Data Corruption bug with Samba's vfs_iouring and Linux 5.6.7/5.7rc3

[Andreas Schneider]

On Wednesday, 6 May 2020 16:08:03 CEST Stefan Metzmacher via samba-technical 
wrote:
> Am 06.05.20 um 14:41 schrieb Anoop C S:
> > On Wed, 2020-05-06 at 12:33 +0200, Stefan Metzmacher wrote:
> >> Hi Anoop,
> >> 
> >>> I could reproduce the difference in SHA256 checksum after copying a
> >>> directory with 100 copies of test file(provided by reporter) from
> >>> io_uring VFS module enabled share using Windows explorer(right-
> >>> click-
> >>> 
> >>>> copy/paste). Only 5 out of 100 files had correct checksum after
> >>>> copy
> >>> 
> >>> operation :-/
> >> 
> >> Great! Can you please try to collect level 1 log files with
> >> the patch https://bugzilla.samba.org/attachment.cgi?id=15955
> >> applied?
> > 
> > I have attached three log files.
> > log.io_uring.smbd -- Copy using Windows explorer
> > log.io_uring-mget.smd -- Copy using smbclient
> > log.io_uring-powershell.smd -- Copy using `Copy-Item`
> 
> Thanks! All of them show short reads like:
> > [2020/05/06 17:27:28.130248,  1]
> > ../../source3/modules/vfs_io_uring.c:103(vfs_io_uring_finish_req)> 
> >   vfs_io_uring_finish_req: pread ofs=0 (0x0) len=32768 (0x8000)
> >   nread=32768 (0x32768) eof=10000000 (0x989680) blks=4096 blocks=19536
> >   dir/1.bin fnum 1607026405> 
> > [2020/05/06 17:27:28.131049,  1]
> > ../../source3/modules/vfs_io_uring.c:103(vfs_io_uring_finish_req)> 
> >   vfs_io_uring_finish_req: pread ofs=9969664 (0x982000) len=30336 (0x7680)
> >   nread=30336 (0x30336) eof=10000000 (0x989680) blks=4096 blocks=19536
> >   dir/1.bin fnum 1607026405> 
> > [2020/05/06 17:27:28.133679,  1]
> > ../../source3/modules/vfs_io_uring.c:103(vfs_io_uring_finish_req)> 
> >   vfs_io_uring_finish_req: pread ofs=61440 (0xf000) len=32768 (0x8000)
> >   nread=32768 (0x32768) eof=10000000 (0x989680) blks=4096 blocks=19536
> >   dir/1.bin fnum 1607026405> 
> > [2020/05/06 17:27:28.140184,  0]
> > ../../source3/modules/vfs_io_uring.c:88(vfs_io_uring_finish_req)> 
> >   vfs_io_uring_finish_req: Invalid pread ofs=0 (0x0) len=1048576
> >   (0x100000) nread=32768 (0x32768) eof=10000000 (0x989680) blks=4096
> >   blocks=19536 dir/1.bin fnum 1607026405
> It seems the first request is at ofs=0 len=32768 (0x8000) and we get
> 32768 back.
> A follow up request with ofs=0 len=1048576 (0x100000) only gets the
> first 32768 bytes which are already in the page cache.
> 
> I can easily reproduce this with the Ubuntu 5.4 kernel once I add
> state->ur.sqe.rw_flags |= RWF_NOWAIT; to vfs_io_uring_pread_send()
> and use this.
> 
> echo 1 > /proc/sys/vm/drop_caches
> head -c 1024 /root/samba-test/ff1.dat | hexdump -C
> 00000000  ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff
> 
> |................|
> 
> *
> 00000400
> smbclient //172.31.9.167/uringff -Uroot%test -c "get ff1.dat"
> 
> results in this log entries:
> > [2020/05/06 06:51:57.069990,  0]
> > ../../source3/modules/vfs_io_uring.c:89(vfs_io_uring_finish_req)> 
> >   vfs_io_uring_finish_req: Invalid pread ofs=0 (0x0) len=8388608
> >   (0x800000) nread=16384 (0x4000) eof=8388608 (0x800000) blks=4096
> >   blocks=16384 ff1.dat fnum 840153065> 
> > [2020/05/06 06:51:57.076882,  1]
> > ../../source3/modules/vfs_io_uring.c:104(vfs_io_uring_finish_req)> 
> >   vfs_io_uring_finish_req: pread ofs=16384 (0x4000) len=8372224 (0x7fc000)
> >   nread=8372224 (0x7fc000) eof=8388608 (0x800000) blks=4096 blocks=16384
> >   ff1.dat fnum 840153065
> smbclient is just smart enough to recover itself from the short read.
> But the windows client isn't.
> 
> The attached test against liburing (git://git.kernel.dk/liburing) should
> be able to demonstrate the problem. It can also be found in
> https://github.com/metze-samba/liburing/tree/implicit-rwf-nowaithttps://gith
> ub.com/metze-samba/liburing/commit/eb06dcfde747e46bd08bedf9def2e6cb536c39e3
> 
> 
> I added the sqe->rw_flags = RWF_NOWAIT; line in order to demonstrate it
> against the Ubuntu 5.3 and 5.4 kernels. They both seem to have the bug.
> 
> Can someone run the unmodified test/implicit-rwf_nowait against
> a newer kernel?

$ uname -a
Linux magrathea 5.6.8-1-default #1 SMP Thu Apr 30 10:47:01 UTC 2020 (63116ab) 
x86_64 x86_64 x86_64 GNU/Linux
$ ./a.out 
cqe res=4096 != 8192


	Andreas

-- 
Andreas Schneider                      asn at samba.org
Samba Team                             www.samba.org
GPG-ID:     8DFF53E18F2ABC8D8F3C92237EE0FC4DCC014E3D