linux-kernel - Re: [PATCH v2] kexec: Use min

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Date:	Mon, 25 Feb 2013 15:35:54 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Simon Horman <horms@...ge.net.au>
Cc:	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] kexec: Use min_t to simplify logic

On Mon, 25 Feb 2013 09:36:51 +0900
Simon Horman <horms@...ge.net.au> wrote:

> On Sun, Feb 24, 2013 at 10:37:21PM +0800, Zhang Yanfei wrote:
> > From: Zhang Yanfei <zhangyanfei@...fujitsu.com>
> > 
> > This is just a tweak: using min_t to simplify logic of variable
> > assignments.
> > 
> > v2:
> > - Rewrite patch description as Simon suggested.
> > - Fix an inappropriate if test introduced by v1. Thanks Simon.
> 
> Hi Zhang,
> 
> thanks for the update.
> 
> Signed-off-by: Simon Horman <horms@...ge.net.au>

Signed-off-by: implies that you were involved in the development or
patch delivery.  Were you?  If not, an Acked-by or Reviewed-by is more
appropriate.

Also, the need to use min_t rather than min is a sign that the types
are screwed up.  Let's take a look.

> > 
> > diff --git a/kernel/kexec.c b/kernel/kexec.c
> > index 2436ffc..effd655 100644
> > --- a/kernel/kexec.c
> > +++ b/kernel/kexec.c
> > @@ -822,13 +822,8 @@ static int kimage_load_normal_segment(struct kimage *image,
> >  		/* Start with a clear page */
> >  		clear_page(ptr);
> >  		ptr += maddr & ~PAGE_MASK;
> > -		mchunk = PAGE_SIZE - (maddr & ~PAGE_MASK);
> > -		if (mchunk > mbytes)
> > -			mchunk = mbytes;
> > -
> > -		uchunk = mchunk;
> > -		if (uchunk > ubytes)
> > -			uchunk = ubytes;
> > +		mchunk = min_t(size_t, mbytes, PAGE_SIZE - (maddr & ~PAGE_MASK));
> > +		uchunk = min_t(size_t, ubytes, mchunk);

The types of ubytes and mbytes are clearly wrong.  They are initialised
from a size_t and they are manipulated alongside size_t's.

The types of PAGE_SIZE and PAGE_MASK are vague - iirc they once had
different types on different architectures, so some form of casting is
unavoidable here.

> >  		result = copy_from_user(ptr, buf, uchunk);
> >  		kunmap(page);
> > @@ -874,13 +869,9 @@ static int kimage_load_crash_segment(struct kimage *image,
> >  		}
> >  		ptr = kmap(page);
> >  		ptr += maddr & ~PAGE_MASK;
> > -		mchunk = PAGE_SIZE - (maddr & ~PAGE_MASK);
> > -		if (mchunk > mbytes)
> > -			mchunk = mbytes;
> > -
> > -		uchunk = mchunk;
> > -		if (uchunk > ubytes) {
> > -			uchunk = ubytes;
> > +		mchunk = min_t(size_t, mbytes, PAGE_SIZE - (maddr & ~PAGE_MASK));
> > +		uchunk = min_t(size_t, ubytes, mchunk);
> > +		if (mchunk > uchunk) {
> >  			/* Zero the trailing part of the page */
> >  			memset(ptr + uchunk, 0, mchunk - uchunk);
> >  		}

Again, mybtes and ubytes have the wrong type.

> > @@ -1461,8 +1452,7 @@ void vmcoreinfo_append_str(const char *fmt, ...)
> >  	r = vsnprintf(buf, sizeof(buf), fmt, args);
> >  	va_end(args);
> >  
> > -	if (r + vmcoreinfo_size > vmcoreinfo_max_size)
> > -		r = vmcoreinfo_max_size - vmcoreinfo_size;
> > +	r = min_t(size_t, r, vmcoreinfo_max_size - vmcoreinfo_size);
> >  
> >  	memcpy(&vmcoreinfo_data[vmcoreinfo_size], buf, r);

vmcoreinfo_max_size is a size_t and vmcoreinfo_size is a size_t and
memcpy's `size' argument is a size_t.  Therefore the type of `r' should
be...  what?  int!  bzzzzt.

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