[PATCH bpf] bpf, arm64: Fix stack frame construction for struct_ops trampoline

[Puranjay Mohan]

On Wed, Oct 23, 2024 at 12:50 AM Alexei Starovoitov
<alexei.starovoitov at gmail.com> wrote:
>
> On Sat, Oct 19, 2024 at 2:15 AM Xu Kuohai <xukuohai at huaweicloud.com> wrote:
> >
> > From: Xu Kuohai <xukuohai at huawei.com>
> >
> > The callsite layout for arm64 fentry is:
> >
> > mov x9, lr
> > nop
> >
> > When a bpf prog is attached, the nop instruction is patched to a call
> > to bpf trampoline:
> >
> > mov x9, lr
> > bl <bpf trampoline>
> >
> > This passes two return addresses to bpf trampoline: the return address
> > for the traced function/prog, stored in x9, and the return address for
> > the bpf trampoline, stored in lr. To ensure stacktrace works properly,
> > the bpf trampoline constructs two fake function stack frames using x9
> > and lr.
> >
> > However, struct_ops progs are used as function callbacks and are invoked
> > directly, without x9 being set as the fentry callsite does. Therefore,
> > only one stack frame should be constructed using lr for struct_ops.
>
> Are you saying that currently stack unwinder on arm64 is
> completely broken for struct_ops progs ?
> or it shows an extra frame that doesn't have to be shown ?
>
> If former then it's certainly a bpf tree material.
> If latter then bpf-next will do.
> Pls clarify.

It is not completely broken, only an extra garbage frame is shown
between the caller of the trampoline and its caller.

So, this can go from the bpf-next tree. But let's wait for Xu to
provide more information.

Thanks,
Puranjay