linux-kernel - Re: [PATCH] kexec: Make a pair of map/unmap reserved pages in error path

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Date:	Mon, 29 Feb 2016 19:06:46 +0800
From:	Minfei Huang <mhuang@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	ebiederm@...ssion.com, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, Minfei Huang <mnfhuang@...il.com>
Subject: Re: [PATCH] kexec: Make a pair of map/unmap reserved pages in error
 path

On 02/26/16 at 04:56pm, Andrew Morton wrote:
> On Thu, 25 Feb 2016 22:02:40 +0800 Minfei Huang <mhuang@...hat.com> wrote:
> 
> > From: Minfei Huang <mnfhuang@...il.com>
> > 
> > For some arch, kexec shall map the reserved pages, then use them, when
> > we try to start the kdump service.
> > 
> > kexec may return directly, without unmaping the reserved pages, if it
> > fails during starting service. To fix it, we make a pair of map/unmap
> > reserved pages both in generic path and error path.
> 
> This patch both refactors the code AND fixes the bug.  It is a
> decent-looking refactoring, but mixing the two together makes it *much*
> harder to review the bugfix.  These two steps should be separated
> please, with the bugfix patch coming first.

I will bisect this patch to make it more simple to be reviewed.
> 
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -103,6 +103,68 @@ out_free_image:
> >  	return ret;
> >  }
> >  
> > +static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
> > +		struct kexec_segment __user *segments, unsigned long flags)
> > +{
> > +	struct kimage **dest_image, *image;
> > +	unsigned long i;
> > +	int ret;
> > +
> > +	if (flags & KEXEC_ON_CRASH)
> > +		dest_image = &kexec_crash_image;
> > +	else
> > +		dest_image = &kexec_image;
> > +
> > +	if (nr_segments == 0) {
> > +		/* Uninstall image */
> > +		kimage_free(xchg(dest_image, NULL));
> > +		return 0;
> > +	}
> > +	if (flags & KEXEC_ON_CRASH) {
> > +		/*
> > +		 * Loading another kernel to switch to if this one
> > +		 * crashes.  Free any current crash dump kernel before
> > +		 * we corrupt it.
> > +		 */
> > +		kimage_free(xchg(&kexec_crash_image, NULL));
> > +	}
> > +
> > +	ret = kimage_alloc_init(&image, entry, nr_segments, segments, flags);
> > +	if (ret)
> > +		return ret;
> 
> This is a bug, isn't it?  Missed kimage_free().

The variable image is NULL, if kimage_alloc_init returns with error. And
kimage_alloc_init will do the cleanup before returning. So it's fine
without calling kimage_free.

Thanks
Minfei