linux-kernel - Re: [RFC PATCH v1 11/16] s390/unwind_user/sframe: Enable HAVE_UNWIND_USER

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Message-ID: <20250801125350.9905B20-hca@linux.ibm.com>
Date: Fri, 1 Aug 2025 14:53:50 +0200
From: Heiko Carstens <hca@...ux.ibm.com>
To: Jens Remus <jremus@...ux.ibm.com>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        bpf@...r.kernel.org, x86@...nel.org,
        Steven Rostedt <rostedt@...nel.org>, Vasily Gorbik <gor@...ux.ibm.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        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>
Subject: Re: [RFC PATCH v1 11/16] s390/unwind_user/sframe: Enable
 HAVE_UNWIND_USER_SFRAME

On Thu, Jul 10, 2025 at 06:35:17PM +0200, Jens Remus wrote:
> Add s390 support for unwinding of user space using SFrame.  This
> leverages the previous commits to address the following s390
> particularities:
> 
> - The CFA is defined as the value of the stack pointer (SP) at call
>   site in the previous frame + 160.  Therefore the SP unwinds as
>   SP = CFA - 160.  Therefore use a SP value offset from CFA of -160.
> 
> - The return address (RA) is not saved on the stack at function entry.
>   It is also not saved in the function prologue, when in leaf functions.
>   Therefore the RA does not necessarily need to be unwound in the first
>   unwinding step for the topmost frame.
> 
> - The frame pointer (FP) and/or return address (RA) may be saved in
>   other registers when in leaf functions.  GCC effectively uses
>   floating-point registers (FPR) for this purpose.  Therefore DWARF
>   register numbers may be encoded in the SFrame FP/RA offsets.

...

> +static inline void __s390_get_dwarf_fpr(unsigned long *val, int regnum)
> +{
> +	switch (regnum) {
> +	case 16:
> +		fpu_std(0, (freg_t *)val);
> +		break;

...

> +static inline int s390_unwind_user_get_reg(unsigned long *val, int regnum)
> +{
> +	if (0 <= regnum && regnum <= 15) {
> +		struct pt_regs *regs = task_pt_regs(current);
> +		*val = regs->gprs[regnum];
> +	} else if (16 <= regnum && regnum <= 31) {
> +		__s390_get_dwarf_fpr(val, regnum);

This won't work with other potential in-kernel fpu users. User space fpr
contents may have been written to the current task's fpu save area and fprs
may have been clobbered by in-kernel users; so you need to get register
contents from the correct location. See arch/s390/include/asm/fpu.h.