linux-kernel - Re: [PATCH RESEND bpf 1/2] x86/bpf: Fix IP after emitting call depth accounting

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Date: Fri, 29 Mar 2024 14:26:38 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: X86 ML <x86@...nel.org>, bpf <bpf@...r.kernel.org>, 
	Network Development <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Joan Bruguera Micó <joanbrugueram@...il.com>, 
	Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH RESEND bpf 1/2] x86/bpf: Fix IP after emitting call depth accounting

On Fri, Mar 29, 2024 at 2:49 AM Uros Bizjak <ubizjak@...il.com> wrote:
>
> From: Joan Bruguera Micó <joanbrugueram@...il.com>
>
> Adjust the IP passed to `emit_patch` so it calculates the correct offset
> for the CALL instruction if `x86_call_depth_emit_accounting` emits code.
> Otherwise we will skip some instructions and most likely crash.
>
> Fixes: b2e9dfe54be4 ("x86/bpf: Emit call depth accounting if required")
> Link: https://lore.kernel.org/lkml/20230105214922.250473-1-joanbrugueram@gmail.com/
> Signed-off-by: Joan Bruguera Micó <joanbrugueram@...il.com>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> ---
>  arch/x86/net/bpf_jit_comp.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index a7ba8e178645..09f7dc9d4d65 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -479,9 +479,10 @@ static int emit_call(u8 **pprog, void *func, void *ip)
>
>  static int emit_rsb_call(u8 **pprog, void *func, void *ip)
>  {
> +       void *adjusted_ip;
>         OPTIMIZER_HIDE_VAR(func);
> -       x86_call_depth_emit_accounting(pprog, func);
> -       return emit_patch(pprog, func, ip, 0xE8);
> +       adjusted_ip = ip + x86_call_depth_emit_accounting(pprog, func);

Why not just
ip += x86_call_depth_emit_accounting(pprog, func);

?

> +       return emit_patch(pprog, func, adjusted_ip, 0xE8);
>  }
>
>  static int emit_jump(u8 **pprog, void *func, void *ip)
> --
> 2.44.0
>