linux-kernel - Re: [PATCH] x86/show_trace_log_lvl: ensure stack pointer is aligned, again

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Date:   Thu, 11 May 2023 19:20:59 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Josh Poimboeuf' <jpoimboe@...nel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vernon Lovejoy <vlovejoy@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/show_trace_log_lvl: ensure stack pointer is aligned,
 again

On Sun, Apr 30, 2023 at 11:59:17AM +0000, David Laight wrote:
> From: Josh Poimboeuf
> > Sent: 29 April 2023 00:58
> > 
> > On Fri, Apr 28, 2023 at 08:55:13AM +0200, Oleg Nesterov wrote:
> > > On 04/27, Josh Poimboeuf wrote:
> > > >
> > > > On Thu, Apr 27, 2023 at 04:00:54PM +0200, Oleg Nesterov wrote:
> > > > > +	stack = PTR_ALIGN(stack, sizeof(long));
> > > > >  	for ( ; stack; stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
> > > > >  		const char *stack_name;
> > > >
> > > > Seems reasonable, though 'stack' is already initialized a few lines
> > > > above this, so it would be cleaner to do the PTR_ALIGN then.  Or even
> > > > better, just move it all to the for loop:
> > > >
> > > > 	for (stack = PTR_ALIGN(stack ? : get_stack_pointer(task, regs));
> > > > 	     stack;
> > > > 	     stack = PTR_ALIGN(stack_info.next_sp, sizeof(long))) {
> > >
> > > We decided to make the simplest one-liner fix, but I was thinking about
> > >
> > > 	for ( stack = stack ? : get_stack_pointer(task, regs);
> > > 	     (stack = PTR_ALIGN(stack, sizeof(long)));
> > > 	      stack = stack_info.next_sp)
> > > 	{
> > > 		...
> > >
> > > to factout out the annoying PTR_ALIGN(). Will it work for you?
> > 
> > I'd rather not, that's a little *too* clever, IMO.
> 
> I'd leave the initialisation outside the loop and move
> the PTR_ALIGN() into the loop so that the 'for' fits on one line:
> 	if (!stack)
> 		stack = get_stack_pointer(task, regs);
> 	for (; stack; stack = stack_info.next_sp) {
> 		const char ...
> 		stack = PTR_ALIGN(stack, sizeof(long));

I do like that better, except... put the initialization in the 'for':

	for (stack = stack ? : get_stack_pointer(task, regs);
	     stack;
	     stack = stack_info.next_sp) {
		const char ...
		stack = PTR_ALIGN(stack, sizeof(long));

A multi-line 'for' is fine, it's better to put the initialization in the
conventional spot.

-- 
Josh