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SubjectRe: [PATCH 2/2] net/9p: fix response size check in p9_check_errors()
Christian Schoenebeck wrote on Tue, Nov 22, 2022 at 12:04:08AM +0100:
> Since 60ece0833b6c (net/9p: allocate appropriate reduced message buffers)
> it is no longer appropriate to check server's response size against
> msize. Check against the previously allocated buffer capacity instead.

Thanks for the follow up!

> - Omit this size check entirely for zero-copy messages, as those always
> allocate 4k (P9_ZC_HDR_SZ) linear buffers which are not used for actual
> payload and can be much bigger than 4k.

[review includes the new flag patch]

hmm, unless there's anywhere else you think we might use these flags it
looks simpler to just pass a flag to p9_check_errors?

In particular adding a bool in this position is not particularly efficient:
-------(pahole)-----
struct p9_fcall {
u32 size; /* 0 4 */
u8 id; /* 4 1 */

/* XXX 1 byte hole, try to pack */

u16 tag; /* 6 2 */
size_t offset; /* 8 8 */
size_t capacity; /* 16 8 */
bool zc; /* 24 1 */

/* XXX 7 bytes hole, try to pack */

struct kmem_cache * cache; /* 32 8 */
u8 * sdata; /* 40 8 */

/* size: 48, cachelines: 1, members: 8 */
/* sum members: 40, holes: 2, sum holes: 8 */
/* last cacheline: 48 bytes */
};
----------------
Not that adding it between id and tag sounds better to me, so this is
probably just as good as anywhere else :-D

Anyway, I'm just nitpicking -- on principle I agree just whitelisting zc
requests from this check makes most sense, happy with either way if you
think this is better for the future.

> - Replace p9_debug() by pr_err() to make sure this message is always
> printed in case this error is triggered.
>
> - Add 9p message type to error message to ease investigation.

Yes to these log changes!

>
> Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
> ---
> net/9p/client.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/net/9p/client.c b/net/9p/client.c
> index 30dd82f49b28..63f13dd1ecff 100644
> --- a/net/9p/client.c
> +++ b/net/9p/client.c
> @@ -514,10 +514,10 @@ static int p9_check_errors(struct p9_client *c, struct p9_req_t *req)
> int ecode;
>
> err = p9_parse_header(&req->rc, NULL, &type, NULL, 0);
> - if (req->rc.size >= c->msize) {
> - p9_debug(P9_DEBUG_ERROR,
> - "requested packet size too big: %d\n",
> - req->rc.size);
> + if (req->rc.size > req->rc.capacity && !req->rc.zc) {
> + pr_err(
> + "requested packet size too big: %d does not fit %ld (type=%d)\n",
> + req->rc.size, req->rc.capacity, req->rc.id);

Haven't seen this style before -- is that what qemu uses?
We normally keep the message on first line and align e.g.
> + pr_err("requested packet size too big: %d does not fit %ld (type=%d)\n",
> + req->rc.size, req->rc.capacity, req->rc.id);

(at least what's what other grep -A 1 'pr_err.*,$' seem to do, and
checkpatch is happier with that)

--
Dominique

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