dir_name, const char *file_name)
-{
-	char *fn = strdup(file_name);
-	char *dn;
-	int fd1, fd2, fd3, ret = -1, top_fd;
+	/* Turn the paths into the canonical form */
+	real_file = malloc(PATH_MAX);
+	if (!real_file) {
+		free(copy);
+		return -1;
+	}
 
-	if (!fn)
+	real_dir = malloc(PATH_MAX);
+	if (!real_dir) {
+		free(real_file);
+		free(copy);
+		return -1;
+	}
+	if (!realpath(file_base, real_file)) {
+		perror("realpath");
 		return -1;
-	top_fd = open("/", O_RDONLY);
-	if (top_fd != -1) {
-		dn = dirname(fn);
-		fd1 = open(dir_name, O_RDONLY);
-		if (fd1 != -1) {
-			fd2 = open(dn, O_RDONLY);
-			if (fd2 != -1) {
-				while (1) {
-					int same;
-
-					same = same_dir(fd1, fd2);
-					if (same) {
-						ret = same;
-						break;
-					}
-					if (same_dir(fd2, top_fd)) {
-						ret = 0;
-						break;
-					}
-					fd3 = do_openat(fd2, "..", O_RDONLY);
-					if (fd3 == -1)
-						break;
-					close(fd2);
-					fd2 = fd3;
-				}
-				close(fd2);
-			}
-			close(fd1);
-		}
-		close(top_fd);
 	}
-	free(fn);
-	return ret;
+	if (!realpath(dir, real_dir)) {
+		perror("realpath");
+		return -1;
+	}
+
+	p = strstr(real_file, real_dir);
+	free(real_dir);
+	free(real_file);
+	free(copy);
+	return !!p;
 }
 
 /**
@@ -376,7 +334,7 @@ static int validate_options(void)
 
 	if (!output)
 		return err_msg("no output file or UBI volume specified");
-	if (root && in_path(root, output))
+	if (root && is_contained(output, root))
 		return err_msg("output file cannot be in the UBIFS root "
 			       "directory");
 	if (!is_power_of_2(c->min_io_size))