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

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Thu, Jun 16, 2022 at 04:45:09PM +0200, Daniel Borkmann wrote:
> 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@cloudflare.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!

Indeed!

Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>

I was wondering if it would be possible to work only on rounded up to 8
stack depth from JIT POV since it's what we do everywhere we use it...