[Bug 1877384] Re: 9pfs file create with mapped-xattr can fail on overlayfs

[Christian Schoenebeck]

Yes, that compile error with QEMU + recent kernel headers is a bit
annoying, and AFAICS it is not fixed in Debian yet.

Would you mind writing a test case for this bug that you fixed, to
prevent this accidentally being broken in future again?

Please note that 9pfs is currently only been taken care of by 2 people,
and both only on a side channel. The 9pfs code base is complex and error
prone to edge cases like this one, so active assistance would be very
much appreciated!

If you might consider writing a test case, I would give you quick, easy
and short instructions how to compile the 9pfs test cases, and which
source files to touch. There is no guest OS installation required for
the test cases.

Thanks!

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https://bugs.launchpad.net/bugs/1877384

Title:
  9pfs file create with mapped-xattr can fail on overlayfs

Status in QEMU:
  New

Bug description:
  QEMU Version: 3.1.0 as packaged in debian buster, but the code appears to do 
the same in master.
  qemu command-line: qemu-system-x86_64 -m 1G -nographic -nic 
"user,model=virtio-net-pci,tftp=$(pwd),net=10.0.2.0/24,host=10.0.2.2" -fsdev 
local,id=fs,path=$thisdir/..,security_model=mapped-xattr -device 
virtio-9p-pci,fsdev=fs,mount_tag=fs -drive 
"file=$rootdisk,if=virtio,format=raw" -kernel "$kernel" -initrd "$initrd" 
-append "$append"

  
  I'm using CI that runs in a Docker container and runs a qemu VM with code and 
results shared via virtio 9p.
  The 9p fsdev is configured with security_model=mapped-xattr
  When the test code attempts to create a log file in an existing directory, 
open with O_CREAT fails with -ENOENT.

  The relevant strace excerpt is:

  28791 openat(11, ".", O_RDONLY|O_NOFOLLOW|O_PATH|O_DIRECTORY) = 20
  28791 openat(20, "src", O_RDONLY|O_NOCTTY|O_NONBLOCK|O_NOFOLLOW|O_DIRECTORY) 
= 21
  28791 fcntl(21, F_SETFL, O_RDONLY|O_DIRECTORY) = 0
  28791 close(20)                         = 0
  28791 openat(21, "client.log", 
O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK|O_NOFOLLOW, 0600) = 20
  28791 fcntl(20, F_SETFL, O_WRONLY|O_CREAT|O_NONBLOCK|O_NOFOLLOW) = 0
  28791 lsetxattr("/proc/self/fd/21/client.log", "user.virtfs.uid", "\0\0\0", 
4, 0) = -1 ENOENT (No such file or directory)

  My hypothesis for what's going wrong is since the Docker container's
  overlayfs copies-up on writes, when it opens the file it's created a
  new version of the `src` directory containing a `client.log`, but this
  new src directory isn't accessible by file descriptor 20 and the
  lsetxattr call is instead attempting to set attributes on the path in
  the old `src` directory.

  Looking at the code, a fix would be to change `hw/9pfs/9p-local.c` and
  change `local_open2` to instead of calling `local_set_xattrat` to set
  the xattrs by directory file descriptor and file name, to have a
  version of local_set_xattrat` which uses `fsetxattr` to set the virtfs
  attributes instead of the `fsetxattrat_nofollow` helper.

  This reliably happened for me in CI, but I don't have access to the CI
  host or the time to strip the test down to make a minimal test case,
  and had difficulty reproducing the error on other machines.

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