linux-kernel - Re: [PATCH] fuse: add support for explicit export disabling

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Date: Wed, 24 Jan 2024 13:16:56 +0100
From: Miklos Szeredi <miklos@...redi.hu>
To: Jingbo Xu <jefflexu@...ux.alibaba.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	amir73il@...il.com
Subject: Re: [PATCH] fuse: add support for explicit export disabling

On Wed, 24 Jan 2024 at 12:30, Jingbo Xu <jefflexu@...ux.alibaba.com> wrote:
>
> open_by_handle_at(2) can fail with -ESTALE with a valid handle returned
> by a previous name_to_handle_at(2) for evicted fuse inodes, which is
> especially common when entry_valid_timeout is 0, e.g. when the fuse
> daemon is in "cache=none" mode.
>
> The time sequence is like:
>
>         name_to_handle_at(2)    # succeed
>         evict fuse inode
>         open_by_handle_at(2)    # fail
>
> The root cause is that, with 0 entry_valid_timeout, the dput() called in
> name_to_handle_at(2) will trigger iput -> evict(), which will send
> FUSE_FORGET to the daemon.  The following open_by_handle_at(2) will send
> a new FUSE_LOOKUP request upon inode cache miss since the previous inode
> eviction.  Then the fuse daemon may fail the FUSE_LOOKUP request with
> -ENOENT as the cached metadata of the requested inode has already been
> cleaned up during the previous FUSE_FORGET.  The returned -ENOENT is
> treated as -ESTALE when open_by_handle_at(2) returns.
>
> This confuses the application somehow, as open_by_handle_at(2) fails
> when the previous name_to_handle_at(2) succeeds.  The returned errno is
> also confusing as the requested file is not deleted and already there.
> It is reasonable to fail name_to_handle_at(2) early in this case, after
> which the application can fallback to open(2) to access files.
>
> Since this issue typically appears when entry_valid_timeout is 0 which
> is configured by the fuse daemon, the fuse daemon is the right person to
> explicitly disable the export when required.
>
> Also considering FUSE_EXPORT_SUPPORT actually indicates the support for
> lookups of "." and "..", and there are existing fuse daemons supporting
> export without FUSE_EXPORT_SUPPORT set, for compatibility, we add a new
> INIT flag for such purpose.

This looks good overall.

>
> Signed-off-by: Jingbo Xu <jefflexu@...ux.alibaba.com>
> ---
> RFC: https://lore.kernel.org/all/20240123093701.94166-1-jefflexu@linux.alibaba.com/
> ---
>  fs/fuse/inode.c           | 11 ++++++++++-
>  include/uapi/linux/fuse.h |  2 ++
>  2 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> index 2a6d44f91729..851940c0e930 100644
> --- a/fs/fuse/inode.c
> +++ b/fs/fuse/inode.c
> @@ -1110,6 +1110,11 @@ static struct dentry *fuse_get_parent(struct dentry *child)
>         return parent;
>  }
>
> +/* only for fid encoding; no support for file handle */
> +static const struct export_operations fuse_fid_operations = {

Nit: I'd call this fuse_no_export_operations (or something else that
emphasizes the fact that this is only for encoding and not for full
export support).

Thanks,
Miklos