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

Jiri Olsa
2021-06-07T18:13:18Z

On Sun, Jun 06, 2021 at 08:07:44PM -0700, Yonghong Song wrote:
>
>
> 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.

sorry if the description is confusing as always, the idea
is to take the function's return address from fentry call:

function
call fentry
xxxx <---- this address

and use it to call the original function body before fexit handler

jirka

>
> > 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
> > */
> >
>