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Date: Fri, 15 May 2026 17:09:02 +0200
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From: Thorsten Blum <thorsten.blum@linux.dev>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Kees Cook <kees@kernel.org>,
	Andy Shevchenko <andy@kernel.org>, linux-kernel@vger.kernel.org,
	linux-hardening@vger.kernel.org
Subject: Re: [PATCH] string: use min in sized_strscpy
Message-ID: <agc3DrODtyc2rF27@linux.dev>
References: <20260514165601.527883-3-thorsten.blum@linux.dev>
 <CAHp75VfQNkqEYsO4Uup0c-uiYuVyAWit=tmCz2BsYLp-sjXsZw@mail.gmail.com>
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On Fri, May 15, 2026 at 09:45:08AM +0300, Andy Shevchenko wrote:
> On Thu, May 14, 2026 at 7:56 PM Thorsten Blum <thorsten.blum@linux.dev> wrote:
> >
> > Use min() and drop the limit variable to simplify sized_strscpy().
> 
> ...
> 
> > -       if ((long)src & (sizeof(long) - 1)) {
> > -               size_t limit = PAGE_SIZE - ((long)src & (PAGE_SIZE - 1));
> > -               if (limit < max)
> > -                       max = limit;
> > -       }
> > +       if ((long)src & (sizeof(long) - 1))
> > +               max = min(PAGE_SIZE - ((long)src & (PAGE_SIZE - 1)), max);
> 
> Side note: Isn't simply
>                max = min(PAGE_SIZE - offset_in_page(src), max);
> 
> ? (One will need to include linux/mm.h for this, though.)

I thought about it, but wasn't sure if pulling in all of linux/mm.h into
lib/string.c is worth it.

> Moreover there are plenty of duplications to count the size in the
> first page and even the similar min() as in
>   fs/iomap/buffered-io.c:869
>   arch/s390/kvm/gaccess.c:976
> and many more...
> 
> Perhaps a new macro with a good (famous last words!) naming?

How about adding

#define offset_in_page(p)	((unsigned long)(p) & ~PAGE_MASK)
#define bytes_to_page_end(p)	(PAGE_SIZE - offset_in_page(p))

to a new header, e.g. include/linux/page_helpers.h? There are about 50+
direct replacements and possibly more.