Re: [PATCH bpf-next v2 1/1] bpf, arm64: support exceptions

[Puranjay Mohan]

Hi Mark,

On Thu, Nov 2, 2023 at 5:59 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
>
> On Sun, Sep 17, 2023 at 12:00:45AM +0000, Puranjay Mohan wrote:
> > Implement arch_bpf_stack_walk() for the ARM64 JIT. This will be used
> > by bpf_throw() to unwind till the program marked as exception boundary and
> > run the callback with the stack of the main program.
> >
> > The prologue generation code has been modified to make the callback
> > program use the stack of the program marked as exception boundary where
> > callee-saved registers are already pushed.
> >
> > As the bpf_throw function never returns, if it clobbers any callee-saved
> > registers, they would remain clobbered. So, the prologue of the
> > exception-boundary program is modified to push R23 and R24 as well,
> > which the callback will then recover in its epilogue.
> >
> > The Procedure Call Standard for the Arm 64-bit Architecture[1] states
> > that registers r19 to r28 should be saved by the callee. BPF programs on
> > ARM64 already save all callee-saved registers except r23 and r24. This
> > patch adds an instruction in prologue of the  program to save these
> > two registers and another instruction in the epilogue to recover them.
> >
> > These extra instructions are only added if bpf_throw() used. Otherwise
> > the emitted prologue/epilogue remains unchanged.
> >
> > [1] https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst
> >
> > Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> > ---
>
> [...]
>
> > +void arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 sp, u64 bp), void *cookie)
> > +{
> > +     struct stack_info stacks[] = {
> > +             stackinfo_get_task(current),
> > +     };
>
> Can bpf_throw() only be used by BPF programs that run in task context, or is it
> possible e.g. for those to run within an IRQ handler (or otherwise on the IRQ
> stack)?

I will get back on this with more information.

>
> > +
> > +     struct unwind_state state = {
> > +             .stacks = stacks,
> > +             .nr_stacks = ARRAY_SIZE(stacks),
> > +     };
> > +     unwind_init_common(&state, current);
> > +     state.fp = (unsigned long)__builtin_frame_address(1);
> > +     state.pc = (unsigned long)__builtin_return_address(0);
> > +
> > +     if (unwind_next_frame_record(&state))
> > +             return;
> > +     while (1) {
> > +             /* We only use the fp in the exception callback. Pass 0 for sp as it's unavailable*/
> > +             if (!consume_fn(cookie, (u64)state.pc, 0, (u64)state.fp))
> > +                     break;
> > +             if (unwind_next_frame_record(&state))
> > +                     break;
> > +     }
> > +}
>
> IIUC you're not using arch_stack_walk() because you need the FP in addition to
> the PC.

Yes,

>
> Is there any other reason you need to open-code this?

No,

>
> If not, I'd rather rework the common unwinder so that it's possible to get at
> the FP. I had patches for that a while back:
>
>   https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64/stacktrace/metadata
>
> ... and I'm happy to rebase that and pull out the minimum necessary to make
> that possible.

It would be great if you can rebase and push the code, I can rebase
this on your work and
not open code this implementation.

>
> Mark.
>

Thanks,
Puranjay