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| From | Stephen Sprunk <stephen@sprunk.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: the language |
| Date | 2014-12-21 23:49 -0600 |
| Organization | A noiseless patient Spider |
| Message-ID | <m78bdf$197$1@dont-email.me> (permalink) |
| References | (9 earlier) <5492c298.300765@news.xs4all.nl> <ifilw.1128836$yK.904633@fx14.iad> <lnlhm29koo.fsf@nuthaus.mib.org> <f0ce9adkg4val4bj6anbrqm3972gg0t2g0@4ax.com> <20141221135927.223@kylheku.com> |
On 21-Dec-14 16:02, Kaz Kylheku wrote:
> On 2014-12-21, Robert Wessel <robertwessel2@yahoo.com> wrote:
>> The Linux kernel NULL pointer thing is an interesting example.
>> Basically something like the following was in the kernel:
>>
>> x somefunction(sometype *p)
>> {
>> ...
>> t = p->a;
>> ...
>> if (p)
>> someotherfunction(p);
>> ...
>> }
>>
>> GCC optimized the test out, and someotherfunction() ended up
>> called with a NULL pointer. The justification was that the
>> assignment would fault if p was NULL, rendering the test
>> superfluous. Which was not actually true in the Linux kernel,
>> where page zero is actually mapped.
>
> Where? Maybe on some no-MMU-architecture ports of Linux?
>
> Page zero is not mapped in the Linux kernel, and null pointer
> dereferences give rise to an "oops", where they are clearly
> identified as such.
There's no special code for page zero.
Remember that kernel code executes in the caller's context*; rather than
reset the page tables on entry and again on exit, the kernel gets mapped
into each process's address space, typically in the top (or negative)
half, but marked unreadable. When kernel code is executing, that
security check is bypassed, otherwise the kernel wouldn't be able to
read its own data--or even its own code!
This security check is what generates a "segmentation violation" when
user code dereferences a null pointer. However, when kernel code does
the same, that security check is bypassed, so it happily reads the data
at that address and continues on--but the GCC folks didn't take that
into account when they added the above optimization.
Not that GCC was technically wrong since this is clearly UB, but GCC's
release process requires compiling a working Linux kernel (among other
things), which puts them on the hook for making all Linux's uses of UB
work _as Linux expects them to_.
(* Aside from oddballs like RedHat's 4GB+4GB kernel, typically used for
large database servers before x86-64 came out, which actually _did_ have
to reset the page tables on every entry and every exit.)
S
--
Stephen Sprunk "God does not play dice." --Albert Einstein
CCIE #3723 "God is an inveterate gambler, and He throws the
K5SSS dice at every possible opportunity." --Stephen Hawking
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the language Rosario193 <Rosario@invalid.invalid> - 2014-11-26 21:54 +0100
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-26 21:56 +0100
Re: the language John Bode <jfbode1029@gmail.com> - 2014-11-26 13:45 -0800
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Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-12-04 14:52 +0100
Re: the language "Osmium" <r124c4u102@comcast.net> - 2014-12-04 08:07 -0600
Re: the language Geoff <geoff@invalid.invalid> - 2014-12-04 10:20 -0800
Re: the language Mark Storkamp <mstorkamp@yahoo.com> - 2014-11-26 15:11 -0600
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-28 12:16 +0100
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-28 12:18 +0100
Re: the language John Bode <jfbode1029@gmail.com> - 2014-11-26 13:12 -0800
Re: the language Kaz Kylheku <kaz@kylheku.com> - 2014-11-27 00:37 +0000
Re: the language Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-11-27 01:13 +0000
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-11-26 22:45 +0000
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-27 07:41 +0100
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-27 08:12 +0100
Re: the language Morris Dovey <mrdovey@iedu.com> - 2014-11-27 01:43 -0600
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-28 11:56 +0100
Re: the language Morris Dovey <mrdovey@iedu.com> - 2014-11-28 09:08 -0600
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-28 20:58 +0100
Re: the language Morris Dovey <mrdovey@iedu.com> - 2014-11-28 19:22 -0600
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-12-02 02:59 +0100
Re: the language Kaz Kylheku <kaz@kylheku.com> - 2014-12-02 02:53 +0000
Re: the language Les Cargill <lcargill99@comcast.com> - 2014-12-01 21:25 -0600
Re: the language Kaz Kylheku <kaz@kylheku.com> - 2014-12-02 06:22 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-11-27 08:37 +0000
Re: the language Reinhardt Behm <rbehm@hushmail.com> - 2014-11-27 16:44 +0800
Re: the language gazelle@shell.xmission.com (Kenny McCormack) - 2014-11-27 15:35 +0000
Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-11-27 11:58 -0500
Re: the language jt@toerring.de (Jens Thoms Toerring) - 2014-11-27 22:44 +0000
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-28 09:52 +0100
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-11-28 12:53 -0800
Re: the language Thomas Jahns <jahns@idontlikespam.dkrz.de> - 2014-11-28 10:07 +0100
Re: the language Thomas Jahns <jahns@idontlikespam.dkrz.de> - 2014-11-28 14:57 +0100
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-11-28 13:09 -0800
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-28 12:01 +0100
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-11-28 14:46 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-11-28 13:16 -0800
Re: the language gazelle@shell.xmission.com (Kenny McCormack) - 2014-11-28 22:09 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-01 03:40 -0800
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-28 14:42 -0800
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-01 03:22 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-11-28 23:09 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-28 16:14 -0800
Re: the language BartC <bc@freeuk.com> - 2014-11-29 01:42 +0000
Re: the language Kaz Kylheku <kaz@kylheku.com> - 2014-11-29 01:42 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-11-29 04:03 -0800
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-01 00:55 -0800
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-02 05:42 +0000
Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-11-28 19:25 -0500
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-01 03:21 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-01 14:02 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-05 10:18 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-07 08:25 +0000
Re: the language Öö Tiib <ootiib@hot.ee> - 2014-12-07 10:44 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-07 19:17 +0000
Re: the language Öö Tiib <ootiib@hot.ee> - 2014-12-07 15:48 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-08 05:45 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-08 03:27 -0800
Re: the language Kaz Kylheku <kaz@kylheku.com> - 2014-12-08 06:00 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-08 06:17 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-08 03:37 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-08 12:53 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-16 04:21 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-16 12:43 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-16 14:46 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-17 00:23 +0000
Re: the language JohnF <john@please.see.sig.for.email.com> - 2014-12-17 07:01 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-17 07:22 +0000
Re: the language Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-17 11:56 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-17 05:43 -0800
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-12-18 18:30 +0100
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-12-18 18:41 +0100
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-17 14:34 +0000
Re: the language Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-18 02:15 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-17 22:31 -0500
Re: the language Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-18 17:19 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-18 07:52 +0000
Re: the language Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-18 17:21 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-18 18:15 +0000
Re: the language Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-19 01:20 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 07:48 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-19 07:40 -0500
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 12:54 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-19 09:54 -0500
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 15:19 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-19 11:00 -0500
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 16:13 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-19 11:27 -0500
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 16:45 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-19 13:48 -0500
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 19:10 +0000
Re: the language Morris Dovey <mrdovey@iedu.com> - 2014-12-19 21:17 -0600
Re: the language Morris Dovey <mrdovey@iedu.com> - 2014-12-19 21:32 -0600
Re: the language Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-19 16:49 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 17:00 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-19 13:57 -0500
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-19 14:31 -0800
Re: the language Ben Bacarisse <ben.usenet@bsb.me.uk> - 2014-12-19 20:29 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 20:36 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-17 07:18 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-17 15:45 +0000
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-17 16:50 +0000
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-17 17:24 +0000
Re: the language Robert Wessel <robertwessel2@yahoo.com> - 2014-12-18 02:07 -0600
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-18 12:04 +0000
Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-12-18 08:06 -0500
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-18 08:30 -0500
Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-12-19 00:11 -0500
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-19 07:00 +0000
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-19 08:43 -0800
Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-12-19 23:29 -0500
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-20 00:37 -0800
Re: the language gazelle@shell.xmission.com (Kenny McCormack) - 2014-12-20 08:45 +0000
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-20 09:45 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-20 03:17 -0800
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-18 06:09 -0800
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-18 14:30 +0000
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-18 14:23 +0000
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-18 08:18 -0800
Re: the language Richard Damon <Richard@Damon-Family.org> - 2014-12-20 12:22 -0500
Re: the language James Kuyper <jameskuyper@verizon.net> - 2014-12-20 13:36 -0500
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-20 11:43 -0800
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-20 11:37 -0800
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-20 20:10 +0000
Re: the language Robert Wessel <robertwessel2@yahoo.com> - 2014-12-21 15:02 -0600
Re: the language Kaz Kylheku <kaz@kylheku.com> - 2014-12-21 22:02 +0000
Re: the language Stephen Sprunk <stephen@sprunk.org> - 2014-12-21 23:49 -0600
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-21 17:45 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-18 10:29 -0800
Re: the language Robert Wessel <robertwessel2@yahoo.com> - 2014-12-20 05:29 -0600
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-20 11:32 -0800
Re: the language Robert Wessel <robertwessel2@yahoo.com> - 2014-12-21 15:23 -0600
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-22 23:39 -0800
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2014-12-20 20:22 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-18 10:24 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-18 18:36 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-19 09:24 -0800
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 17:53 +0000
Re: the language Tim Rentsch <txr@alumni.caltech.edu> - 2014-12-20 11:48 -0800
Re: the language JohnF <john@please.see.sig.for.email.com> - 2014-12-19 02:20 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 08:31 +0000
Re: the language JohnF <john@please.see.sig.for.email.com> - 2014-12-19 09:31 +0000
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-19 10:10 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-19 07:08 -0800
Re: the language JohnF <john@please.see.sig.for.email.com> - 2014-12-20 06:39 +0000
Re: the language John Forkosh <forkosh@panix.com> - 2014-12-20 07:32 +0000
Re: the language Greg Martin <greg.at.softsprocket@dot.com> - 2015-01-04 17:02 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2015-01-04 12:40 -0500
Re: the language Greg Martin <greg.at.softsprocket@dot.com> - 2015-01-04 18:02 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2015-01-05 09:47 -0500
Re: the language Greg Martin <greg.at.softsprocket@dot.com> - 2015-01-05 15:14 +0000
Re: the language Keith Thompson <kst-u@mib.org> - 2015-01-05 08:42 -0800
Re: the language glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-01-06 00:14 +0000
Re: the language James Kuyper <jameskuyper@verizon.net> - 2015-01-05 13:19 -0500
Re: the language Richard Damon <Richard@Damon-Family.org> - 2015-01-04 15:59 -0500
Re: the language Keith Thompson <kst-u@mib.org> - 2015-01-04 14:30 -0800
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-01-04 15:44 -0800
Re: the language James Kuyper <jameskuyper@verizon.net> - 2015-01-05 13:36 -0500
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-11-28 12:07 +0100
Re: the language "ocq (an evil corporate man)" <ocq@maili.ng> - 2014-12-02 23:16 +0100
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-03 02:23 -0800
Re: the language "ocq (an evil corporate man)" <ocq@maili.ng> - 2014-12-03 15:55 +0100
Re: the language Nobody <nobody@nowhere.invalid> - 2014-12-03 15:42 +0000
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-04 04:49 -0800
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-04 09:07 -0800
Re: the language Malcolm McLean <malcolm.mclean5@btinternet.com> - 2014-12-04 10:08 -0800
Re: the language Keith Thompson <kst-u@mib.org> - 2014-12-04 11:49 -0800
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-12-05 12:32 +0100
Re: the language raltbos@xs4all.nl (Richard Bos) - 2014-12-04 22:17 +0000
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-12-18 18:17 +0100
Re: the language Richard Heathfield <invalid@see.sig.invalid> - 2014-12-18 18:12 +0000
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-12-18 23:15 +0100
Re: the language Rosario193 <Rosario@invalid.invalid> - 2014-12-18 23:50 +0100
Re: the language "ocq (an evil corporate man)" <ocq@maili.ng> - 2014-12-19 02:34 +0100
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