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

[Puranjay Mohan <puranjay@kernel.org>]

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Xu Kuohai <xukuohai@huaweicloud.com> writes:

> From: Xu Kuohai <xukuohai@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.
>
> Fixes: efc9909fdce0 ("bpf, arm64: Add bpf trampoline for arm64")
> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>

Tested-by: Puranjay Mohan <puranjay@kernel.org>

Thanks,
Puranjay

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