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Message-ID: <06cd383a-d1c3-d59c-ddd0-c0bd38ad2962@openwall.com>
Date: Sun, 19 Jan 2020 19:33:08 +0300
From: Alexander Cherepanov <ch3root@...nwall.com>
To: musl@...ts.openwall.com
Subject: Re: Minor style patch to exit.c
On 19/01/2020 17.46, Alexander Monakov wrote:
> On Sun, 19 Jan 2020, 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.
>
> Yeah, I think usually such linker-provided symbols are declared as
> extern arrays. I'm surprised that isn't the case in musl. I don't think
> declaring them as arrays helps with making casts pedantically suitable for
> calculating number of elements though - as you said, any bijection between
> intptr_t and pointers would be a valid implementation of a cast, you're not
Well, we want use from C some outside info, there could be no pedantic
way to do this. Let's see, we know that the _end array follows the
_start array in memory. This means that &_start[i] == &_end[0] for some
i. But different provenance of the pointers means that we cannot do it
just like that. Adding a cast should fix this. Summarizing, it should
look like this:
for (size_t i = 0; (uintptr_t)&_start[i] != (uintptr_t)&_end[0]; i++)
or
for (type *p = _start; (uintptr_t)p != (uintptr_t)_end; p++)
> guaranteed that (intptr_t)&a[i] == (intptr_t)a + i * sizeof *a.
While you are inside one object, I think this should be safe in
practice. For gcc, this is more or less guaranteed by [3]. BTW there is
an explicit restriction there:
"When casting from pointer to integer and back again, the resulting
pointer must reference the same object as the original pointer,
otherwise the behavior is undefined. That is, one may not use integer
arithmetic to avoid the undefined behavior of pointer arithmetic as
proscribed in C99 and C11 6.5.6/8."
[3]
https://gcc.gnu.org/onlinedocs/gcc/Arrays-and-pointers-implementation.html
--
Alexander Cherepanov
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