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

From: Jiri Olsa <[email-masked]>
Date: Mon, 7 Jun 2021 20:13:07 +0200

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 <[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
> >    */
> > 
>