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Message-ID: <ad60275b-fd62-2e38-7a17-a64f4bc0a9f0@openwall.com>
Date: Sun, 19 Jan 2020 18:53:49 +0300
From: Alexander Cherepanov <ch3root@...nwall.com>
To: musl@...ts.openwall.com
Subject: Re: Minor style patch to exit.c
On 19/01/2020 17.24, Markus Wichmann wrote:
> On Sun, Jan 19, 2020 at 04:33:47PM +0300, Alexander Cherepanov wrote:
>> Couldn't _start defined as an array? Then separate values could be accessed
>> simply as elements of this array. And casts to integers could be limited to
>> calculating the number of elements, the terminating value or something.
>
> That reminds me of something I read in the C standard: Two pointers must
> compare equal if, among other possibilities, one is a pointer to
> one-past its underlying array, and the other is a pointer to the start
> of its array, and the arrays happen to lie behind one another in address
> space.
One[1] of the gcc bug reports I mentioned is exactly about this issue.
DR 260[2] allows to take the provenance of the pointers into account
when comparing them and gcc really does this.
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61502
[2] http://open-std.org/jtc1/sc22/wg14/www/docs/dr_260.htm
As a side note, I thinks this is the wildest gcc bug report, it contains
really mind-blowing comments (like comment 3). I don't mean it in a bad
way at all and if you want to turn your understanding of C language
inside-out you can try to read it. OTOH I think it's all wrong after all
and I have some hope for it to be settled after my recent comments
there. But I don't hold my breath.
> Therefore, if _start and _end were arrays, even the GCC devs must agree
> that there might be an integer i such that _start + i == _end. For the C
> language, _start and _end would be arrays that happen to lie adjacent in
> address space.
>
> And if we have guarantees from the outside attesting to that, then
> _end - _start is no longer an undefined expression, right?
Even if we know that _start + k == _end it doesn't mean that we allowed
to subtract them.
--
Alexander Cherepanov
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