Re: [PATCH 5/6] 9pfs: fix 'Tgetattr' after unlink

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

On Sunday, November 24, 2024 4:50:03 PM CET Christian Schoenebeck wrote:
> With a valid file ID (FID) of an open file, it should be possible to send
> a 'Tgettattr' 9p request and successfully receive a 'Rgetattr' response,
> even if the file has been removed in the meantime. Currently this would
> fail with ENOENT.
> 
> I.e. this fixes the following misbehaviour with a 9p Linux client:
> 
>   open("/home/tst/filename", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
>   unlink("/home/tst/filename") = 0
>   fstat(3, 0x23aa1a8) = -1 ENOENT (No such file or directory)
> 
> Expected results:
> 
>   open("/home/tst/filename", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
>   unlink("/home/tst/filename") = 0
>   fstat(3, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
> 
> This is because 9p server is always using a path name based stat() call
> which fails as soon as the file got removed. So to fix this, use fstat()
> whenever we have an open file descriptor already.
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/103
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---

Fixes: 00ede4c2529b ('virtio-9p: getattr server implementation...')

>  hw/9pfs/9p.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
> index 851e36b9a1..578517739a 100644
> --- a/hw/9pfs/9p.c
> +++ b/hw/9pfs/9p.c
> @@ -1596,7 +1596,13 @@ static void coroutine_fn v9fs_getattr(void *opaque)
>          retval = -ENOENT;
>          goto out_nofid;
>      }
> -    retval = v9fs_co_lstat(pdu, &fidp->path, &stbuf);
> +    if ((fidp->fid_type == P9_FID_FILE && fidp->fs.fd != -1) ||
> +        (fidp->fid_type == P9_FID_DIR && fidp->fs.dir.stream))
> +    {
> +        retval = v9fs_co_fstat(pdu, fidp, &stbuf);
> +    } else {
> +        retval = v9fs_co_lstat(pdu, &fidp->path, &stbuf);
> +    }
>      if (retval < 0) {
>          goto out;
>      }
>