netdev - Re: [PATCH bpf-next 1/2] bpf, x86: Fix tail call count offset calculation on bpf2bpf call

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Message-ID: <c19ed052-90ea-3bf5-c57c-7879844579ea@iogearbox.net>
Date:   Thu, 16 Jun 2022 16:45:09 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jakub Sitnicki <jakub@...udflare.com>, bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>,
        kernel-team@...udflare.com
Subject: Re: [PATCH bpf-next 1/2] bpf, x86: Fix tail call count offset
 calculation on bpf2bpf call

On 6/15/22 5:17 PM, Jakub Sitnicki wrote:
[...]
> int entry(struct __sk_buff * skb):
>     0xffffffffa0201788:  nop    DWORD PTR [rax+rax*1+0x0]
>     0xffffffffa020178d:  xor    eax,eax
>     0xffffffffa020178f:  push   rbp
>     0xffffffffa0201790:  mov    rbp,rsp
>     0xffffffffa0201793:  sub    rsp,0x8
>     0xffffffffa020179a:  push   rax
>     0xffffffffa020179b:  xor    esi,esi
>     0xffffffffa020179d:  mov    BYTE PTR [rbp-0x1],sil
>     0xffffffffa02017a1:  mov    rax,QWORD PTR [rbp-0x9]	!!! tail call count
>     0xffffffffa02017a8:  call   0xffffffffa02017d8       !!! is at rbp-0x10
>     0xffffffffa02017ad:  leave
>     0xffffffffa02017ae:  ret
> 
> Fix it by rounding up the BPF stack depth to a multiple of 8, when
> calculating the tail call count offset on stack.
> 
> Fixes: ebf7d1f508a7 ("bpf, x64: rework pro/epilogue and tailcall handling in JIT")
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.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 f298b18a9a3d..c98b8c0ed3b8 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -1420,8 +1420,9 @@ st:			if (is_imm8(insn->off))
>   		case BPF_JMP | BPF_CALL:
>   			func = (u8 *) __bpf_call_base + imm32;
>   			if (tail_call_reachable) {
> +				/* mov rax, qword ptr [rbp - rounded_stack_depth - 8] */
>   				EMIT3_off32(0x48, 0x8B, 0x85,
> -					    -(bpf_prog->aux->stack_depth + 8));
> +					    -round_up(bpf_prog->aux->stack_depth, 8) - 8);

Lgtm, great catch by the way!