linux-kernel - Re: [PATCH v13 02/14] unwind_user: Add frame pointer support

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Message-ID: <70803af5-369c-4de2-af30-70d74f1e6256@linux.ibm.com>
Date: Thu, 10 Jul 2025 14:28:06 +0200
From: Jens Remus <jremus@...ux.ibm.com>
To: Steven Rostedt <rostedt@...nel.org>, linux-kernel@...r.kernel.org,
        linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org,
        x86@...nel.org
Cc: Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrii Nakryiko <andrii@...nel.org>,
        Indu Bhagat <indu.bhagat@...cle.com>,
        "Jose E. Marchesi" <jemarch@....org>,
        Beau Belgrave <beaub@...ux.microsoft.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Florian Weimer <fweimer@...hat.com>,
        Sam James <sam@...too.org>, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH v13 02/14] unwind_user: Add frame pointer support

On 09.07.2025 12:01, Jens Remus wrote:
> On 08.07.2025 03:22, Steven Rostedt wrote:
>> From: Josh Poimboeuf <jpoimboe@...nel.org>

>> diff --git a/kernel/unwind/user.c b/kernel/unwind/user.c

>>  static int unwind_user_next(struct unwind_user_state *state)
>>  {
>> -	/* no implementation yet */
>> +	struct unwind_user_frame *frame;
>> +	unsigned long cfa = 0, fp, ra = 0;
>> +	unsigned int shift;
>> +
>> +	if (state->done)
>> +		return -EINVAL;
>> +
>> +	if (fp_state(state))
>> +		frame = &fp_frame;
>> +	else
>> +		goto done;
>> +
>> +	if (frame->use_fp) {
>> +		if (state->fp < state->sp)

The initial check above is correct.  I got the logic wrong.  Sorry for
the fuss!  Do not change the check to what I came up with yesterday:

> 		if (state->fp <= state->sp)
> 

Below s390 particularity, that FP may be equal to FP in any frame,
is only allowed with the initial check.

> I meanwhile came to the conclusion that for architectures, such as s390,
> where SP at function entry == SP at call site, the FP may be equal to
> the SP.  At least for the brief period where the FP has been setup and
> stack allocation did not yet take place.  For most architectures this
> can probably only occur in the topmost frame.  For s390 the FP is setup
> after static stack allocation, so --fno-omit-frame-pointer would enforce
> FP==SP in any frame that does not perform dynamic stack allocation.
> 
>> +			goto done;
>> +		cfa = state->fp;
>> +	} else {
>> +		cfa = state->sp;
>> +	}

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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