It might be, I revisited a month back and could no longer trigger the
bug, so it's possible unrelated changes or kernel changes have fixed
the overlayfs copy-up semantics in cases where it would cause issues
with QEMU. If I ever see it again I can resubmit evidence, so it may
be better off closed.

On Thu, 10 Dec 2020 at 12:01, Thomas Huth <email address hidden> wrote:
>
> Closed by accident, Christian just told me that this is not fixed yet.
> Sorry for the inconvenience.
>
> ** Changed in: qemu
>        Status: Fix Released => Confirmed
>
> --
> You received this bug notification because you are subscribed to the bug
> report.
> https://bugs.launchpad.net/bugs/1877384
>
> Title:
>   9pfs file create with mapped-xattr can fail on overlayfs
>
> Status in QEMU:
>   Confirmed
>
> 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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