Re: [PATCH bpf] bpf: fix some bad __rcu annotations in bpf/core.c

[Roman Gushchin]

On Tue, Jul 10, 2018 at 10:03:19AM +0200, Daniel Borkmann wrote:
> 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.

Sure.

The only problem which I have with these sparse warnings,
is that my cgroup local storage patchset touches some of
these lines, and I'm receiving automatic complains.

> 
> 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 ?

Yep, fixed.

> 
> >    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?

No, it's not. But fixing it causes to use rcu_access_pointer()
for almost every bpf_prog_array_delete_safe() call.
Still better probably, will go this path in v2.

> 
> > +
> > +	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?

But here it's fine. Please, look at bpf_prog_array_length() and bpf_prog_array_copy_to_user().
Same applies here.

If we want to be more precise, the "progs" field in the bpf_prog_array should
also be marked with __rcu, but this is beyond the scope of this patch(set).

> 
> >  	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.

Same here. old_array->progs field is not marked with __bpf currently.

Thanks!