Turn libc.so into an ld script

[Bryan Drewery]

I've been running my systems with this modification since Feb 2012 and
have seen no problems beyond file(1) usage on /usr/lib/libc.so in
openssl's configure.

I've taken ports/168010 and ports/138228 for exp-runs. I want to get
(optional) SSP support into ports this year.

I'll start a libc.ld exp-run tomorrow. It will be ran against
9.1-RELEASE since HEAD currently only has 1/2 of the ports tree passing
due to the clang switch.

Bryan

On 5/25/2013 3:06 AM, Jeremie Le Hen wrote:
> Hi,
> 
> There has been quite a while since the SSP glue has been committed in
> the tree.  Yet there has always been a gloomy corner case since then
> that was reported from time to time, mainly by port maintainers, which
> has been hard to reproduce.  This is the main showstopper to enable SSP
> for ports by default.
> 
> On i386 for PIC objects, gcc uses the __stack_chk_fail_local hidden
> symbol instead of calling __stack_chk_fail directly [1].  This happen
> not only with our gcc-4.2.1 but also with the latest gcc-4.8.  If you
> want the very nasty details, see [2].
> 
> OTOH the problem doesn't exist on other architectures.  It also doesn't
> exist with Clang as the latter will somehow manage to create the
> function in the object file at compile time (contrary to only
> referencing it through a symbol that will be brought in at link time).
> 
> In a perfect world, when an object file is compiled with
> -fstack-protector, it will be linked into a binary or a DSO with this
> same flag as well, so GCC will add libssp_nonshared.a to the linker
> command-line.  Unfortunately, we don't control softwares in ports and we
> may have such broken DSO.  This is the whole point of this patch.
> 
> I wrote a specific test that exhibits the error:
> 	http://people.freebsd.org/~jlh/twisted_ssp_linktime_fail.shar
> If you run "make main" on i386, it will fail.  More details at [3].
> 
> So the attached patch turns libc.so into an ld script which will
> automatically _propose_ libssp_nonshared.a along with the real libc DSO
> to the linker.  It is important to understand that the object file
> contained in this library will be pulled in the resulting binary
> _only if_ the linker notices one of its symbols is needed (i.e. one of
> the SSP symbol is missing).  Otherwise nothing is changed, except a
> slight theorical overhead that I wasn't able to measure on my Core 2
> developement machine with -j 4:
> 
> x current
> + current_libc_ldscript
> +------------------------------------------------------------------------------+
> |           ++ x+ xx                                             +            x|
> |||_____________M__M_______A____A___________________|__________|               |
> +------------------------------------------------------------------------------+
>     N           Min           Max        Median           Avg        Stddev
> x   4          9130          9227          9136          9157     46.740418
> +   4          9126          9207          9132          9148     39.420807
> 
> 
> Any objection to the patch?
> 
> Thanks for reading,
> 
> 
> [1] See comment here if you wonder why:
>     sed -n 19460,+3p src/contrib/gcc/config/i386/i386.c
> 
> [2] When compiling a source file to an object file, if you use something
>     which is external to the compilation unit, GCC doesn't know yet if
>     this symbol will be inside or outside the DSO.  So it expects the
>     worst case and routes the symbol through the GOT, which means
>     additional space and extra relocation for rtld(1).
> 
>     Declaring a symbol has hidden tells GCC to use the optimal route (no
>     GOT), but on the other hand this means the symbol has to be provided
>     in the same DSO (namely libssp_nonshared.a).
> 
>     On i386, GCC actually uses an hidden symbol for SSP in PIC objects
>     to save PIC register setup, as said in [1].
> 
> [3] As abstractly explained in [2], the problem shows up as long as you
>     compile a PIC (or PIE) object but you don't link it directly with
>     libssp_nonshared.a.
>     
>     So in the test I gave, you can also trigger the problem by setting
>     "BIN_CFLAGS= -fstack-protector-all -fPIE" and leaving BIN_LDFLAGS
>     blank, whatever you did with LIB_{CFLAGS,LDFLAGS}.
> 
>     This won't happen without -fPIE here, because a non-hidden symbol
>     will be emitted in that case.
> 
> 
> 
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-- 
Regards,
Bryan Drewery

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