Re: [PATCH] ceph: Annotate struct ceph_monmap with __counted_by

[Gustavo A. R. Silva]

On 9/17/23 18:25, Xiubo Li wrote:
> On 9/16/23 04:15, Kees Cook wrote:
> > Prepare for the coming implementation by GCC and Clang of the __counted_by
> > attribute. Flexible array members annotated with __counted_by can have
> > their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
> > (for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
> > functions).
> >
> > As found with Coccinelle[1], add __counted_by for struct ceph_monmap.
> > Additionally, since the element count member must be set before accessing
> > the annotated flexible array member, move its initialization earlier.
> >
> > [1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci
> >
> > Cc: Ilya Dryomov <idryomov@xxxxxxxxx>
> > Cc: Xiubo Li <xiubli@xxxxxxxxxx>
> > Cc: Jeff Layton <jlayton@xxxxxxxxxx>
> > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> > Cc: Eric Dumazet <edumazet@xxxxxxxxxx>
> > Cc: Jakub Kicinski <kuba@xxxxxxxxxx>
> > Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
> > Cc: ceph-devel@xxxxxxxxxxxxxxx
> > Cc: netdev@xxxxxxxxxxxxxxx
> > Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > ---
> >   include/linux/ceph/mon_client.h | 2 +-
> >   net/ceph/mon_client.c           | 2 +-
> >   2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/ceph/mon_client.h b/include/linux/ceph/mon_client.h
> > index b658961156a0..7a9a40163c0f 100644
> > --- a/include/linux/ceph/mon_client.h
> > +++ b/include/linux/ceph/mon_client.h
> > @@ -19,7 +19,7 @@ struct ceph_monmap {
> >       struct ceph_fsid fsid;
> >       u32 epoch;
> >       u32 num_mon;
> > -    struct ceph_entity_inst mon_inst[];
> > +    struct ceph_entity_inst mon_inst[] __counted_by(num_mon);
> >   };
> >   struct ceph_mon_client;
> > diff --git a/net/ceph/mon_client.c b/net/ceph/mon_client.c
> > index faabad6603db..f263f7e91a21 100644
> > --- a/net/ceph/mon_client.c
> > +++ b/net/ceph/mon_client.c
> > @@ -1136,6 +1136,7 @@ static int build_initial_monmap(struct ceph_mon_client *monc)
> >                      GFP_KERNEL);
> >       if (!monc->monmap)
> >           return -ENOMEM;
> > +    monc->monmap->num_mon = num_mon;
> >       for (i = 0; i < num_mon; i++) {
> >           struct ceph_entity_inst *inst = &monc->monmap->mon_inst[i];
> > @@ -1147,7 +1148,6 @@ static int build_initial_monmap(struct ceph_mon_client *monc)
> >           inst->name.type = CEPH_ENTITY_TYPE_MON;
> >           inst->name.num = cpu_to_le64(i);
> >       }
> > -    monc->monmap->num_mon = num_mon;
>
> BTW, is this change related ?

Yes, it is, and it's described in the changelog text.

`num_mon` must be updated before the first access to flex-array `mon_inst`.
Otherwise the compiler cannot properly instrument the code to catch any
out-of-bounds access to `mon_inst`.

--
Gustavo

> >       return 0;
> >   }
>
> Else LGTM.
>
> Reviewed-by: Xiubo Li <xiubli@xxxxxxxxxx>
>
> Thanks!
>
> - Xiubo