Re: [PATCH] 9pfs: local: ignore O_NOATIME if we don't have permissions

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Donnerstag, 16. April 2020 02:44:33 CEST Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> QEMU's local 9pfs server passes through O_NOATIME from the client. If
> the QEMU process doesn't have permissions to use O_NOATIME (namely, it
> does not own the file nor have the CAP_FOWNER capability), the open will
> fail. This causes issues when from the client's point of view, it
> believes it has permissions to use O_NOATIME (e.g., a process running as
> root in the virtual machine). Additionally, overlayfs on Linux opens
> files on the lower layer using O_NOATIME, so in this case a 9pfs mount
> can't be used as a lower layer for overlayfs (cf.
> https://github.com/osandov/drgn/blob/dabfe1971951701da13863dbe6d8a1d172ad965
> 0/vmtest/onoatimehack.c and https://github.com/NixOS/nixpkgs/issues/54509).
> 
> Luckily, O_NOATIME is effectively a hint, and is often ignored by, e.g.,
> network filesystems. open(2) notes that O_NOATIME "may not be effective
> on all filesystems. One example is NFS, where the server maintains the
> access time." This means that we can honor it when possible but fall
> back to ignoring it.

I am not sure whether NFS would simply silently ignore O_NOATIME i.e. without 
returning EPERM. I don't read it that way. Fact is on Linux the expected 
behaviour is returning EPERM if O_NOATIME cannot be satisfied, consistent 
since its introduction 22 years ago:
http://lkml.iu.edu/hypermail/linux/kernel/9811.2/0118.html

> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>  hw/9pfs/9p-util.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/hw/9pfs/9p-util.h b/hw/9pfs/9p-util.h
> index 79ed6b233e..50842d540f 100644
> --- a/hw/9pfs/9p-util.h
> +++ b/hw/9pfs/9p-util.h
> @@ -37,9 +37,14 @@ static inline int openat_file(int dirfd, const char
> *name, int flags, {
>      int fd, serrno, ret;
> 
> +again:
>      fd = openat(dirfd, name, flags | O_NOFOLLOW | O_NOCTTY | O_NONBLOCK,
>                  mode);
>      if (fd == -1) {
> +        if (errno == EPERM && (flags & O_NOATIME)) {
> +            flags &= ~O_NOATIME;
> +            goto again;
> +        }
>          return -1;
>      }

It would certainly fix the problem in your use case. But it would also unmask 
O_NOATIME for all other ones (i.e. regular users on guest).

I mean I understand your point, but I also have to take other use cases into 
account which might expect to receive EPERM if O_NOATIME cannot be granted.

May I ask how come that file/dir in question does not share the same uid in 
your specific use case? Are the file(s) created outside of QEMU, i.e. directly 
by some app on host?

Best regards,
Christian Schoenebeck