netdev - Re: [PATCH bpf-next 07/29] bpf, x64: Allow to use caller address from stack

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Date:   Thu, 18 Nov 2021 20:14:09 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>
Subject: Re: [PATCH bpf-next 07/29] bpf, x64: Allow to use caller address
 from stack

On Thu, Nov 18, 2021 at 12:24:33PM +0100, 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.
> 
> Adding support to retrieve the original function address from the stack
> by adding new BPF_TRAMP_F_ORIG_STACK flag for arch_prepare_bpf_trampoline
> function.
> 
> Basically we take the return address of the 'fentry' call:
> 
>    function + 0: call fentry    # stores 'function + 5' address on stack
>    function + 5: ...
> 
> The 'function + 5' address will be used as the address for the
> original function to call.
> 
> Signed-off-by: Jiri Olsa <jolsa@...nel.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 67e8ac9aaf0d..d87001073033 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -2035,10 +2035,15 @@ 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);
> +			EMIT2(0xff, 0xd0); /* call *rax */

Either return an eror if repoline is on
or use emit_indirect_jump().