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From:   Puranjay Mohan <puranjay12@gmail.com>
Date:   Mon, 6 Nov 2023 10:04:09 +0100
Message-ID: <CANk7y0g8SOrSAY2jqZ22v6Duu9yhHY-d39g5gJ2vA2j2Y-v53Q@mail.gmail.com>
Subject: Re: [PATCH bpf-next v2 1/1] bpf, arm64: support exceptions
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>,
        Yonghong Song <yonghong.song@linux.dev>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Zi Shen Lim <zlim.lnx@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, bpf@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Kumar Kartikeya Dwivedi <memxor@gmail.com>
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Hi Mark,

On Thu, Nov 2, 2023 at 5:59=E2=80=AFPM Mark Rutland <mark.rutland@arm.com> =
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-save=
d
> > 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 o=
n
> > 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.rs=
t
> >
> > Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
> > ---
>
> [...]
>
> > +void arch_bpf_stack_walk(bool (*consume_fn)(void *cookie, u64 ip, u64 =
sp, u64 bp), void *cookie)
> > +{
> > +     struct stack_info stacks[] =3D {
> > +             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 =3D {
> > +             .stacks =3D stacks,
> > +             .nr_stacks =3D ARRAY_SIZE(stacks),
> > +     };
> > +     unwind_init_common(&state, current);
> > +     state.fp =3D (unsigned long)__builtin_frame_address(1);
> > +     state.pc =3D (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 f=
or 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 additi=
on 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 ge=
t 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=
=3Darm64/stacktrace/metadata
>
> ... and I'm happy to rebase that and pull out the minimum necessary to ma=
ke
> 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