netdev - Re: [PATCH bpf] bpf: use NOP_ATOMIC5 instead of emit_nops(&prog, 5) for BPF_TRAMP_F_CALL

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Date:   Fri, 19 Mar 2021 17:14:02 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH bpf] bpf: use NOP_ATOMIC5 instead of emit_nops(&prog, 5)
 for BPF_TRAMP_F_CALL_ORIG

On Fri, Mar 19, 2021 at 5:00 PM Stanislav Fomichev <sdf@...gle.com> wrote:
>
> __bpf_arch_text_poke does rewrite only for atomic nop5, emit_nops(xxx, 5)
> emits non-atomic one which breaks fentry/fexit with k8 atomics:
>
> P6_NOP5 == P6_NOP5_ATOMIC (0f1f440000 == 0f1f440000)
> K8_NOP5 != K8_NOP5_ATOMIC (6666906690 != 6666666690)
>
> Can be reproduced by doing "ideal_nops = k8_nops" in "arch_init_ideal_nops()
> and running fexit_bpf2bpf selftest.
>
> Fixes: e21aa341785c ("bpf: Fix fexit trampoline.")
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>
> ---
>  arch/x86/net/bpf_jit_comp.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 72b5a57e9e31..b35fc8023884 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -2012,7 +2012,8 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
>                 /* remember return value in a stack for bpf prog to access */
>                 emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
>                 im->ip_after_call = prog;
> -               emit_nops(&prog, 5);
> +               memcpy(prog, ideal_nops[NOP_ATOMIC5], X86_PATCH_SIZE);
> +               prog += X86_PATCH_SIZE;

I'm well aware, but ideal_nops are pretty much gone already.
The changes are already in the -tip tree.
So I decided to reduce the conflicts for the merge window.

Do you actually see the breakage or it's purely theoretical?