netdev - Re: [RFC PATCH bpf-next 1/2] bpf: add bpf

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Message-ID: <20211106192602.knmfk2x7ogcjuzvw@ast-mbp.dhcp.thefacebook.com>
Date:   Sat, 6 Nov 2021 12:26:02 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Hou Tao <houtao1@...wei.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        Song Liu <songliubraving@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [RFC PATCH bpf-next 1/2] bpf: add bpf_strncmp helper

On Sat, Nov 06, 2021 at 09:28:21PM +0800, Hou Tao wrote:
> The helper compares two strings: one string is a null-terminated
> read-only string, and another one has const max storage size. And
> it can be used to compare file name in tracing or LSM program.
> 
> We don't check whether or not s2 in bpf_strncmp() is null-terminated,
> because its content may be changed by malicous program, and we only
> ensure the memory accessed is bounded by s2_sz.

I think "malicous" adjective is unnecessary and misleading.
It's also misspelled.
Just mention that 2nd argument doesn't have to be null terminated.

> + * long bpf_strncmp(const char *s1, const char *s2, u32 s2_sz)
...
> +BPF_CALL_3(bpf_strncmp, const char *, s1, const char *, s2, size_t, s2_sz)

probably should match u32 instead of size_t.

> @@ -1210,6 +1210,8 @@ bpf_tracing_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
>  		return &bpf_get_branch_snapshot_proto;
>  	case BPF_FUNC_trace_vprintk:
>  		return bpf_get_trace_vprintk_proto();
> +	case BPF_FUNC_strncmp:
> +		return &bpf_strncmp_proto;

why tracing only?
Should probably be in bpf_base_func_proto.

I was thinking whether the proto could be:
long bpf_strncmp(const char *s1, u32 s1_sz, const char *s2)
but I think your version is better though having const string as 1st arg
is a bit odd in normal C.

Would it make sense to add bpf_memchr as well while we are at it?
And
static inline bpf_strnlen(const char *s, u32 sz)
{
  return bpf_memchr(s, sz, 0);
}
to bpf_helpers.h ?