Re: [PATCH v3 1/6] NFSD: Encode COMPOUND operation status on page boundaries

Cedric Blancher
2024-12-26T17:18:12Z

On Thu, 26 Dec 2024 at 17:29, <cel@kernel.org> wrote:
>
> From: Chuck Lever <chuck.lever@oracle.com>
>
> J. David reports an odd corruption of a READDIR reply sent to a
> FreeBSD client.
>
> xdr_reserve_space() has to do a special trick when the @nbytes value
> requests more space than there is in the current page of the XDR
> buffer.
>
> In that case, xdr_reserve_space() returns a pointer to the start of
> the next page, and then the next call to xdr_reserve_space() invokes
> __xdr_commit_encode() to copy enough of the data item back into the
> previous page to make that data item contiguous across the page
> boundary.
>
> But we need to be careful in the case where buffer space is reserved
> early for a data item whose value will be inserted into the buffer
> later.
>
> One such caller, nfsd4_encode_operation(), reserves 8 bytes in the
> encoding buffer for each COMPOUND operation. However, a READDIR
> result can sometimes encode file names so that there are only 4
> bytes left at the end of the current XDR buffer page (though plenty
> of pages are left to handle the remaining encoding tasks).
>
> If a COMPOUND operation follows the READDIR result (say, a GETATTR),
> then nfsd4_encode_operation() will reserve 8 bytes for the op number
> (9) and the op status (usually NFS4_OK). In this weird case,
> xdr_reserve_space() returns a pointer to byte zero of the next buffer
> page, as it assumes the data item will be copied back into place (in
> the previous page) on the next call to xdr_reserve_space().
>
> nfsd4_encode_operation() writes the op num into the buffer, then
> saves the next 4-byte location for the op's status code. The next
> xdr_reserve_space() call is part of GETATTR encoding, so the op num
> gets copied back into the previous page, but the saved location for
> the op status continues to point to the wrong spot in the current
> XDR buffer page because __xdr_commit_encode() moved that data item.
>
> After GETATTR encoding is complete, nfsd4_encode_operation() writes
> the op status over the first XDR data item in the GETATTR result.
> The NFS4_OK status code (0) makes it look like there are zero items
> in the GETATTR's attribute bitmask.
>
> The patch description of commit 2825a7f90753 ("nfsd4: allow encoding
> across page boundaries") [2014] remarks that NFSD "can't handle a
> new operation starting close to the end of a page." This bug appears
> to be one reason for that remark.
>
> Reported-by: J David <j.david.lists@gmail.com>
> Closes: https://lore.kernel.org/linux-nfs/3998d739-c042-46b4-8166-dbd6c5f0e804@oracle.com/T/#t
> Tested-by: Rick Macklem <rmacklem@uoguelph.ca>
> Reviewed-by: NeilBrown <neilb@suse.de>
> X-Cc: stable@vger.kernel.org
> Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> ---

We would appreciate it if this patch series (esp the "insulate"
patches) could be backported to (at least) the 6.6 LTS branch, as this
kind of data corruption is haunting NFSv4.x clients since years#

Ced
--
Cedric Blancher <cedric.blancher@gmail.com>
[https://plus.google.com/u/0/+CedricBlancher/]
Institute Pasteur