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Date: Wed, 05 Nov 2014 16:25:03 +0100
From: John Spencer <maillist-musl@...fooze.de>
To: musl@...ts.openwall.com
CC: Gregor Richards <gr@...due.edu>
Subject: fixing -fPIE + -fstack-protector-all
using -fPIE + -fstack-protector-all is currently broken for a number of
architectures (most notably i386) in the default gcc setup (including
the musl-cross patches), as it depends on a libssp_nonshared.a which
provides __stack_chk_fail_local().
even when gcc's version is built from ssp-local.c and installed via
`make install-target-libssp`, gcc won't use it to link programs compiled
with -fstack-protector[-all].
the reason is that (since we provide the rest of the ssp functionality
in musl) we set gcc_cv_libc_provides_ssp=yes, which, contrary to our
earlier expectations means: "libc provides ssp *and* ssp_nonshared":
(from gcc/gcc.c)
#ifdef TARGET_LIBC_PROVIDES_SSP
#define LINK_SSP_SPEC "%{fstack-protector:}"
#else
#define LINK_SSP_SPEC
"%{fstack-protector|fstack-protector-all:-lssp_nonshared -lssp}"
#endif
so my conclusion is that in order to fix this issue cleanly and get musl
support into upstream, we need to either (on the gcc side) define a new
gcc_cv_libc_provides_ssp_but_not_ssp_nonshared conditional and an
install target that installs only libssp_nonshared but not libssp, and
links only to libssp_nonshared if ssp was used, or somehow get that
symbol (__stack_chk_fail_local()) into musl and linked to binaries in a
way that doesn't require additional library flags on the linker command
line.
in the former case, the above snippet would look like
#ifdef TARGET_LIBC_PROVIDES_SSP
#define LINK_SSP_SPEC "%{fstack-protector:}"
#elif TARGET_LIBC_PROVIDES_SSP_BUT_NOT_SSP_NONSHARED
#define LINK_SSP_SPEC
"%{fstack-protector|fstack-protector-all:-lssp_nonshared}"
#else
#define LINK_SSP_SPEC
"%{fstack-protector|fstack-protector-all:-lssp_nonshared -lssp}"
#endif
--JS
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