netdev - Re: [PATCH bpf-next 1/2] bpf: track contents of read-only maps as scalars

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Message-ID: <20191008212945.GG27307@pc-66.home>
Date:   Tue, 8 Oct 2019 23:29:45 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        andrii.nakryiko@...il.com, kernel-team@...com
Subject: Re: [PATCH bpf-next 1/2] bpf: track contents of read-only maps as
 scalars

On Tue, Oct 08, 2019 at 12:45:47PM -0700, Andrii Nakryiko wrote:
> Maps that are read-only both from BPF program side and user space side
> have their contents constant, so verifier can track referenced values
> precisely and use that knowledge for dead code elimination, branch
> pruning, etc. This patch teaches BPF verifier how to do this.
> 
> Signed-off-by: Andrii Nakryiko <andriin@...com>
> ---
>  kernel/bpf/verifier.c | 58 +++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 56 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index ffc3e53f5300..1e4e4bd64ca5 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2739,6 +2739,42 @@ static void coerce_reg_to_size(struct bpf_reg_state *reg, int size)
>  	reg->smax_value = reg->umax_value;
>  }
>  
> +static bool bpf_map_is_rdonly(const struct bpf_map *map)
> +{
> +	return (map->map_flags & BPF_F_RDONLY_PROG) &&
> +	       ((map->map_flags & BPF_F_RDONLY) || map->frozen);

This is definitely buggy. Testing for 'map->map_flags & BPF_F_RDONLY'
to assume it's RO from user space side is not correct as it's just
related to the current fd, but not the map itself. So the second part
definitely /must/ only be: && map->frozen

Thanks,
Daniel