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

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Message-ID: <YZgbS0GUpEzAeGNu@krava>
Date:   Fri, 19 Nov 2021 22:46:51 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.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 08:14:09PM -0800, Alexei Starovoitov wrote:
> 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().
> 

ok, will check

thanks,
jirka