linux-kernel - Re: [PATCH] ceph: Delete unused variables in mds

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Date:   Thu, 12 Oct 2017 15:45:42 -0700
From:   Gregory Farnum <gfarnum@...hat.com>
To:     Christos Gkekas <chris.gekas@...il.com>
Cc:     "Yan, Zheng" <zyan@...hat.com>, Sage Weil <sage@...hat.com>,
        Ilya Dryomov <idryomov@...il.com>,
        ceph-devel <ceph-devel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ceph: Delete unused variables in mds_client

On Thu, Oct 12, 2017 at 2:55 PM, Christos Gkekas <chris.gekas@...il.com> wrote:
> Remove variables in mds_client that are set but never used.

Hmm, it looks like all the values removed here (except err) *are* used
and this patch would break the world. Specifically...

>
> Signed-off-by: Christos Gkekas <chris.gekas@...il.com>
> ---
>  fs/ceph/mds_client.c | 11 -----------
>  1 file changed, 11 deletions(-)
>
> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> index f23c820..cadf9b6 100644
> --- a/fs/ceph/mds_client.c
> +++ b/fs/ceph/mds_client.c
> @@ -3797,11 +3797,8 @@ void ceph_mdsc_handle_fsmap(struct ceph_mds_client *mdsc, struct ceph_msg *msg)
>         void *p = msg->front.iov_base;
>         void *end = p + msg->front.iov_len;
>         u32 epoch;
> -       u32 map_len;
>         u32 num_fs;
>         u32 mount_fscid = (u32)-1;
> -       u8 struct_v, struct_cv;
> -       int err = -EINVAL;
>
>         ceph_decode_need(&p, end, sizeof(u32), bad);
>         epoch = ceph_decode_32(&p);
> @@ -3809,10 +3806,6 @@ void ceph_mdsc_handle_fsmap(struct ceph_mds_client *mdsc, struct ceph_msg *msg)
>         dout("handle_fsmap epoch %u\n", epoch);
>
>         ceph_decode_need(&p, end, 2 + sizeof(u32), bad);
> -       struct_v = ceph_decode_8(&p);
> -       struct_cv = ceph_decode_8(&p);
> -       map_len = ceph_decode_32(&p);
> -

p here is a pointer into a buffer which was sent to us over the
network (msg->front.iov_base, shown at top of patch). ceph_decode is
reading data out of that buffer into these variables and advancing p
for the subsequent readers. If you drop these statements, we start
trying to read values at the wrong offsets, and obviously nothing
makes sense.

>         ceph_decode_need(&p, end, sizeof(u32) * 3, bad);
>         p += sizeof(u32) * 2; /* skip epoch and legacy_client_fscid */
>
> @@ -3820,12 +3813,9 @@ void ceph_mdsc_handle_fsmap(struct ceph_mds_client *mdsc, struct ceph_msg *msg)
>         while (num_fs-- > 0) {
>                 void *info_p, *info_end;
>                 u32 info_len;
> -               u8 info_v, info_cv;
>                 u32 fscid, namelen;
>
>                 ceph_decode_need(&p, end, 2 + sizeof(u32), bad);
> -               info_v = ceph_decode_8(&p);
> -               info_cv = ceph_decode_8(&p);
>                 info_len = ceph_decode_32(&p);
>                 ceph_decode_need(&p, end, info_len, bad);
>                 info_p = p;
> @@ -3852,7 +3842,6 @@ void ceph_mdsc_handle_fsmap(struct ceph_mds_client *mdsc, struct ceph_msg *msg)
>                                    0, true);
>                 ceph_monc_renew_subs(&fsc->client->monc);
>         } else {
> -               err = -ENOENT;
>                 goto err_out;

Amusingly, this bit does seem to be correct, as the err_out block just
unconditionally sets mdsc->mdsmap_err to -ENOENT (and is only
reachable from this goto).

>         }
>         return;

I assume this patch was generated by some code cleanliness tool, so it
appears to need some work... :)
-Greg