linux-kernel - Re: [PATCH] tracing: Fix the bug where bpf_get

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Message-ID: <aNTmMGC3iYoAlfUU@krava>
Date: Thu, 25 Sep 2025 08:50:24 +0200
From: Jiri Olsa <olsajiri@...il.com>
To: Feng Yang <yangfeng59949@....com>
Cc: rostedt@...dmis.org, mhiramat@...nel.org, mark.rutland@....com,
	catalin.marinas@....com, will@...nel.org, revest@...omium.org,
	alexei.starovoitov@...il.com, olsajiri@...il.com, andrii@...nel.org,
	ast@...nel.org, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, bpf@...r.kernel.org
Subject: Re: [PATCH] tracing: Fix the bug where bpf_get_stackid returns
 -EFAULT on the ARM64

On Thu, Sep 25, 2025 at 10:08:22AM +0800, Feng Yang wrote:
> From: Feng Yang <yangfeng@...inos.cn>
> 
> When using bpf_program__attach_kprobe_multi_opts on ARM64 to hook a BPF program
> that contains the bpf_get_stackid function, the BPF program fails
> to obtain the stack trace and returns -EFAULT.
> 
> This is because ftrace_partial_regs omits the configuration of the pstate register,
> leaving pstate at the default value of 0. When get_perf_callchain executes,
> it uses user_mode(regs) to determine whether it is in kernel mode.
> This leads to a misjudgment that the code is in user mode,
> so perf_callchain_kernel is not executed and the function returns directly.
> As a result, trace->nr becomes 0, and finally -EFAULT is returned.
> 
> Therefore, the assignment of the pstate register is added here.
> 
> Fixes: b9b55c8912ce ("tracing: Add ftrace_partial_regs() for converting ftrace_regs to pt_regs")
> Closes: https://lore.kernel.org/bpf/20250919071902.554223-1-yangfeng59949@163.com/
> Signed-off-by: Feng Yang <yangfeng@...inos.cn>

Tested-by: Jiri Olsa <jolsa@...nel.org>

thanks,
jirka

> ---
>  arch/arm64/include/asm/ftrace.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> index bfe3ce9df197..ba7cf7fec5e9 100644
> --- a/arch/arm64/include/asm/ftrace.h
> +++ b/arch/arm64/include/asm/ftrace.h
> @@ -153,6 +153,7 @@ ftrace_partial_regs(const struct ftrace_regs *fregs, struct pt_regs *regs)
>  	regs->pc = afregs->pc;
>  	regs->regs[29] = afregs->fp;
>  	regs->regs[30] = afregs->lr;
> +	regs->pstate = PSR_MODE_EL1h;
>  	return regs;
>  }
>  
> -- 
> 2.25.1
>