netdev - Re: [PATCH net-next] bpf: fix loading of BPF

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Date:   Wed, 7 Dec 2016 12:42:22 +0300
From:   Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
To:     Daniel Borkmann <daniel@...earbox.net>, davem@...emloft.net
Cc:     alexei.starovoitov@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] bpf: fix loading of BPF_MAXINSNS sized programs

Hello!

On 12/7/2016 3:15 AM, Daniel Borkmann wrote:

> General assumption is that single program can hold up to BPF_MAXINSNS,
> that is, 4096 number of instructions. It is the case with cBPF and

    Up to BPF_MAXINSNS (that is 4096) instructions.

> that limit was carried over to eBPF. When recently testing digest, I
> noticed that it's actually not possible to feed 4096 instructions
> via bpf(2).
>
> The check for > BPF_MAXINSNS was added back then to bpf_check() in
> cbd357008604 ("bpf: verifier (add ability to receive verification log)").
> However, 09756af46893 ("bpf: expand BPF syscall with program load/unload")
> added yet another check that comes before that into bpf_prog_load(),
> but this time bails out already in case of >= BPF_MAXINSNS.
>
> Fix it up and perform the check early in bpf_prog_load(), so we can drop
> the second one in bpf_check(). It makes sense, because also a 0 insn
> program is useless and we don't want to waste any resources doing work
> up to bpf_check() point. The existing bpf(2) man page documents E2BIG
> as the official error for such cases, so just stick with it as well.
>
> Fixes: 09756af46893 ("bpf: expand BPF syscall with program load/unload")
> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> Acked-by: Alexei Starovoitov <ast@...nel.org>
[...]

MBR, Sergei