netdev - Re: [PATCH bpf-next 3/4] bpf: Fix net.core.bpf_jit

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Date:   Wed, 9 Mar 2022 19:29:03 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Hou Tao <houtao1@...wei.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        Yonghong Song <yhs@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        KP Singh <kpsingh@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next 3/4] bpf: Fix net.core.bpf_jit_harden race

On Wed, Mar 9, 2022 at 5:01 PM Hou Tao <houtao1@...wei.com> wrote:
>
> Hi,
>
> On 3/10/2022 7:22 AM, Alexei Starovoitov wrote:
> > On Wed, Mar 09, 2022 at 08:33:20PM +0800, Hou Tao wrote:
> >> It is the bpf_jit_harden counterpart to commit 60b58afc96c9 ("bpf: fix
> >> net.core.bpf_jit_enable race"). bpf_jit_harden will be tested twice
> >> for each subprog if there are subprogs in bpf program and constant
> >> blinding may increase the length of program, so when running
> >> "./test_progs -t subprogs" and toggling bpf_jit_harden between 0 and 2,
> >> jit_subprogs may fail because constant blinding increases the length
> >> of subprog instructions during extra passs.
> >>
> >> So cache the value of bpf_jit_blinding_enabled() during program
> >> allocation, and use the cached value during constant blinding, subprog
> >> JITing and args tracking of tail call.
> > Looks like this patch alone is enough.
> > With race fixed. Patches 1 and 2 are no longer necessary, right?
> Yes and no. With patch 3 applied, the problems described in patch 1 and patch 2
> are gone, but it may recur due to other issue in JIT. So I post these two patch
> together and hope these fixes can also be merged.

What kind of 'issues in JIT'?
I'd rather fix them than do defensive programming.
patch 2 is a hack that should not happen in a correct JIT.