Several months ago we reported an ESTALE bug when mounting NFS2/3
partitions. The tests which trigger this bug perform lots of small mkfs's
on small /dev/loop partitions, after which destroying the loop device and
creating a new f/s. This bug was reported in a message subject "nfs2/3
ESTALE bug on mount point (v2.6.24-rc8)" here:
http://www.mail-archive.com/[email protected]/msg00877.html
The message we posted also provides a simple shell script which triggers the
bug quite consistently on a variety of systems. The problem started for us
when we began using a newer version of nfs-utils, and until now we couldn't
use newer versions of nfs-utils. After some testing, we tracked it down to
a bug in libblkid (part of e2fsprogs), which newer versions of nfs-utils
began using: ALL versions of nfs-utils using libblkid have this bug.
Here's what happens.
1. Mountd calls get_uuid() (which is #ifdef'ed USE_BLKID). There, it
initializes a static blkid_cache variable as follows:
static blkid_cache cache = NULL;
if (cache == NULL)
blkid_get_cache(&cache, NULL);
get_uuid then calls
dev = blkid_get_dev(cache, devname, BLKID_DEV_NORMAL);
to find the specific device out of the cache, and if found, it extracts
the UUID from the device structure returned (using the appropriate tag).
It should be noted that getting the uuid is a filesystems-specific
procedure, and libblkid knows how to handle ext2/3/4, reiserfs, xfs, and
more.
2. Each time mountd needs to get the uuid of a device, after the first time,
it then calls blkid_get_dev with the BLKID_DEV_NORMAL flag (which is
defined as
#define BLKID_DEV_NORMAL (BLKID_DEV_CREATE | BLKID_DEV_VERIFY)
The code in blkid_get_dev (libblkid has three parts):
(a) check if the device is in the known cached list;, if so, return it.
(b) if not found, then create a new blkid device structure, store it in
the cache, and mark the cache as having changed
(BLKID_BIC_FL_CHANGED).
(c) finally, since flags includes BLKID_DEV_VERIFY (which is part of
BLKID_DEV_NORMAL), then blkid_get_dev also calls blkid_verify() to
check that the existing device about to be returned to the caller
(get_uuid) is valid.
3. blkid_verify can verify that a device is valid. To do so, it open(2)'s
the device, reads some filesystem-specific info from it (e.g., the ext2
superblock header, which includes the uuid), and caches all that
filesystem-specific information for later reuse. Of course, reading the
f/s superblock each time is expensive, so blkid_verify has some code
which is intended to prevent excessive verifications as follows:
now = time(0);
diff = now - dev->bid_time;
if ((now > dev->bid_time) && (diff > 0) &&
((diff < BLKID_PROBE_MIN) ||
(dev->bid_flags & BLKID_BID_FL_VERIFIED &&
diff < BLKID_PROBE_INTERVAL)))
return dev;
It should be noted that when a blkid device structure is created/probed,
the time it was last probed is saved in dev->bid_time. IOW, this
bid_time is the last time it was probed, *not* the mtime of the /dev
itself.
Therefore, the above "if" wants to ensure that a device won't be
re-probed unless enough time had passed since the last time it was
probed: BLKID_PROBE_INTERVAL is 200 seconds. That check alone is wrong.
A device could have changed numerous times within 200 seconds, and the
result is that libblkid happily returns the old dev structure, with the
old UUID, *until* 200 seconds have passed. We verified it by sticking
printf's in nfs-utils and libblkid, and also using tcpdumps b/t an nfs
client and server.
If the above "if" condition is false, blkid_verify falls through and
re-probes the device: call open(2), get the new superblock info, and
return a new UUID. This is the point at which our script aborted with
ESTALE, because the on disk /dev's UUID (which NFS uses in the fhandle)
and the cached one did not match. In other words, although our scripts
mkfs's ext2 file systems many many times within 200 seconds, it takes
that long for mountd to detect the changed UUID (I have to think that
this can be a security problem, esp. if UUIDs are used for such
purposes).
Therefore, the above "if" test in blkid_verify must also check to see if
the on-disk device itself has changed, by checking to see if its *mtime*
of the device is older than the last probe time: if the mtime is older,
then it's safe to return the existing cached device; otherwise, we must
reprobe and update the cached info
(BTW, I think that the check for "diff > 0" is redundant in the above
"if" b/c if "now > dev->bid_time" is true, then "diff > 0" is also true.)
The following patch (against e2fsprogs v1.40.8-215-g9817a2b) fixes the bug.
It's a small patch, but perhaps not the cleanest one: it has to goto the
middle of another if statement; a better patch would perhaps consolidate the
stat() and fstat() calls into one, and rewrite the code so the second 'if'
doesn't need the fstat(). We've tested this patch with our script and we
cannot reproduce the ESTALE bug even after thousands of iterations.
Enjoy,
Erez and Himanshu.
--cut-here----cut-here----cut-here----cut-here----cut-here----cut-here--
BLKID: don't uses stale cache/uuid values for recreated devices
Signed-off-by: Himanshu Kanda <[email protected]>
Signed-off-by: Erez Zadok <[email protected]>
diff --git a/lib/blkid/probe.c b/lib/blkid/probe.c
index d8457a8..838c54b 100644
--- a/lib/blkid/probe.c
+++ b/lib/blkid/probe.c
@@ -1220,7 +1220,11 @@ blkid_dev blkid_verify(blkid_cache cache, blkid_dev dev)
now = time(0);
diff = now - dev->bid_time;
+ if (stat(dev->bid_name, &st) < 0)
+ goto out_err;
+
if ((now > dev->bid_time) && (diff > 0) &&
+ (st.st_mtime <= dev->bid_time) &&
((diff < BLKID_PROBE_MIN) ||
(dev->bid_flags & BLKID_BID_FL_VERIFIED &&
diff < BLKID_PROBE_INTERVAL)))
@@ -1233,6 +1237,7 @@ blkid_dev blkid_verify(blkid_cache cache, blkid_dev dev)
if (((probe.fd = open(dev->bid_name, O_RDONLY)) < 0) ||
(fstat(probe.fd, &st) < 0)) {
if (probe.fd >= 0) close(probe.fd);
+out_err:
if ((errno != EPERM) && (errno != EACCES) &&
(errno != ENOENT)) {
DBG(DEBUG_PROBE,
--cut-here----cut-here----cut-here----cut-here----cut-here----cut-here--
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