Re: [PATCH 09/19] bpf, x64: Allow to use caller address from stack

Yonghong Song
2021-06-07T03:08:59Z

On 6/5/21 4:10 AM, Jiri Olsa wrote:
> Currently we call the original function by using the absolute address
> given at the JIT generation. That's not usable when having trampoline
> attached to multiple functions. In this case we need to take the
> return address from the stack.

Here, it is mentioned to take the return address from the stack.

>
> Adding support to retrieve the original function address from the stack

Here, it is said to take original funciton address from the stack.

> by adding new BPF_TRAMP_F_ORIG_STACK flag for arch_prepare_bpf_trampoline
> function.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> arch/x86/net/bpf_jit_comp.c | 13 +++++++++----
> include/linux/bpf.h | 5 +++++
> 2 files changed, 14 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index 2a2e290fa5d8..b77e6bd78354 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -2013,10 +2013,15 @@ int arch_prepare_bpf_trampoline(struct bpf_tramp_image *im, void *image, void *i
> if (flags & BPF_TRAMP_F_CALL_ORIG) {
> restore_regs(m, &prog, nr_args, stack_size);
>
> - /* call original function */
> - if (emit_call(&prog, orig_call, prog)) {
> - ret = -EINVAL;
> - goto cleanup;
> + if (flags & BPF_TRAMP_F_ORIG_STACK) {
> + emit_ldx(&prog, BPF_DW, BPF_REG_0, BPF_REG_FP, 8);

This is load double from base_pointer + 8 which should be func return
address for x86, yet we try to call it.
I guess I must have missed something
here. Could you give some explanation?

> + EMIT2(0xff, 0xd0); /* call *rax */
> + } else {
> + /* call original function */
> + if (emit_call(&prog, orig_call, prog)) {
> + ret = -EINVAL;
> + goto cleanup;
> + }
> }
> /* remember return value in a stack for bpf prog to access */
> emit_stx(&prog, BPF_DW, BPF_REG_FP, BPF_REG_0, -8);
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 86dec5001ae2..16fc600503fb 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -554,6 +554,11 @@ struct btf_func_model {
> */
> #define BPF_TRAMP_F_SKIP_FRAME BIT(2)
>
> +/* Get original function from stack instead of from provided direct address.
> + * Makes sense for fexit programs only.
> + */
> +#define BPF_TRAMP_F_ORIG_STACK BIT(3)
> +
> /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
> * bytes on x86. Pick a number to fit into BPF_IMAGE_SIZE / 2
> */
>