linux-kernel - Re: [PATCH 6/9] s390/bug: Implement __WARN

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Message-ID: <20251209123540.GE3707837@noisy.programming.kicks-ass.net>
Date: Tue, 9 Dec 2025 13:35:40 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: Alexander Gordeev <agordeev@...ux.ibm.com>,
	Sven Schnelle <svens@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Christian Borntraeger <borntraeger@...ux.ibm.com>,
	Mark Rutland <mark.rutland@....com>, Arnd Bergmann <arnd@...db.de>,
	Jens Remus <jremus@...ux.ibm.com>,
	Stefan Schulze Frielinghaus <stefansf@...ux.ibm.com>,
	Juergen Christ <jchrist@...ux.ibm.com>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org
Subject: Re: [PATCH 6/9] s390/bug: Implement __WARN_printf()

On Tue, Dec 09, 2025 at 01:16:58PM +0100, Heiko Carstens wrote:

> +#define __WARN_print_arg(flags, format, arg...)				\
> +do {									\
> +	int __flags = (flags) | BUGFLAG_WARNING | BUGFLAG_ARGS;		\
> +									\
> +	__WARN_trap(__WARN_bug_entry(__flags, format), ## arg);		\
> +} while (0)

So on x86 I had to add:

	asm("");

after the __WARN_trap() call above, to inhibit tail call optimization,
because:

> +void *__warn_args(struct arch_va_list *args, struct pt_regs *regs)
> +{
> +	struct stack_frame *stack_frame;
> +
> +	/*
> +	 * Generate va_list from pt_regs. See ELF Application Binary Interface
> +	 * s390x Supplement documentation for details.
> +	 *
> +	 * - __overflow_arg_area needs to point to the parameter area, which
> +	 *   is right above the standard stack frame (160 bytes)
> +	 *
> +	 * - __reg_save_area needs to point to a register save area where
> +	 *   general registers (%r2 - %r6) can be found at offset 16. Which
> +	 *   means that the gprs save area of pt_regs can be used
> +	 *
> +	 * - __gpr must be set to one, since the first parameter has been
> +	 *   processed (pointer to bug_entry)
> +	 */
> +	stack_frame = (struct stack_frame *)regs->gprs[15];
> +	args->__overflow_arg_area = stack_frame + 1;
> +	args->__reg_save_area = regs->gprs;
> +	args->__gpr = 1;
> +	return args;
> +}

that would affect the stack layout here. You don't suffer this because
you have a link register like setup?