Re: [PATCH RFC bpf-next 4/7] bpf,x86: adjust the "jmp" mode for bpf trampoline

Alexei Starovoitov
2025-11-14T18:22:25Z

On Fri, Nov 14, 2025 at 1:25 AM Menglong Dong <menglong8.dong@gmail.com> wrote:
>
> In the origin call case, if BPF_TRAMP_F_SKIP_FRAME is not set, it means
> that the trampoline is not called, but "jmp".
>
> Introduce the function bpf_trampoline_need_jmp() to check if the
> trampoline is in "jmp" mode.
>
> Do some adjustment on the "jmp" mode for the x86_64. The main adjustment
> that we make is for the stack parameter passing case, as the stack
> alignment logic changes in the "jmp" mode without the "rip". What's more,
> the location of the parameters on the stack also changes.
>
> Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
> ---
> arch/x86/net/bpf_jit_comp.c | 15 ++++++++++-----
> include/linux/bpf.h | 12 ++++++++++++
> 2 files changed, 22 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2d300ab37cdd..21ce2b8457ec 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -2830,7 +2830,7 @@ static int get_nr_used_regs(const struct btf_func_model *m)
> }
>
> static void save_args(const struct btf_func_model *m, u8 **prog,
> - int stack_size, bool for_call_origin)
> + int stack_size, bool for_call_origin, bool jmp)

I have an allergy to bool args.

Please pass flags and do
boll jmp_based_tramp = bpf_trampoline_uses_jmp(flags);

I think bpf_trampoline_uses_jmp() is more descriptive than
bpf_trampoline_need_jmp().

The actual math lgtm.