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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Wed, 3 Feb 2021 09:37:37 -0800
Message-ID: <CAADnVQL4S_XbyNEFrX9+6ew_6wyMZfQXW8t7pHu1eLdY0mgtJQ@mail.gmail.com>
Subject: Re: [PATCH bpf-next v3] bpf: Propagate stack bounds to registers in
 atomics w/ BPF_FETCH
To:     Yonghong Song <yhs@fb.com>
Cc:     Brendan Jackman <jackmanb@google.com>, bpf <bpf@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii.nakryiko@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        Florent Revest <revest@chromium.org>,
        John Fastabend <john.fastabend@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
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On Wed, Feb 3, 2021 at 9:07 AM Yonghong Song <yhs@fb.com> wrote:
>
>
>
> On 2/2/21 5:50 AM, Brendan Jackman wrote:
> > When BPF_FETCH is set, atomic instructions load a value from memory
> > into a register. The current verifier code first checks via
> > check_mem_access whether we can access the memory, and then checks
> > via check_reg_arg whether we can write into the register.
> >
> > For loads, check_reg_arg has the side-effect of marking the
> > register's value as unkonwn, and check_mem_access has the side effect
> > of propagating bounds from memory to the register. This currently only
> > takes effect for stack memory.
> >
> > Therefore with the current order, bounds information is thrown away,
> > but by simply reversing the order of check_reg_arg
> > vs. check_mem_access, we can instead propagate bounds smartly.
> >
> > A simple test is added with an infinite loop that can only be proved
> > unreachable if this propagation is present. This is implemented both
> > with C and directly in test_verifier using assembly.
> >
> > Suggested-by: John Fastabend <john.fastabend@gmail.com>
> > Signed-off-by: Brendan Jackman <jackmanb@google.com>
>
> Ack with a nit below.

Sorry it was already applied yesterday.
patchbot just didn't send auto-reply.