From: Larry McVoy <lm@mcvoy.com>
To: The Unix Heritage Society mailing list <tuhs@tuhs.org>
Subject: Re: [TUHS] History of popularity of C
Date: Wed, 27 May 2020 13:13:47 -0700 [thread overview]
Message-ID: <20200527201347.GY22882@mcvoy.com> (raw)
In-Reply-To: <m1je23J-0036tPC@more.local>
So I may have just gotten lucky in my 30+ years of writing C code but
I have yet to hit a single instance of this doom and gloom.
On Wed, May 27, 2020 at 12:49:25PM -0700, Greg A. Woods wrote:
> At Wed, 27 May 2020 18:11:33 +0200, "Thomas Paulsen" <thomas.paulsen@firemail.de> wrote:
> Subject: Re: [TUHS] History of popularity of C
> >
> > When I'm doing C I always have the CPU and its instructions in mind.
>
> And that's exactly what might trip you up unless you _exactly_
> understand how the language standard defines the operations of the
> abstract virtual machine (right down to the implications of every
> sequence point in the code); how compilers and optimizers do and (more
> importantly) do not work when mapping the abstract virtual machine
> operations into real-world machine instructions; and what how _all_
> instances of "undefined behaviour" can arise, and exactly what the
> optimizer is allowed to do when and if it spots UB conditions in the
> code.
>
> A big part of the problem is that the C Standard mandates compilation
> will and must succeed (and allows this success to be totally silent too)
> even if the code contains instances of undefined behaviour. This means
> that the successful execution of the generated code may depend on what
> optimization level was chosen. Code that does security tests on input
> values might be entirely and silently eliminated by the optimizer
> because of some innocuous-seeming UB instance, and this is exactly what
> has happened in the Linux kernel, for example (probably more than once).
>
> UB can be introduced quite innocently just by moving sequence points in
> variable references in ways that are not necessarily obvious even to
> seasoned programmers (and indeed "seasoned" programmers are often the
> ones who's old-fashioned coding habits might lead to introduction of
> serious problems in such a way).
>
> I've found dozens of instances of UB in mature and well tested code, and
> sometimes only by luck of having chosen the "right" compiler and enabled
> its feature of introducing illegal instructions in places where UB might
> occur, _and_ having had the luck to test in such a way as to encounter
> the specific code path where this UB occurred.
>
> I would claim it's truly safer now to write C without understanding the
> underlying mechanics of the CPU and memory, but rather by just paying
> very close attention to the detailed semantics of the language,
> understanding only the abstract virtual C machine, and hoping your
> compiler will at least warn if anything even remotely suspicious is done
> in your code; and lastly (but perhaps most importantly) avoiding like
> the plague any coding constructs which might make UB harder to spot
> (e.g. never ever initialize local variables with their definition when
> pointers are involved).
>
> Unfortunately the new "most advanced" C compilers also make it quite a
> bit more difficult for those of us writing C code that must have
> specific actions on the bare metal hardware, e.g. in embedded systems,
> kernels, hardware drivers, etc.; including especially where UB detection
> tools are far more difficult to use.
>
> --
> Greg A. Woods <gwoods@acm.org>
>
> Kelowna, BC +1 250 762-7675 RoboHack <woods@robohack.ca>
> Planix, Inc. <woods@planix.com> Avoncote Farms <woods@avoncote.ca>
--
---
Larry McVoy lm at mcvoy.com http://www.mcvoy.com/lm
next prev parent reply other threads:[~2020-05-27 20:14 UTC|newest]
Thread overview: 105+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-21 15:27 Tyler Adams
2020-05-21 16:10 ` Toby Thain
2020-05-21 16:30 ` Larry McVoy
2020-05-21 17:22 ` John Foust
2020-05-21 20:17 ` Toby Thain
2020-05-21 16:43 ` Tony Finch
2020-05-21 17:35 ` arnold
2020-05-21 19:16 ` CHARLES KESTER
2020-05-21 20:33 ` Thomas Paulsen
2020-05-21 20:09 ` Toby Thain
2020-05-21 20:12 ` Tony Finch
2020-05-22 8:28 ` David Arnold
2020-05-21 20:07 ` Toby Thain
2020-05-21 20:56 ` Clem Cole
2020-05-21 23:45 ` Toby Thain
2020-05-21 23:57 ` Richard Salz
2020-05-22 0:17 ` Toby Thain
2020-05-22 4:10 ` John Gilmore
2020-05-22 14:11 ` Larry McVoy
2020-05-22 14:34 ` Richard Salz
2020-05-22 14:17 ` Larry McVoy
2020-05-22 7:42 ` arnold
2020-05-22 23:50 ` Greg A. Woods
2020-05-23 7:28 ` Andy Kosela
2020-05-23 17:08 ` Clem Cole
2020-05-23 17:22 ` Richard Salz
2020-05-23 18:42 ` Derek Fawcus
2020-05-23 19:28 ` Michael Kjörling
2020-05-26 4:21 ` Dave Horsfall
2020-05-26 4:32 ` Ed Carp
2020-05-26 8:21 ` Rob Pike
2020-05-26 14:44 ` Clem Cole
2020-05-26 14:32 ` Clem Cole
2020-05-26 19:50 ` Greg A. Woods
2020-05-26 21:48 ` Thomas Paulsen
2020-05-26 22:36 ` Greg A. Woods
2020-05-27 14:37 ` Ronald Natalie
2020-05-27 15:09 ` Clem Cole
2020-05-27 16:11 ` Thomas Paulsen
2020-05-27 19:49 ` Greg A. Woods
2020-05-27 20:13 ` Larry McVoy [this message]
2020-05-27 20:23 ` Richard Salz
2020-05-27 21:00 ` Nevin Liber
2020-05-27 23:17 ` Greg A. Woods
2020-06-05 20:57 ` Dave Horsfall
2020-06-05 21:40 ` Nemo Nusquam
2020-06-05 21:47 ` Richard Salz
2020-06-05 22:01 ` Bakul Shah
2020-06-06 20:49 ` Ed Carp
2020-06-06 21:08 ` Thomas Paulsen
2020-06-06 21:13 ` Larry McVoy
2020-06-06 22:27 ` Ed Carp
2020-06-06 23:14 ` Tyler Adams
2020-06-07 5:57 ` arnold
2020-06-07 9:22 ` Andy Kosela
2020-06-07 9:39 ` Ed Carp
2020-06-07 10:02 ` Brantley Coile
2020-06-07 11:30 ` Thomas Paulsen
2020-06-07 15:26 ` Clem Cole
2020-06-07 15:52 ` Larry McVoy
2020-06-08 1:02 ` Adam Thornton
2020-06-08 8:04 ` Thomas Paulsen
2020-06-07 17:26 ` Bakul Shah
2020-06-07 17:35 ` Bakul Shah
2020-06-07 18:50 ` Nemo Nusquam
2020-06-07 21:15 ` Chris Torek
2020-06-07 22:16 ` Dan Cross
2020-06-07 22:56 ` Chris Torek
2020-06-07 23:14 ` [TUHS] Comparative languages Warren Toomey
2020-06-08 0:24 ` [TUHS] History of popularity of C Bram Wyllie
2020-06-08 5:48 ` Lars Brinkhoff
2020-06-06 23:31 ` Bakul Shah
2020-06-07 0:12 ` Greg A. Woods
2020-06-07 11:04 ` emanuel stiebler
2020-06-07 11:33 ` Thomas Paulsen
2020-05-26 15:19 ` Toby Thain
2020-05-26 16:00 ` Thomas Paulsen
2020-05-26 16:21 ` Christopher Browne
2020-05-26 19:29 ` Thomas Paulsen
2020-05-26 19:55 ` Dan Cross
2020-05-26 20:00 ` Jon Steinhart
2020-05-21 16:18 ` Jim Capp
2020-05-21 18:58 ` A. P. Garcia
2020-05-21 19:02 ` Clem Cole
2020-05-21 18:28 Noel Chiappa
2020-05-21 18:44 ` Thomas Paulsen
2020-05-21 19:06 ` Paul Winalski
2020-05-21 20:27 ` Thomas Paulsen
2020-05-22 8:52 ` Tom Ivar Helbekkmo via TUHS
2020-05-22 9:51 ` Tyler Adams
2020-05-22 11:09 ` arnold
2020-05-22 11:15 ` Tyler Adams
2020-05-22 18:40 ` John Gilmore
2020-05-22 19:01 ` Toby Thain
2020-05-22 19:35 ` Larry McVoy
2020-05-22 19:31 ` Larry McVoy
2020-05-22 20:19 ` Michael Kjörling
2020-05-22 14:59 ` Toby Thain
2020-05-22 11:58 ` A. P. Garcia
2020-06-06 21:49 Doug McIlroy
2020-06-06 21:55 ` Warner Losh
2020-06-08 13:56 ` Derek Fawcus
2020-06-08 15:20 ` Richard Salz
2020-06-08 15:30 ` Dan Cross
2020-06-08 16:32 ` Tony Finch
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