linux-kernel - Re: [PATCH bpf-next v3 1/7] bpf: Refactor trampoline update code

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Message-ID: <20200304190855.GA31073@chromium.org>
Date:   Wed, 4 Mar 2020 20:08:55 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        linux-security-module@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Andrii Nakryiko <andriin@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Paul Turner <pjt@...gle.com>, Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>
Subject: Re: [PATCH bpf-next v3 1/7] bpf: Refactor trampoline update code

On 04-Mär 10:47, Andrii Nakryiko wrote:
> On Wed, Mar 4, 2020 at 10:44 AM KP Singh <kpsingh@...omium.org> wrote:
> >
> > On 04-Mär 19:37, Daniel Borkmann wrote:
> > > On 3/4/20 4:47 PM, KP Singh wrote:
> > > > From: KP Singh <kpsingh@...gle.com>
> > > >
> > > > As we need to introduce a third type of attachment for trampolines, the
> > > > flattened signature of arch_prepare_bpf_trampoline gets even more
> > > > complicated.
> > > >
> > > > Refactor the prog and count argument to arch_prepare_bpf_trampoline to
> > > > use bpf_tramp_progs to simplify the addition and accounting for new
> > > > attachment types.
> > > >
> > > > Signed-off-by: KP Singh <kpsingh@...gle.com>
> > > > Acked-by: Andrii Nakryiko <andriin@...com>
> > >
> > > [...]
> > > > diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
> > > > index c498f0fffb40..9f7e0328a644 100644
> > > > --- a/kernel/bpf/bpf_struct_ops.c
> > > > +++ b/kernel/bpf/bpf_struct_ops.c
> > > > @@ -320,6 +320,7 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
> > > >     struct bpf_struct_ops_value *uvalue, *kvalue;
> > > >     const struct btf_member *member;
> > > >     const struct btf_type *t = st_ops->type;
> > > > +   struct bpf_tramp_progs *tprogs = NULL;
> > > >     void *udata, *kdata;
> > > >     int prog_fd, err = 0;
> > > >     void *image;
> > > > @@ -425,10 +426,18 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
> > > >                     goto reset_unlock;
> > > >             }
> > > > +           tprogs = kcalloc(BPF_TRAMP_MAX, sizeof(*tprogs), GFP_KERNEL);
> > > > +           if (!tprogs) {
> > > > +                   err = -ENOMEM;
> > > > +                   goto reset_unlock;
> > > > +           }
> > > > +
> > >
> > > Looking over the code again, I'm quite certain that here's a memleak
> > > since the kcalloc() is done in the for_each_member() loop in the ops
> > > update but then going out of scope and in the exit path we only kfree
> > > the last tprogs.
> >
> > You're right, nice catch. Fixing it.
> 
> There is probably no need to do many allocations as well, just one
> outside of the loop and reuse?

Yeah moved it out of the loop and before we grab the mutex, returning
an -ENOMEM directly.

Thanks for noticing this. Sending v4 now.

- KP

> 
> >
> > - KP
> >
> > >
> > > > +           tprogs[BPF_TRAMP_FENTRY].progs[0] = prog;
> > > > +           tprogs[BPF_TRAMP_FENTRY].nr_progs = 1;
> > > >             err = arch_prepare_bpf_trampoline(image,
> > > >                                               st_map->image + PAGE_SIZE,
> > > >                                               &st_ops->func_models[i], 0,
> > > > -                                             &prog, 1, NULL, 0, NULL);
> > > > +                                             tprogs, NULL);
> > > >             if (err < 0)
> > > >                     goto reset_unlock;
> > > > @@ -469,6 +478,7 @@ static int bpf_struct_ops_map_update_elem(struct bpf_map *map, void *key,
> > > >     memset(uvalue, 0, map->value_size);
> > > >     memset(kvalue, 0, map->value_size);
> > > >   unlock:
> > > > +   kfree(tprogs);
> > > >     mutex_unlock(&st_map->lock);
> > > >     return err;
> > > >   }