Re: [PATCH] 9p: fix EBADF errors in cached mode

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Dienstag, 14. Juni 2022 05:41:40 CEST Dominique Martinet wrote:
> Dominique Martinet wrote on Tue, Jun 14, 2022 at 12:38:02PM +0900:
> > cached operations sometimes need to do invalid operations (e.g. read
> > on a write only file)
> > Historic fscache had added a "writeback fid" for this, but the conversion
> > to new fscache somehow lost usage of it: use the writeback fid instead
> > of normal one.
> > 
> > Note that the way this works (writeback fid being linked to inode) means
> > we might use overprivileged fid for some operations, e.g. write as root
> > when we shouldn't.
> > Ideally we should keep both fids handy, and only use the writeback fid
> > when really required e.g. reads to a write-only file to fill in the page
> > cache (read-modify-write); but this is the situation we've always had
> > and this commit only fixes an issue we've had for too long.
> > 
> > Fixes: eb497943fa21 ("9p: Convert to using the netfs helper lib to do
> > reads and caching") Cc: stable@vger.kernel.org
> > Cc: David Howells <dhowells@redhat.com>
> > Reported-By: Christian Schoenebeck <linux_oss@crudebyte.com>
> > Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> > ---
> > Ok so finally had time to look at this, and it's not a lot so this is
> > the most straight forward way to do: just reverting to how the old
> > fscache worked.
> > 
> > This appears to work from quick testing, Chiristian could you test it?
> > 
> > I think the warnings you added in p9_client_read/write that check
> > fid->mode might a lot of sense, if you care to resend it as
> > WARN_ON((fid->mode & ACCMODE) == O_xyz);
> > instead I'll queue that for 5.20
> > 
> > 
> > @Stable people, I've checked it applies to 5.17 and 5.18 so should be
> > good to grab once I submit it for inclusion (that commit was included in
> > 5.16, which is no longer stable)
> > 
> >  fs/9p/vfs_addr.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
> > index 7382c5227e94..262968d02f55 100644
> > --- a/fs/9p/vfs_addr.c
> > +++ b/fs/9p/vfs_addr.c
> > @@ -58,7 +58,11 @@ static void v9fs_issue_read(struct netfs_io_subrequest
> > *subreq)> 
> >   */
> >  
> >  static int v9fs_init_request(struct netfs_io_request *rreq, struct file
> >  *file) {
> > 
> > -	struct p9_fid *fid = file->private_data;
> > +	struct inode *inode = file_inode(file);
> > +	struct v9fs_inode *v9inode = V9FS_I(inode);
> > +	struct p9_fid *fid = v9inode->writeback_fid;
> > +
> 
> Sorry for mails back-to-back (grmbl I hate git commit --amend not
> warning I only have unstaged changes), this is missing the following
> here:

I think git does actually. It shows you staged and unstaged changes as comment 
below the commit log text inside the editor. Not as a big fat warning, but the 
info is there.

> +    /* If there is no writeback fid this file only ever has had
> +     * read-only opens, so we can use file's fid which should
> +     * always be set instead */
> +    if (!fid)
> +        fid = file->private_data;
> 
> Christian, you can find it here to test:
> https://github.com/martinetd/linux/commit/a6e033c41cc9f0ec105f5d208b0a820118
> e2bda8
> > +	BUG_ON(!fid);
> > 
> >  	p9_fid_get(fid);
> >  	rreq->netfs_priv = fid;

It definitely goes into the right direction, but I think it's going a bit too 
far by using writeback_fid also in cases where it is not necessary and wasn't 
used before in the past.

What about something like this in v9fs_init_request() (yet untested):

    /* writeback_fid is always opened O_RDWR (instead of just O_WRONLY) 
     * explicitly for this case: partial write backs that require a read
     * prior to actual write and therefore requires a fid with read
     * capability.
     */
    if (rreq->origin == NETFS_READ_FOR_WRITE)
        fid = v9inode->writeback_fid;

If desired, this could be further constrained later on like:

    if (rreq->origin == NETFS_READ_FOR_WRITE &&
        (fid->mode & O_ACCMODE) == O_WRONLY)
    {
        fid = v9inode->writeback_fid;
    }

I will definitely give these options some test spins here, a short feedback 
ahead would be appreciated though.

Thanks Dominique!

Best regards,
Christian Schoenebeck