Re: [PATCH] ceph: use correct index when encoding client supported features

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2022-05-24 at 17:06 +0100, Luís Henriques wrote:
> Feature bits have to be encoded into the correct locations.  This hasn't
> been an issue so far because the only hole in the feature bits was in bit
> 10 (CEPHFS_FEATURE_RECLAIM_CLIENT), which is located in the 2nd byte.  When
> adding more bits that go beyond the this 2nd byte, the bug will show up.
> 
> Fixes: 9ba1e224538a ("ceph: allocate the correct amount of extra bytes for the session features")
> Signed-off-by: Luís Henriques <lhenriques@suse.de>
> ---
>  fs/ceph/mds_client.c | 7 +++++--
>  fs/ceph/mds_client.h | 2 --
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> I hope I got this code right.  I found this issue when trying to add an
> extra feature bit that would go beyond bit 15 and that wasn't showing up
> on the MDS side.
> 
> diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> index 1bd3e1bb0fdf..77e742b6fd30 100644
> --- a/fs/ceph/mds_client.c
> +++ b/fs/ceph/mds_client.c
> @@ -1220,14 +1220,17 @@ static int encode_supported_features(void **p, void *end)
>  	if (count > 0) {
>  		size_t i;
>  		size_t size = FEATURE_BYTES(count);
> +		unsigned long bit;
>  
>  		if (WARN_ON_ONCE(*p + 4 + size > end))
>  			return -ERANGE;
>  
>  		ceph_encode_32(p, size);
>  		memset(*p, 0, size);
> -		for (i = 0; i < count; i++)
> -			((unsigned char*)(*p))[i / 8] |= BIT(feature_bits[i] % 8);
> +		for (i = 0; i < count; i++) {
> +			bit = feature_bits[i];
> +			((unsigned char *)(*p))[bit / 8] |= BIT(bit % 8);
> +		}

I wish we could just use __set_bit/__clear_bit here. It would be a lot
easier to reason this out. Your logic looks correct to me though.


>  		*p += size;
>  	} else {
>  		if (WARN_ON_ONCE(*p + 4 > end))
> diff --git a/fs/ceph/mds_client.h b/fs/ceph/mds_client.h
> index 33497846e47e..12901e3a6823 100644
> --- a/fs/ceph/mds_client.h
> +++ b/fs/ceph/mds_client.h
> @@ -45,8 +45,6 @@ enum ceph_feature_type {
>  	CEPHFS_FEATURE_MULTI_RECONNECT,		\
>  	CEPHFS_FEATURE_DELEG_INO,		\
>  	CEPHFS_FEATURE_METRIC_COLLECT,		\
> -						\
> -	CEPHFS_FEATURE_MAX,			\
>  }
>  #define CEPHFS_FEATURES_CLIENT_REQUIRED {}
>  

Reviewed-by: Jeff Layton <jlayton@kernel.org>