[PATCH v3] blkdiscard: check that the file is a block device before opening

[Ari Sundholm]

Opening a file with the O_EXCL flag on and the O_CREAT flag off has
undefined behavior unless the file is a block device. Let's try and
avoid this case on a best-effort basis. The undefined behavior can
still be hit if the block device is replaced with something else
inbetween.

v2: Fix typo: O_CREAT, not O_CREATE.
v3: Change the comment, as the condition is not actually enforced,
    only checked on a best-effort basis.

Signed-off-by: Ari Sundholm <ari at tuxera.com>
---
 util-linux/blkdiscard.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/util-linux/blkdiscard.c b/util-linux/blkdiscard.c
index ace88a1..b97f716 100644
--- a/util-linux/blkdiscard.c
+++ b/util-linux/blkdiscard.c
@@ -38,7 +38,7 @@ int blkdiscard_main(int argc UNUSED_PARAM, char **argv)
 	uint64_t offset; /* Leaving these two variables out does not  */
 	uint64_t length; /* shrink code size and hampers readability. */
 	uint64_t range[2];
-//	struct stat st;
+	struct stat st;
 	int fd;
 
 	enum {
@@ -51,11 +51,17 @@ int blkdiscard_main(int argc UNUSED_PARAM, char **argv)
 	opts = getopt32(argv, "o:l:s", &offset_str, &length_str);
 	argv += optind;
 
+	/* Opening a file with the O_EXCL flag on and the O_CREAT flag off has
+	 * undefined behavior unless the file is a block device. Let's try and
+	 * avoid this case on a best-effort basis. The undefined behavior can
+	 * still be hit if the block device is replaced with something else
+	 * inbetween.
+	 */
+	xstat(argv[0], &st);
+	if (!S_ISBLK(st.st_mode))
+		bb_error_msg_and_die("%s: not a block device", argv[0]);
+
 	fd = xopen(argv[0], O_RDWR|O_EXCL);
-//Why bother, BLK[SEC]DISCARD will fail on non-blockdevs anyway?
-//	xfstat(fd, &st);
-//	if (!S_ISBLK(st.st_mode))
-//		bb_error_msg_and_die("%s: not a block device", argv[0]);
 
 	offset = xatoull_sfx(offset_str, kMG_suffixes);
 
-- 
1.9.1