Re: [PATCH 6/9] x86/bpf: Fix JIT frame pointer usage

[Alexei Starovoitov]

On Thu, Jun 13, 2019 at 08:22:48PM -0500, Josh Poimboeuf wrote:
> On Thu, Jun 13, 2019 at 02:58:09PM -0700, Alexei Starovoitov wrote:
> > On Thu, Jun 13, 2019 at 08:21:03AM -0500, Josh Poimboeuf wrote:
> > > The BPF JIT code clobbers RBP.  This breaks frame pointer convention and
> > > thus prevents the FP unwinder from unwinding through JIT generated code.
> > > 
> > > RBP is currently used as the BPF stack frame pointer register.  The
> > > actual register used is opaque to the user, as long as it's a
> > > callee-saved register.  Change it to use R12 instead.
> > > 
> > > Fixes: d15d356887e7 ("perf/x86: Make perf callchains work without CONFIG_FRAME_POINTER")
> > > Reported-by: Song Liu <songliubraving@xxxxxx>
> > > Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> > > ---
> > >  arch/x86/net/bpf_jit_comp.c | 43 +++++++++++++++++++++----------------
> > >  1 file changed, 25 insertions(+), 18 deletions(-)
> > > 
> > > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > > index e649f977f8e1..bb1968fea50a 100644
> > > --- a/arch/x86/net/bpf_jit_comp.c
> > > +++ b/arch/x86/net/bpf_jit_comp.c
> > > @@ -100,9 +100,8 @@ static int bpf_size_to_x86_bytes(int bpf_size)
> > >  /*
> > >   * The following table maps BPF registers to x86-64 registers.
> > >   *
> > > - * x86-64 register R12 is unused, since if used as base address
> > > - * register in load/store instructions, it always needs an
> > > - * extra byte of encoding and is callee saved.
> > > + * RBP isn't used; it needs to be preserved to allow the unwinder to move
> > > + * through generated code stacks.
> > 
> > Extra register save/restore is kinda annoying just to fix ORC.
> 
> It's not just for the ORC unwinder.  It also fixes the frame pointer
> unwinder (see above commit msg).  And it's standard frame pointer
> practice to not clobber RBP.

not true.
generated JITed code has no issues with regular stack unwinder.
it breaks down under ORC only.

> > Also every stack access from bpf prog will be encoded via r12 and consume
> > extra byte of encoding. I really don't like this approach.
> 
> Do you have another callee-saved register you'd prefer to use as the
> stack pointer?

RBP must be used.

> > Can you teach ORC to understand JIT-ed frames instead?
> 
> We could, but it would add a lot more complexity than this.  And anyway,
> the frame pointer unwinder would still be broken.

I disagree. See above. Only ORC is broken. Hence ORC should be fixed.