linux-kernel - Re: [PATCH bpf-next v3 1/2] bpf: make the verifier tracks the "not equal" for regs

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Message-ID: <CAADnVQ+kKxj2hg33CzH_iXdH5fs8wjwpkPP-Jjh41weqf9BEwA@mail.gmail.com>
Date:   Thu, 14 Dec 2023 05:49:24 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Menglong Dong <menglong8.dong@...il.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, Eddy Z <eddyz87@...il.com>,
        Yonghong Song <yonghong.song@...ux.dev>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next v3 1/2] bpf: make the verifier tracks the "not
 equal" for regs

On Wed, Dec 13, 2023 at 10:28 PM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> We can derive some new information for BPF_JNE in regs_refine_cond_op().
> Take following code for example:
>
>   /* The type of "a" is u16 */
>   if (a > 0 && a < 100) {
>     /* the range of the register for a is [0, 99], not [1, 99],
>      * and will cause the following error:
>      *
>      *   invalid zero-sized read
>      *
>      * as a can be 0.
>      */
>     bpf_skb_store_bytes(skb, xx, xx, a, 0);
>   }

Please craft a selftest from above with inline asm
(C might not work as compiler might optimize it)

Also we call:
        /* fallthrough (FALSE) branch */
        regs_refine_cond_op(false_reg1, false_reg2,
rev_opcode(opcode), is_jmp32);
        /* jump (TRUE) branch */
        regs_refine_cond_op(true_reg1, true_reg2, opcode, is_jmp32);

so despite BPF_JNE is not handled explicitly it still should have
caught above due to rev_opcode() ?