netdev - Re: [PATCH 1/3] bpf: Add support to check if BTF object is nested in another object

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Date:   Tue, 7 Apr 2020 11:37:28 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Yonghong Song <yhs@...com>,
        Martin KaFai Lau <kafai@...com>,
        David Miller <davem@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Wenbo Zhang <ethercflow@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Andrii Nakryiko <andriin@...com>, bgregg@...flix.com,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 1/3] bpf: Add support to check if BTF object is nested in
 another object

On Mon, Apr 06, 2020 at 06:16:01PM -0700, Alexei Starovoitov wrote:

SNIP

> > +			continue;
> > +
> > +		/* the 'off' we're looking for is either equal to start
> > +		 * of this field or inside of this struct
> > +		 */
> > +		if (btf_type_is_struct(mtype)) {
> > +			/* our field must be inside that union or struct */
> > +			t = mtype;
> > +
> > +			/* adjust offset we're looking for */
> > +			off -= moff;
> > +			goto again;
> > +		}
> 
> Looks like copy-paste that should be generalized into common helper.

right, I think we could have some common code with btf_struct_access,
but id not want to complicate the change for rfc

> 
> > +
> > +		bpf_log(log, "struct %s doesn't have struct field at offset %d\n", tname, off);
> > +		return -EACCES;
> > +	}
> > +
> > +	bpf_log(log, "struct %s doesn't have field at offset %d\n", tname, off);
> > +	return -EACCES;
> > +}
> > +
> >  static int __btf_resolve_helper_id(struct bpf_verifier_log *log, void *fn,
> >  				   int arg)
> >  {
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 04c6630cc18f..6eb88bef4379 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -3103,6 +3103,18 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env,
> >  	return 0;
> >  }
> >  
> > +static void check_ptr_in_btf(struct bpf_verifier_env *env,
> > +			     struct bpf_reg_state *reg,
> > +			     u32 exp_id)
> > +{
> > +	const struct btf_type *t = btf_type_by_id(btf_vmlinux, reg->btf_id);
> > +
> > +	if (!btf_struct_address(&env->log, t, reg->off, exp_id)) {
> > +		reg->btf_id = exp_id;
> > +		reg->off = 0;
> 
> This doesn't look right.
> If you simply overwrite btf_id and off in the reg it will contain wrong info
> in any subsequent instruction.
> Typically it would be ok, since this reg is a function argument and will be
> scratched after the call, but consider:
> bpf_foo(&file->f_path, &file->f_owner);
> The same base register will be used to construct R1 and R2
> and above re-assign will screw up R1.

ok.. I'll use the 'new btf id' just to do check on the helper args
and not change the reg's btf id

thanks,
jirka