Subject: Re: [PATCH bpf-next v4 1/2] bpf: allow map helpers access to map values directly

From: Paul Chaignon via iovisor-dev <iovisor-dev-9jONkmmOlFHEE9lA1F8Ukti2O/[email-masked]>
Date: Tue, 24 Apr 2018 15:50:15 +0200

On 04/23/2018 11:18 PM +0200, Daniel Borkmann wrote:
> On 04/22/2018 11:52 PM, Paul Chaignon wrote:
> > Helpers that expect ARG_PTR_TO_MAP_KEY and ARG_PTR_TO_MAP_VALUE can only
> > access stack and packet memory.  Allow these helpers to directly access
> > map values by passing registers of type PTR_TO_MAP_VALUE.
> > 
> > This change removes the need for an extra copy to the stack when using a
> > map value to perform a second map lookup, as in the following:
> > 
> > struct bpf_map_def SEC("maps") infobyreq = {
> >     .type = BPF_MAP_TYPE_HASHMAP,
> >     .key_size = sizeof(struct request *),
> >     .value_size = sizeof(struct info_t),
> >     .max_entries = 1024,
> > };
> > struct bpf_map_def SEC("maps") counts = {
> >     .type = BPF_MAP_TYPE_HASHMAP,
> >     .key_size = sizeof(struct info_t),
> >     .value_size = sizeof(u64),
> >     .max_entries = 1024,
> > };
> > SEC("kprobe/blk_account_io_start")
> > int bpf_blk_account_io_start(struct pt_regs *ctx)
> > {
> >     struct info_t *info = bpf_map_lookup_elem(&infobyreq, &ctx->di);
> >     u64 *count = bpf_map_lookup_elem(&counts, info);
> >     (*count)++;
> > }
> > 
> > Signed-off-by: Paul Chaignon <[email-masked]>
> > ---
> >  kernel/bpf/verifier.c | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 5dd1dcb902bf..70e00beade03 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -1914,7 +1914,7 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
> >  	if (arg_type == ARG_PTR_TO_MAP_KEY ||
> >  	    arg_type == ARG_PTR_TO_MAP_VALUE) {
> >  		expected_type = PTR_TO_STACK;
> > -		if (!type_is_pkt_pointer(type) &&
> > +		if (!type_is_pkt_pointer(type) && type != PTR_TO_MAP_VALUE &&
> >  		    type != expected_type)
> >  			goto err_type;
> >  	} else if (arg_type == ARG_CONST_SIZE ||
> > @@ -1970,6 +1970,9 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
> >  			err = check_packet_access(env, regno, reg->off,
> >  						  meta->map_ptr->key_size,
> >  						  false);
> > +		else if (type == PTR_TO_MAP_VALUE)
> > +			err = check_map_access(env, regno, reg->off,
> > +					       meta->map_ptr->key_size, false);
> >  		else
> >  			err = check_stack_boundary(env, regno,
> >  						   meta->map_ptr->key_size,
> 
> We should reuse check_helper_mem_access() here which covers all three cases
> from above already and simplifies the code a bit.

Thanks for the review.
I've sent a refactored patchset that uses check_helper_mem_access().

> 
> > @@ -1987,6 +1990,10 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
> >  			err = check_packet_access(env, regno, reg->off,
> >  						  meta->map_ptr->value_size,
> >  						  false);
> > +		else if (type == PTR_TO_MAP_VALUE)
> > +			err = check_map_access(env, regno, reg->off,
> > +					       meta->map_ptr->value_size,
> > +					       false);
> >  		else
> >  			err = check_stack_boundary(env, regno,
> >  						   meta->map_ptr->value_size,
> > 
> 
> Ditto.
> 
> Thanks,
> Daniel