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@xxxxxxxxxx>
> ---
> 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().
