Re: [PATCH bpf] bpf: fix some bad __rcu annotations in bpf/core.c
From: Daniel Borkmann
Date: Tue Jul 10 2018 - 04:03:27 EST
Hi Roman,
On 07/10/2018 03:59 AM, Roman Gushchin wrote:
> Sparse shows some "incorrect type" warnings in the bpf core code.
Thanks for taking a stab at these! It would really help if you could
split the patch into a small series and fix each individual case that
is problematic here.
Please also add Fixes tags to the patches.
More below.
> They are caused by bad __rcu annotations:
> 1) bpf_prog_array_alloc() returns an __rcu pointer, which isn't true.
> At that moment it's obviously an exclusive "owning" pointer,
> which is valid for an infinite amount of time, so __rcu is
> meaningless.
> 2) The progs local variable in compute_effective_progs should be
> marked as __bpf too, it's a local variable, not shared with anyone
Typo: __bpf ?
> else at all. The real __rcu variable is array pointer, which should
> be assigned with rcu_assign_pointer.
> 3) __rcu progs argument of bpf_prog_array_free() should be casted
> to a simple pointer before calling kfree_rcu().
> 4) There is a missing rcu_dereference() annotation in
> bpf_prog_array_copy_to_user().
> 5) old_array __rcu pointer in bpf_prog_array_copy() is used as
> a "normal" non-__rcu pointer.
>
> These changes remove the following sparse warnings:
> kernel/bpf/core.c:1544:31: warning: incorrect type in return expression (different address spaces)
> kernel/bpf/core.c:1544:31: expected struct bpf_prog_array [noderef] <asn:4>*
> kernel/bpf/core.c:1544:31: got void *
> kernel/bpf/core.c:1548:17: warning: incorrect type in return expression (different address spaces)
> kernel/bpf/core.c:1548:17: expected struct bpf_prog_array [noderef] <asn:4>*
> kernel/bpf/core.c:1548:17: got struct bpf_prog_array *<noident>
> kernel/bpf/core.c:1556:9: warning: incorrect type in argument 1 (different address spaces)
> kernel/bpf/core.c:1556:9: expected struct callback_head *head
> kernel/bpf/core.c:1556:9: got struct callback_head [noderef] <asn:4>*<noident>
> kernel/bpf/core.c:1629:34: warning: incorrect type in initializer (different address spaces)
> kernel/bpf/core.c:1629:34: expected struct bpf_prog **prog
> kernel/bpf/core.c:1629:34: got struct bpf_prog *[noderef] <asn:4>*<noident>
> kernel/bpf/core.c:1653:31: warning: incorrect type in assignment (different address spaces)
> kernel/bpf/core.c:1653:31: expected struct bpf_prog **existing_prog
> kernel/bpf/core.c:1653:31: got struct bpf_prog *[noderef] <asn:4>*<noident>
> kernel/bpf/core.c:1681:15: warning: incorrect type in assignment (different address spaces)
> kernel/bpf/core.c:1681:15: expected struct bpf_prog_array *array
> kernel/bpf/core.c:1681:15: got struct bpf_prog_array [noderef] <asn:4>*
> kernel/bpf/core.c:1687:31: warning: incorrect type in assignment (different address spaces)
> kernel/bpf/core.c:1687:31: expected struct bpf_prog **[assigned] existing_prog
> kernel/bpf/core.c:1687:31: got struct bpf_prog *[noderef] <asn:4>*<noident>
>
> Signed-off-by: Roman Gushchin <guro@xxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> ---
> include/linux/bpf.h | 2 +-
> kernel/bpf/cgroup.c | 7 +++----
> kernel/bpf/core.c | 14 ++++++++------
> 3 files changed, 12 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 8827e797ff97..943fb08d8287 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -352,7 +352,7 @@ struct bpf_prog_array {
> struct bpf_prog *progs[0];
> };
>
> -struct bpf_prog_array __rcu *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags);
> +struct bpf_prog_array *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags);
> void bpf_prog_array_free(struct bpf_prog_array __rcu *progs);
> int bpf_prog_array_length(struct bpf_prog_array __rcu *progs);
> int bpf_prog_array_copy_to_user(struct bpf_prog_array __rcu *progs,
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 3d83ee7df381..badabb0b435c 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -95,7 +95,7 @@ static int compute_effective_progs(struct cgroup *cgrp,
> enum bpf_attach_type type,
> struct bpf_prog_array __rcu **array)
> {
> - struct bpf_prog_array __rcu *progs;
> + struct bpf_prog_array *progs;
> struct bpf_prog_list *pl;
> struct cgroup *p = cgrp;
> int cnt = 0;
> @@ -120,13 +120,12 @@ static int compute_effective_progs(struct cgroup *cgrp,
> &p->bpf.progs[type], node) {
> if (!pl->prog)
> continue;
> - rcu_dereference_protected(progs, 1)->
> - progs[cnt++] = pl->prog;
> + progs->progs[cnt++] = pl->prog;
> }
> p = cgroup_parent(p);
> } while (p);
>
> - *array = progs;
> + rcu_assign_pointer(*array, progs);
> return 0;
> }
>
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 1e5625d46414..f6e5b207a0d7 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -1538,7 +1538,7 @@ static struct {
> .null_prog = NULL,
> };
>
> -struct bpf_prog_array __rcu *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags)
> +struct bpf_prog_array *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags)
> {
> if (prog_cnt)
> return kzalloc(sizeof(struct bpf_prog_array) +
> @@ -1550,10 +1550,11 @@ struct bpf_prog_array __rcu *bpf_prog_array_alloc(u32 prog_cnt, gfp_t flags)
Looks good to me to here.
> void bpf_prog_array_free(struct bpf_prog_array __rcu *progs)
> {
> - if (!progs ||
> - progs == (struct bpf_prog_array __rcu *)&empty_prog_array.hdr)
> + struct bpf_prog_array *array = rcu_access_pointer(progs);
Can you elaborate on the rcu_access_pointer() part? This looks odd, at minimum
this needs a comment explaining why it's needed. Is the __rcu annotation above
even correct?
> +
> + if (!array || array == &empty_prog_array.hdr)
> return;
> - kfree_rcu(progs, rcu);
> + kfree_rcu(array, rcu);
> }
>
> int bpf_prog_array_length(struct bpf_prog_array __rcu *progs)
> @@ -1626,7 +1627,7 @@ int bpf_prog_array_copy_to_user(struct bpf_prog_array __rcu *progs,
> void bpf_prog_array_delete_safe(struct bpf_prog_array __rcu *progs,
> struct bpf_prog *old_prog)
> {
> - struct bpf_prog **prog = progs->progs;
> + struct bpf_prog **prog = rcu_dereference(progs)->progs;
Can you elaborate here as well? __rcu annotation buggy instead?
> for (; *prog; prog++)
> if (*prog == old_prog) {
> @@ -1635,11 +1636,12 @@ void bpf_prog_array_delete_safe(struct bpf_prog_array __rcu *progs,
> }
> }
>
> -int bpf_prog_array_copy(struct bpf_prog_array __rcu *old_array,
> +int bpf_prog_array_copy(struct bpf_prog_array __rcu *__old_array,
> struct bpf_prog *exclude_prog,
> struct bpf_prog *include_prog,
> struct bpf_prog_array **new_array)
> {
> + struct bpf_prog_array *old_array = rcu_access_pointer(__old_array);
Same comment here, this doesn't look right. We even fetch old_array->progs
from it later on in this path.
> int new_prog_cnt, carry_prog_cnt = 0;
> struct bpf_prog **existing_prog;
> struct bpf_prog_array *array;
>
Thanks,
Daniel