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

[Alexei Starovoitov]

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.