svn commit: r251668 - head/lib/libc

[Nikolai Lifanov]

On 2013-06-12 17:12, Jeremie Le Hen wrote:
> Author: jlh
> Date: Wed Jun 12 21:12:05 2013
> New Revision: 251668
> URL: http://svnweb.freebsd.org/changeset/base/251668
> 
> Log:
>   Turn libc.so into an ld script rather than a symlink pointing to the
>   real shared object and libssp_nonshared.a.
> 
>   This was the last showstopper that prevented from enabling SSP for 
> ports
>   by default.  portmgr@ performed a buildworld which showed no 
> significant
>   breakage with this patch.
> 
>   Details:
> 
>   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.
> 
>   You can reproduce the problem on i386 by compiling a source file into 
> an
>   object file with "-fstack-protector-all -fPIE" and linking it
>   into a binary without "-fstack-protector".
> 
>   This ld script automatically proposes 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).
> 
>   A theorical performance impact could be when compiling, but my 
> testing
>   showed less than 0.1% of difference.
> 
>   [1] For 32-bit code gcc saves the PIC register setup by using
>       __stack_chk_fail_local hidden function instead of calling
>       __stack_chk_fail directly.  See comment line 19460 in:
>       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].
> 
>   PR:		ports/138228
>   PR:		ports/168010
>   Reviewed by:	kib, kan
> 
> Added:
>   head/lib/libc/libc.ldscript   (contents, props changed)
> Modified:
>   head/lib/libc/Makefile
> 
> Modified: head/lib/libc/Makefile
>  
>  
>  
>  
>  
>  
>  
> =======================================================================
> --- head/lib/libc/Makefile	Wed Jun 12 20:11:49 2013	(r251667)
> +++ head/lib/libc/Makefile	Wed Jun 12 21:12:05 2013	(r251668)
> @@ -23,6 +23,7 @@ LIBC_ARCH=${MACHINE_CPUARCH}
>  # to CFLAGS below.  -DSYSLIBC_SCCS affects just the system call stubs.
>  LIB=c
>  SHLIB_MAJOR= 7
> +SHLIB_LDSCRIPT=libc.ldscript
>  WARNS?=	2
>  CFLAGS+=-I${.CURDIR}/include -I${.CURDIR}/../../include
>  CFLAGS+=-I${.CURDIR}/${LIBC_ARCH}
> 
> Added: head/lib/libc/libc.ldscript
>  
>  
>  
>  
>  
>  
>  
> =======================================================================
> --- /dev/null	00:00:00 1970	(empty, because file is newly added)
> +++ head/lib/libc/libc.ldscript	Wed Jun 12 21:12:05 2013	(r251668)
> @@ -0,0 +1,2 @@
> +/* $FreeBSD$ */
> +GROUP ( @@SHLIB@@ @@LIBDIR@@/libssp_nonshared.a )
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This breaks installworld with read-only obj: something that was possible 
before.

install -s -o root -g wheel -m 444   -fschg -S  libc.so.7 /lib
sed -e 's,@@SHLIB@@,/lib/libc.so.7,g'  -e 's,@@LIBDIR@@,/usr/lib,g'  
/usr/src/lib/libc/libc.ldscript > libc.ld
cannot create libc.ld: Read-only file system
*** Error code 2

- Nikolai Lifanov