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Message-ID: <539E11E5.2080004@midipix.org>
Date: Sun, 15 Jun 2014 17:36:37 -0400
From: "writeonce@...ipix.org" <writeonce@...ipix.org>
To: musl@...ts.openwall.com
Subject: gcc'c crtstuff.c: a musl-related experience
Greetings,
I thought I should share a recent experience that I had while building a
cross compiler for i686-unknown-linux with x86_64 as a host. The
problem I encountered (and now solved) is yet another "penalty" for me
deviating from the norm, which in this case is gcc's way of hard-coding
libc-specific features into the toolchain (mglibc, muclibc, etc.) As my
toolchain does not provide such options, the compiler was mistakenly
thinking that glibc was the default, which led to the "incident" I'd
like to describe.
The two files at stake are gcc/linux.h, and libgcc/crtstuff.c. In the
former, a built-in macro named __gnu_linux__ is defined when glibc is
the toolchain's default libc (OPTION_GLIBC). In the latter, a macro
named USE_PT_GNU_EH_FRAME is defined when __gnu_linux__ is defined,
which accordingly prevents USE_EH_FRAME_REGISTRY from being defined
shortly thereafter. The absence of this last macro results in a
crtbegin.o that is incompatible with musl, at least in some cases (a
simple c++ application crashes after an exception has been thrown, which
is how I came to notice it).
The resulting file crtbegin.o is installed in one of the toolchain's
directories, and is thus shared between all the libc's targeted by the
toolchain. This means that when the toolchain's default libc is glibc,
specifying -mmusl would result in a binary that has an incompatible
crtbegin.o, at least in the above environment (i686 running on x86_64).
To complete the picture: in the musl-cross toolchains, glibc is _not_
the default libc, and __gnu_linux_ is accordingly not defined. All you
need to do in order to reproduce the above bug is moving around the
options so that glibc becomes the default. With the current code of
crtstuff.c, one easy way to avoid the bug while still keeping -mglibc,
-mmusl, etc. is to "manually" define USE_EH_FRAME_REGISTRY. Not ideal,
but still works.
zg
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