Re: FAILED: patch "[PATCH] bpf, x86: Fix encoding for lower 8-bit registers in BPF_STX" failed to apply to 5.4-stable tree

[Sasha Levin <sashal@kernel.org>]

On Thu, Apr 30, 2020 at 05:06:18PM +0200, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 5.4-stable tree.
>If someone wants it applied there, or to any other stable or longterm
>tree, then please email the backport, including the original git commit
>id to <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From aee194b14dd2b2bde6252b3acf57d36dccfc743a Mon Sep 17 00:00:00 2001
>From: Luke Nelson <lukenels@cs.washington.edu>
>Date: Sat, 18 Apr 2020 16:26:53 -0700
>Subject: [PATCH] bpf, x86: Fix encoding for lower 8-bit registers in BPF_STX
> BPF_B
>
>This patch fixes an encoding bug in emit_stx for BPF_B when the source
>register is BPF_REG_FP.
>
>The current implementation for BPF_STX BPF_B in emit_stx saves one REX
>byte when the operands can be encoded using Mod-R/M alone. The lower 8
>bits of registers %rax, %rbx, %rcx, and %rdx can be accessed without using
>a REX prefix via %al, %bl, %cl, and %dl, respectively. Other registers,
>(e.g., %rsi, %rdi, %rbp, %rsp) require a REX prefix to use their 8-bit
>equivalents (%sil, %dil, %bpl, %spl).
>
>The current code checks if the source for BPF_STX BPF_B is BPF_REG_1
>or BPF_REG_2 (which map to %rdi and %rsi), in which case it emits the
>required REX prefix. However, it misses the case when the source is
>BPF_REG_FP (mapped to %rbp).
>
>The result is that BPF_STX BPF_B with BPF_REG_FP as the source operand
>will read from register %ch instead of the correct %bpl. This patch fixes
>the problem by fixing and refactoring the check on which registers need
>the extra REX byte. Since no BPF registers map to %rsp, there is no need
>to handle %spl.
>
>Fixes: 622582786c9e0 ("net: filter: x86: internal BPF JIT")
>Signed-off-by: Xi Wang <xi.wang@gmail.com>
>Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>
>Signed-off-by: Alexei Starovoitov <ast@kernel.org>
>Link: https://lore.kernel.org/bpf/20200418232655.23870-1-luke.r.nels@gmail.com

The code got shuffled around in 3b2744e66520 ("bpf: Refactor x86 JIT
into helpers"). I've fixed it and queued for all branches.

-- 
Thanks,
Sasha