Comment 4 for bug 1877384

Swamped with other work at the moment, but this hasn't been forgotten.
I might be able to take a look at it next week.

On Sat, 16 May 2020 at 12:55, Christian Schoenebeck
<email address hidden> wrote:
>
> 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!
>
> --
> 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:
> 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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