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

From: Yonghong Song <[email-masked]>
Date: Sun, 6 Jun 2021 20:07:44 -0700

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 <[email-masked]>
> ---
>   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
>    */
>