linux-kernel - Re: [PATCH] MIPS: Fix incorrect NULL check in local_flush_tlb

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Date:	Fri, 4 Jul 2014 23:52:51 +0200
From:	Jonas Gorski <jogo@...nwrt.org>
To:	Emil Goode <emilgoode@...il.com>
Cc:	Ralf Baechle <ralf@...ux-mips.org>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	John Crispin <blogic@...nwrt.org>,
	MIPS Mailing List <linux-mips@...ux-mips.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] MIPS: Fix incorrect NULL check in local_flush_tlb_page()

On Fri, Jul 4, 2014 at 7:07 PM, Emil Goode <emilgoode@...il.com> wrote:
> We check that the struct vm_area_struct pointer vma is NULL and
> then dereference it. The intent must have been to check that
> vma is not NULL before we dereference it in the next condition.

Actually if it is NULL, then it will short-cut and won't dereference
it (because !vma is true it can never become false again), so the
condition would be fine previously.

But, looking at the code a few lines into branch:

        if (!vma || cpu_context(cpu, vma->vm_mm) != 0) {
                unsigned long flags;
                int oldpid, newpid, idx;

#ifdef DEBUG_TLB
                printk("[tlbpage<%lu,0x%08lx>]", cpu_context(cpu,
vma->vm_mm), page);
#endif
                newpid = cpu_context(cpu, vma->vm_mm) & ASID_MASK;

it will be then dereferenced here, so the change is actually sensible,
even if the description isn't quite spot-on where it breaks.


Jonas
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