Re: [gentoo-user] gcc 6 PIE use flag

[Walter Dnes]

On Mon, May 08, 2017 at 08:46:54AM +1000, Adam Carter wrote
> Since an update to the default USE flags on gcc 6 turned on PIE and SSP,
> i'm getting these errors;
>
> /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
> atof-generic.o: relocation R_X86_64_32 against `.rodata' can not be used
> when making a shared object; recompile with -fPIC
> /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
> dw2gencfi.o: relocation R_X86_64_32 against `.rodata.str1.8' can not be
> used when making a shared object; recompile with -fPIC
> /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
> frags.o: relocation R_X86_64_32 against `.rodata.str1.8' can not be used
> when making a shared object; recompile with -fPIC
> /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
> messages.o: relocation R_X86_64_32 against `.rodata.str1.1' can not be used
> when making a shared object; recompile with -fPIC
> /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
> subsegs.o: relocation R_X86_64_32 against `.rodata' can not be used when
> making a shared object; recompile with -fPIC
> /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
> symbols.o: relocation R_X86_64_32S against symbol `_sch_toupper' can not be
> used when making a shared object; recompile with -fPIC
> /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
> atof-ieee.o: relocation R_X86_64_32S against `.rodata' can not be used when
> making a shared object; recompile with -fPIC
> /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
> symbols.o: warning: relocation against `strlen@@GLIBC_2.2.5' in readonly
> section `.text'
> /usr/lib/gcc/x86_64-pc-linux-gnu/6.3.0/../../../../x86_64-pc-linux-gnu/bin/ld:
> final link failed: Nonrepresentable section on output
> collect2: error: ld returned 1 exit status
>
> If I add -fPIC to CFLAGS it will compile. Interestingly some packages (eg
> PHP) will compile without the -fPIC. Also the man page says; "This option
> makes a difference on AArch64, m68k, PowerPC and SPARC:, whereas i'm
> running amd64.
>
> What's going on here?

  It's a feature, not a bug. <G> There's a news item being worked on in
the Gentoo dev list.  I'd normally just link to a mail archive, but it's
too new to have been archived yet.  Here's a copy of the proposed news
item.  Note the example error message about relocation against .rodata

#######################################################################
Title: GCC 6 defaults to USE="pie ssp"
Author: Matthias Maier <tamiko@g.o>
Content-Type: text/plain
Posted: 2017-05-07
Revision: 1
News-Item-Format: 1.0
Display-If-Installed: >=sys-devel/gcc-6.3.0
Display-If-Keyword: amd64

In Gentoo, several GCC features can be default disabled or enabled
via use-flags of sys-devel/gcc. Starting with gcc-4.8.3 we have already
enabled default SSP [1]. Since the PIE patchset for default position
independent executable support was integrated upstream [2,3], starting
with gcc-6.3 we are also enabling PIE by default (via a default-enabled
use-flag pie) in regular (non-hardened) profiles.

[Additionally, following Gentoo policies, the default-off use-flags
nopie (only present in Hardened) and nossp are replaced starting with
gcc-6 by default-on use-flags pie and ssp.]

Be advised that switching from an older version to GCC 6 will enable the
PIE feature by default. This should not cause many problems, but it may
be necessary to recompile parts of your userland. An indicator are
linker errors of the form [4]

  relocation R_X86_64_32 against `.rodata.str1.1' can not be used when
  making a shared object; recompile with -fPIC

[1] https://www.gentoo.org/support/news-items/2014-06-15-gcc48_ssp.html
[2] https://gcc.gnu.org/gcc-6/changes.html
[3] A big thanks to all developers and members of the Gentoo community that
    made upstreaming the pie patchset and other hardening options possible!
[4] https://bugs.gentoo.org/617698
#######################################################################


-- 
Walter Dnes <waltdnes@××××××××.org>
I don't run "desktop environments"; I run useful applications