RE: [RFC PATCH 03/13] usb: remove the usage of the list iterator after the loop

From: David Laight <David.Laight@ACULAB.COM>
To: 'Linus Torvalds' <torvalds@linux-foundation.org>,
	Segher Boessenkool <segher@kernel.crashing.org>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	Arnd Bergmann <arnd@arndb.de>, Jakob <jakobkoschel@gmail.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Kees Cook <keescook@chromium.org>,
	Mike Rapoport <rppt@kernel.org>,
	"Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	Brian Johannesmeyer <bjohannesmeyer@gmail.com>,
	Cristiano Giuffrida <c.giuffrida@vu.nl>,
	"Bos, H.J." <h.j.bos@vu.nl>
Subject: RE: [RFC PATCH 03/13] usb: remove the usage of the list iterator after the loop
Date: Mon, 28 Feb 2022 06:15:59 +0000	[thread overview]
Message-ID: <6729109ae6ad429f87270e2bef2eed2f@AcuMS.aculab.com> (raw)
In-Reply-To: <CAHk-=wijh=SQ_9_-H6O08HgmXrWz37_vcdm55oECo+31LUs2EQ@mail.gmail.com>

From: Linus Torvalds
> Sent: 27 February 2022 21:05
...
> And then the C standards people decided that "because our job isn't to
> describe all the architectural issues you can hit, we'll call it
> undefined, and in the process let compiler people intentionally break
> it".
> 
> THAT is a problem.

I'm waiting for them to decide that memset(ptr, 0, len) of
any structure that contains a pointer is UB (because a NULL
pointer need not be the all zero bit pattern) so decide
to discard the call completely (or some such).

Non-zero NULL pointers is the only reason arithmetic on NULL
pointers isn't valid.

Or maybe that character range tests are UB because '0' to '9'
don't have to be adjacent - they are even adjacent in EBCDIC.

Some of the 'strict aliasing' bits are actually useful since
they let the compiler reorder reads and writes.
But the definition is brain-dead.
Sometimes it would be nice to have byte writes reordered,
but even using int:8 doesn't work.

I have never worked out what 'restrict' actually does,
in any places I've tried it did nothing.
Although I may have been hoping it would still help when
the function got inlined.

	David

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