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SubjectRe: [RFC][PATCH 0/3] add FALLOC_FL_NO_HIDE_STALE flag in fallocate
On Tue, Apr 17, 2012 at 12:40:14PM -0500, Eric Sandeen wrote:
> On 4/17/12 11:53 AM, Zheng Liu wrote:
> > Hi list,
> >
> > fallocate is a useful system call because it can preallocate some disk blocks
> > for a file and keep blocks contiguous. However, it has a defect that file
> > system will convert an uninitialized extent to be an initialized when the user
> > wants to write some data to this file, because file system create an
> > unititalized extent while it preallocates some blocks in fallocate (e.g. ext4).
>
> That's a security-driven design feature, not a defect. :)
>
> > Especially, it causes a severe degradation when the user tries to do some
> > random write operations, which frequently modifies the metadata of this file.
> > We meet this problem in our product system at Taobao. Last month, in ext4
> > workshop, we discussed this problem and the Google faces the same problem. So
> > a new flag, FALLOC_FL_NO_HIDE_STALE, is added in order to solve this problem.
>
> Which then opens up severe security problems.
>
> > When this flag is set, file system will create an inititalized extent for this
> > file. So it avoids the conversion from uninitialized to initialized. If users
> > want to use this flag, they must guarantee that file has been initialized by
> > themselves before it is read at the same offset. This flag is added in vfs so
> > that other file systems can also support this flag to improve the performance.
> >
> > I try to make ext4 support this new flag, and run a simple test in my own
> > desktop to verify it. The machine has a Intel(R) Core(TM)2 Duo CPU E8400, 4G
> > memory and a WDC WD1600AAJS-75M0A0 160G SATA disk. I use the following script
> > to tset the performance.
> >
> > #/bin/sh
> > mkfs.ext4 ${DEVICE}
> > mount -t ext4 ${DEVICE} ${TARGET}
> > fallocate -l 27262976 ${TARGET}/test # the size of the file is 256M (*)
>
> That's only 26MB, but the below loop writes to a max offset of around
> 256M.

Yes, you are right. I preallocate a file that is 256MB.

>
> > time for((i=0;i<2000;i++)); do dd if=/dev/zero of=/mnt/sda1/test_256M \
> > conv=notrunc bs=4k count=1 seek=`expr $i \* 16` oflag=sync,direct \
> > 2>/dev/null; done
>
> You fallocate ${TARGET}/test but dd to /mnt/sda1/test_256M ? I presume
> those should be the same file.

Yes, it is the same file.

>
> So the testcase as shown above seems fairly wrong, no? Is that what you
> used for the numbers below?
>
> > * I write a wrapper program to call fallocate(2) with FALLOC_FL_NO_HIDE_STALE
> > flag because the userspace tool doesn't support the new flag.
> >
> > The result:
> > w/o w/
> > real 1m16.043s 0m17.946s -76.4%
> > user 0m0.195s 0m0.192s -1.54%
> > sys 0m0.468s 0m0.462s -1.28%
>
> I think that the missing information here is some profiling to show where
> the time was spent in the "w/o" case. What, exactly, in ext4 extent
> management is so darned slow that we have to poke security holes in the
> filesytem to get decent performance?
>
> However,, the above also seems like an alarmingly large difference, and
> one that I can't attribute to unwritten extent conversion overhead.
>
> If I test the seeky dd to a prewritten file (to eliminate extent
> conversion):
>
> # dd if=/dev/zero of=/mnt/scratch/test bs=1M count=256
> # sync
>
> vs. to a fallocated file (which requires extent conversion):
>
> # fallocate -l 256m /mnt/scratch/test
>
> and then do your seeky dd test after each of the above:
>
> # time for((i=0;i<2000;i++)); do dd if=/dev/zero of=/mnt/scratch/test \
> conv=notrunc bs=4k count=1 seek=`expr $i \* 16` oflag=sync,direct \
> 2>/dev/null; done
>
> On ext4, I get about 59.9 seconds in the pre-written case, 65.2 seconds in the fallocated case.
>
> On xfs, I get about 52.5 seconds in the pre-written case, 52.7 seconds in the fallocated case.
>
> I don't see anywhere near the slowdown you show above, certainly
> nothing bad enough to warrant opening the big security hole.
> Am I missing something?

I will run more detailed benchmarks to trace this issue. If I have a
lastest result, I will send a new mail to let you know. :)

I fully understand that this flag will bring a security hole, and I
totally agree that we should dig *root cause* in ext4. But, IMHO, a
proper interface, which is limited by a proper capabilities, might be
useful for the user.

Regards,
Zheng

>
> The ext4 delta is a bit larger, though, so it seems worth investigating
> the *root cause* of the extra overhead if it's problematic in your
> workloads...
>
> -Eric
>
>
> > Obviously, this flag will bring an secure issue because the malicious user
> > could use this flag to get other user's data if (s)he doesn't do a
> > initialization before reading this file. Thus, a sysctl parameter
> > 'fs.falloc_no_hide_stale' is defined in order to let administrator to determine
> > whether or not this flag is enabled. Currently, this flag is disabled by
> > default. I am not sure whether this is enough or not. Another option is that
> > a new Kconfig entry is created to remove this flag during the kernel is
> > complied. So any suggestions or comments are appreciated.
> >
> > Regards,
> > Zheng
> >
> > Zheng Liu (3):
> > vfs: add FALLOC_FL_NO_HIDE_STALE flag in fallocate
> > vfs: add security check for _NO_HIDE_STALE flag
> > ext4: add FALLOC_FL_NO_HIDE_STALE support
> >
> > fs/ext4/extents.c | 7 +++++--
> > fs/open.c | 12 +++++++++++-
> > include/linux/falloc.h | 5 +++++
> > include/linux/sysctl.h | 1 +
> > kernel/sysctl.c | 10 ++++++++++
> > 5 files changed, 32 insertions(+), 3 deletions(-)
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at http://vger.kernel.org/majordomo-info.html
>


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