linux-kernel - Re: [patch V3 01/32] mm/slab: Fix broken stack trace storage

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Message-ID: <alpine.DEB.2.21.1904141832400.4917@nanos.tec.linutronix.de>
Date:   Sun, 14 Apr 2019 18:34:14 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        Linux-MM <linux-mm@...ck.org>
Subject: Re: [patch V3 01/32] mm/slab: Fix broken stack trace storage

On Sun, 14 Apr 2019, Andy Lutomirski wrote:
> > +               struct stack_trace trace = {
> > +                       .max_entries    = size - 4;
> > +                       .entries        = addr;
> > +                       .skip           = 3;
> > +               };
> 
> This looks correct, but I think that it would have been clearer if you
> left the size -= 3 above.  You're still incrementing addr, but you're
> not decrementing size, so they're out of sync and the resulting code
> is hard to follow.

What about the below?

--- a/mm/slab.c
+++ b/mm/slab.c
@@ -1480,10 +1480,12 @@ static void store_stackinfo(struct kmem_
 	*addr++ = 0x12345678;
 	*addr++ = caller;
 	*addr++ = smp_processor_id();
+	size -= 3;
 #ifdef CONFIG_STACKTRACE
 	{
 		struct stack_trace trace = {
-			.max_entries	= size - 4;
+			/* Leave one for the end marker below */
+			.max_entries	= size - 1;
 			.entries	= addr;
 			.skip		= 3;
 		};