Re: Does reading an uninitialized object have undefined behavior?

From	Kaz Kylheku <864-117-4973@kylheku.com>
Newsgroups	comp.std.c
Subject	Re: Does reading an uninitialized object have undefined behavior?
Date	2023-08-16 21:08 +0000
Organization	A noiseless patient Spider
Message-ID	<20230816134842.416@kylheku.com> (permalink)
References	<87zg3pq1ym.fsf@nosuchdomain.example.com> <864jlfj34p.fsf@linuxsc.com> <871qgjlqe9.fsf@nosuchdomain.example.com> <ubja3a$3e365$1@dont-email.me> <87350ilnv1.fsf@nosuchdomain.example.com>

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On 2023-08-16, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:
> Kaz Kylheku <864-117-4973@kylheku.com> writes:
>> On 2023-08-03, Keith Thompson <Keith.S.Thompson+u@gmail.com> wrote:
>>> Tim Rentsch <tr.17687@z991.linuxsc.com> writes:
>>>> Repeating the question stated in the Subject line:
>>>>
>>>> Does reading an uninitialized object [always] have undefined
>>>> behavior?
>>>
>>> Thank you for taking the time to write that.
>> [ ... ]
>>> I'm not criticizing the author of the standard for making this mistake.
>>> Stuff happens.  It was likely a result of an oversight during the
>>> transition from C90 to C99.
>>
>> [Supersede attempt to reduce quoted material.]
>>
>> I would be in favor of a formal model of what "uninitialized" means
>> which could be summarized as below.
>>
>> Implementors wishing to develop tooling to catch uses of uninitialized
>> data can refer to the model; if their tooling diagnoses only
>> what the model deems undefined, then the tool can be integrated
>> into a conforming implementation.
>>
>> - Certain objects are unintialized, like auto variables without
>>   an initializer, or new bytes coming from malloc or realloc.
>>
>> - What is undefined behavior is when an uninitialized value is used
>>   to make a control-flow decision, or when it is output, or otherwise
>>   passed to the host environment.
>
> Why restrict it to those particular uses, rather than saying that any
> attempt to read an uninitialized value has undefined behavior?

Because that then brings back complications like

- unsigned char access has to be exempt

- what happens if we copy through in intermediate values:

  int ch = *src++; // *src is uninitialized, therefore so is ch
  *dst++ = ch;     // ch is uninitialized and not unsigned char

  Is the second access to ch uninitialized?

- structures: when a struct is access which has uninitialized
  padding, what happens: we need a rule like if those bytes
  are accessed, they are accessed as if unsigned char.

The idea of trapping only control flow decisions or output is inspired
by Valgrind. 

Valgrind does not "spaz out" just because an uninitialized value is
accessed, because it would result in useless false positives.

Not all of the reasoning applies to C; part of it is that Valgrind is
working with machine, with no source language knowledge. The basic idea
makes sense though.

Valgrind usefully finds uninitialized data bugs, while allowing you to
write your own memcpy which can copy a structure full of uninitialized
bytes: and it does so without knowing anything about unsigned char.

We could make the rule that only visible behavior depending on
an uninitialized byte is undefined; the rule about control flows
makes it a bit tighter, while allowing the copying of of uninited
data.

> For example, something like:
>     {
>         int uninit;
>         int copy = uninit + 1;
>     }
> might cause a hardware trap on some systems (for example Itanium if
> uninit is stored in a register and the NaT bit is set).

Right, so the model above doesn't speak to traps. We still have those.

You can copy an object using unsigned char not because it's specially
blessed for access (other than in regard to aliasing rules), but because
it has no trap representation.

On a machine without traps, the above code would just result
in copy being uninitialized.

If that value isn't printed, or used in if, or switch, then it
doesn't matter.

If the type int has trap representations, then it's undefined on that
implementation; it's basically just a matter of luck whether uninit is a
trap or a value, so it has to be regarded as undefined.

I believe that the model can be used to implement useful diagnostics
even without realizing the actual shadow bytes. A subset of the
bugs can be diagnosed within a lexical scope, like uses of
uninitialized auto locals. When the compiler is doing data flow
analysis, it just propagates that uninited info around the program
graph. If an uninited data flow reaches certain nodes in the program
graph, like where control decisions are made or certain functions
are called that are known to pass the datum to the host environment,
then it can diagnose.

-- 
TXR Programming Language: http://nongnu.org/txr
Cygnal: Cygwin Native Application Library: http://kylheku.com/cygnal
Mastodon: @Kazinator@mstdn.ca

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Thread

Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-20 22:16 -0700
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-21 16:33 +0100
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-21 11:56 -0700
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-21 20:54 +0100
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-07-21 14:26 -0700
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-07-21 23:39 +0100
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-12 17:00 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-13 23:41 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-15 21:06 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-15 22:40 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-16 23:13 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-17 07:08 +0000
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-18 12:44 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-19 05:04 +0000
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-19 01:36 -0700
Re: Does reading an uninitialized object have undefined behavior? Richard Damon <Richard@Damon-Family.org> - 2023-08-19 09:18 -0400
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-19 11:12 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-18 20:20 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-19 05:23 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-18 22:56 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-18 12:52 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-26 19:25 -0700
Re: Does reading an uninitialized object have undefined behavior? Spiros Bousbouras <spibou@gmail.com> - 2023-08-27 08:31 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-29 04:35 -0700
Re: Does reading an uninitialized object have undefined behavior? Spiros Bousbouras <spibou@gmail.com> - 2023-08-30 19:53 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-30 17:40 -0700
Re: Does reading an uninitialized object have undefined behavior? Spiros Bousbouras <spibou@gmail.com> - 2023-08-31 18:18 +0000
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-05 05:39 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-09-05 17:03 -0700
Re: Does reading an uninitialized object have undefined behavior? Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2023-09-07 17:09 +0200
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-07 17:19 +0100
Re: Does reading an uninitialized object have undefined behavior? Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2023-09-08 23:12 +0200
Re: Does reading an uninitialized object have undefined behavior? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2023-09-08 22:31 +0100
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-07-22 06:40 +0000
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-07-22 06:03 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-25 21:53 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-16 11:11 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-07-21 17:42 +0000
Re: Does reading an uninitialized object have undefined behavior? Jakob Bohm <jb-usenet@wisemo.com.invalid> - 2023-07-24 07:53 +0200
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-07-25 21:57 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-03 13:13 -0700
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-08-03 15:20 -0700
Re: Does reading an uninitialized object have undefined behavior? Martin Uecker <ma.uecker@gmail.com> - 2023-08-05 01:15 -0700
Re: Does reading an uninitialized object have undefined behavior? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2023-08-16 09:19 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-16 19:51 +0000
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-16 20:03 +0000
Re: Does reading an uninitialized object have undefined behavior? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2023-08-16 13:43 -0700
Re: Does reading an uninitialized object have undefined behavior? Kaz Kylheku <864-117-4973@kylheku.com> - 2023-08-16 21:08 +0000

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