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Date:   Tue, 14 Jun 2022 12:41:40 +0900
From:   Dominique Martinet <asmadeus@codewreck.org>
To:     Eric Van Hensbergen <ericvh@gmail.com>,
        Latchesar Ionkov <lucho@ionkov.net>,
        Christian Schoenebeck <linux_oss@crudebyte.com>,
        David Howells <dhowells@redhat.com>
Cc:     linux-fsdevel@vger.kernel.org, stable@vger.kernel.org,
        v9fs-developer@lists.sourceforge.net, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] 9p: fix EBADF errors in cached mode
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 <20220614033802.1606738-1-asmadeus@codewreck.org>
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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:

+    /* 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/a6e033c41cc9f0ec105f5d208b0a820118e2bda8

> +	BUG_ON(!fid);
>  
>  	p9_fid_get(fid);
>  	rreq->netfs_priv = fid;

Thanks,
-- 
Dominique