linux-kernel - Re: [PATCH -mm] -funsigned-char, x86: make struct p4_event

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Message-ID: <Y1JMCqv5QucIfNCS@kroah.com>
Date:   Fri, 21 Oct 2022 09:36:42 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        mm-commits@...r.kernel.org, masahiroy@...nel.org,
        andriy.shevchenko@...ux.intel.com,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH -mm] -funsigned-char, x86: make struct
 p4_event_bind::cntr signed array

On Fri, Oct 21, 2022 at 03:24:27AM -0400, Jason A. Donenfeld wrote:
> Hi Greg,
> 
> On Fri, Oct 21, 2022 at 2:48 AM Greg KH <gregkh@...uxfoundation.org> wrote:
> >
> > On Thu, Oct 20, 2022 at 01:17:33PM -0700, Linus Torvalds wrote:
> > > And in other cases, there's no actual difference at all, just
> > > different register usage, so the diff looks fairly big, but doesn't
> > > seem to be real.  In one case I looked at, it started with a 'movzbl',
> > > but it was that in both cases, because the type was actually 'unsigned
> > > char' to begin with. But for some reason it just used different
> > > registers. Example:
> > >
> > >  - handle_control_request() in drivers/usb/gadget/udc/dummy_hcd.c
> > >
> > >    The reason here *seems* to be that
> > >
> > >                         char *buf;
> > >                         buf = (char *)urb->transfer_buffer;
> > >
> > >    where it really probably should be 'u8 *buf', since it actually
> > > does a cast to 'u8' in one place, but there isn't even any read of
> > > that 'buf' pointer. So the difference seems to be entirely just some
> > > "different type in assignment" cast internal to gcc that then
> > > incidentally generated a random other choice in register allocation.
> >
> > I've send a patch for this now:
> >         https://lore.kernel.org/r/20221021064453.3341050-1-gregkh@linuxfoundation.org
> > and will take it through the USB tree, unless Jason wants to grab it
> > through his tree.
> 
> This doesn't appear to have any actual effect, but just changes gcc's
> register allocation unexpectedly. So feel free to take it, as it
> doesn't seem like it's "one of those bad cases" that I'm keeping track
> of.

Great, will take it through my tree, thanks!

greg k-h