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Message-ID: <20160113173049.GF13558@port70.net>
Date: Wed, 13 Jan 2016 18:30:50 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: string word-at-a-time and atomic.h FAQ on twitter
* Szabolcs Nagy <nsz@...t70.net> [2016-01-13 00:07:39 +0100]:
> * Alexander Cherepanov <ch3root@...nwall.com> [2016-01-13 00:09:56 +0300]:
> > On 2016-01-13 00:02, Alexander Cherepanov wrote:
> > >On 2016-01-05 19:46, Szabolcs Nagy wrote:
> > >>i think compiler attributes should be used here on compilers that
> > >>might break the code, but there is no attribute for this kind of
> > >>oob access yet (although may_alias attribute is missing here too
> > >>and should be added like in other string functions).
> > >
> > >Perhaps the noclone function attribute could be used in the meantime?
> > >
> > >https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-g_t_0040code_007bnoclone_007d-function-attribute-3205
> >
> > Probably together with the noinline attribute...
> >
> > Another attribute which looks relevant is no_sanitize_address.
> >
> > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-g_t_0040code_007bno_005fsanitize_005faddress_007d-function-attribute-3199
> >
>
> i think a no-lto attr should be used, maybe noinline
> can achieve that.
>
i tried to do it with -fno-lto
but it seems gcc-6 miscompiles musl with -flto anyway:
lto incorrectly dead code eliminates _dlstart_c.
(the libc entry point, _dlstart, is defined in toplevel
inline asm in ldso/dlstart.c and it jumps to _dlstart_c)
lto breaks symbol binding for environ, _environ, ___environ.
(they should be weak, without that environ in a main binary
has different address than in libc.so)
libc.so built with -flto:
$ readelf --dyn-syms -W libc.so |grep envi
22: 000000000028eb90 8 OBJECT GLOBAL DEFAULT 15 __environ
398: 000000000028eb90 8 OBJECT GLOBAL PROTECTED 15 ___environ
1034: 000000000028eb90 8 OBJECT GLOBAL PROTECTED 15 _environ
1107: 000000000028eb90 8 OBJECT GLOBAL DEFAULT 15 environ
libc.so without -flto:
$ readelf --dyn-syms -W libc.so |grep envi
22: 000000000028d2d8 8 OBJECT GLOBAL DEFAULT 15 __environ
398: 000000000028d2d8 8 OBJECT WEAK PROTECTED 15 ___environ
1034: 000000000028d2d8 8 OBJECT WEAK PROTECTED 15 _environ
1107: 000000000028d2d8 8 OBJECT WEAK DEFAULT 15 environ
so i tried to -fno-lto to crt/*, dlstart.c and __environ.c
and then libc seemed to build correctly, but during tests
gcc lto1 ICE crashed. (i havent reported the bugs yet)
given these issues i'm not convinced that lto build of
libc is a good idea, but i attached a patch how the
string issues might be worked around.
View attachment "lto.diff" of type "text/x-diff" (1780 bytes)
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