[Virtio-fs] [PATCH] Fix file descriptor passed for setupmapping

[Fotis Xenakis]

From: Vivek Goyal <vgoyal at redhat.com>
Sent: Monday, May 18, 2020 15:37
To: Fotis Xenakis <foxen at windowslive.com>
Cc: bo.liu at linux.alibaba.com <bo.liu at linux.alibaba.com>; virtio-fs at redhat.com <virtio-fs at redhat.com>
Subject: Re: [Virtio-fs] [PATCH] Fix file descriptor passed for setupmapping

On Fri, May 15, 2020 at 04:50:31PM +0000, Fotis Xenakis wrote:
> From: Liu Bo <bo.liu at linux.alibaba.com>
> Sent: Friday, May 15, 2020 06:48
> To: Fotis Xenakis <foxen at windowslive.com>
> Cc: virtio-fs at redhat.com <virtio-fs at redhat.com>
> Subject: Re: [Virtio-fs] [PATCH] Fix file descriptor passed for setupmapping
>
> On Wed, May 13, 2020 at 07:33:56PM +0300, Fotis Xenakis wrote:
> > Currently, during FUSE_SETUPMAPPING, virtiofsd passes the wrong file
> > descriptor for the file to mmap() to QEMU (specifically, it passes the
> > file handle as requested from the guest).
> >
> > This fixes it, using lo_fi_fd() to map the file handle to the right file
> > descriptor on the host.
> >
>
> Looks good, guest has been setting fi as NULL though.
> As far as I can see, this function is only called from [1], which passes fi as NULL only when the file handle passed from the guest is (uint64_t)-1. In my testing this was never the case though.

Hi Fotis,

Do you know what's different about your setup. This has been working for
me so clearly I am seeing fi as NULL.
Hello,

This was on my mind as well: if others were affected by this bug, it wouldn't have gone unnoticed. Yet, I can't say I know what's different in my setup.

Unfortunately, I have a less-than-basic understanding of virtiofsd, but as far as I can see:
​- virtiofsd differentiates on the file handler passed by the guest [1]. In my case fh != (uint64_t)-1 (which makes sense tbh). fh is whatever FUSE_OPEN returned.
​- Guest-side, the open() flags are passed through to FUSE_OPEN [2]. In my case that's O_RDONLY.
- The only flag set for FUSE_INIT is FUSE_MAP_ALIGNMENT [3].
​-​ For what it's worth, virtiofsd runs with "-o cache=always".

References:
[1] https://gitlab.com/virtio-fs/qemu/-/blob/21336c0f3d05a97f5c409bbc894c19d87259655c/tools/virtiofsd/fuse_lowlevel.c#L1910
[2] https://github.com/foxeng/osv/blob/46255086015a4c801baaf61d431e3aa7eda9e64b/fs/virtiofs/virtiofs_vnops.cc#L111
[3] https://github.com/foxeng/osv/blob/46255086015a4c801baaf61d431e3aa7eda9e64b/fs/virtiofs/virtiofs_vfsops.cc#L91
Vivek

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