linker error with gcc flag -mfunction-return=thunk

[Corinna Vinschen]

On Jun 25 14:16, Corinna Vinschen wrote:
> On Jun 25 14:08, Michael Haubenwallner wrote:
> > Hi,
> > 
> > I'm encountering a package's configure script (openssh-7.7p1) that
> > successfully tests for the compiler flag "-mfunction-return=thunk",
> > which causes subsequent linker errors (and fails to identify zlib):
> >  relocation truncated to fit: R_X86_64_32S against `.text'
> > 
> > The compiler used is gcc-7.3.0 with binutils-2.29.1.20171006 (current),
> > and the easiest test case here is:
> > 
> > $ echo 'int main() { return 0; }' > conftest.c
> > $ gcc -mfunction-return=thunk conftest.c
> > /tmp/ccv1Ird0.o:conftest.c:(.text+0x1a): relocation truncated to fit: R_X86_64_32S against `.text'
> > collect2: error: ld returned 1 exit status
> > 
> > Not sure though where to finally report this problem:
> > * openssh, as they should do a link-test rather than compile-test
> > * gcc, as their generated code is wrong for cygwin
> > * binutils, as their ld is unable to link that code
> > * cygwin, as the maintainers of cygwin support in binutils+gcc
> > 
> > After all, openssh-7.7p1-1.src cygwin package currently fails to compile.
> 
> configure with --without-hardening for the time being.

First of all, I think this needs fixing in gcc/binutils.

However, interesting problem!  I just tested this with your above
testcase and it failed, as expected.  Then I tweaked the openssh
configure.ac file to use OSSH_CHECK_CFLAG_LINK rather than
OSSH_CHECK_CFLAG_COMPILE and to my surprise the link stage
succeeded!  So I tried this again with the conftest.c file used
by the OpenSSH configure.ac and yes, it works!

$ cat > conftest.c <<EOF
#include <stdlib.h>
#include <stdio.h>
int main(int argc, char **argv) {
      /* Some math to catch -ftrapv problems in the toolchain */
      int i = 123 * argc, j = 456 + argc, k = 789 - argc;
      float l = i * 2.1;
      double m = l / 0.5;
      long long int n = argc * 12345LL, o = 12345LL * (long long int)argc;
      printf("%d %d %d %f %f %lld %lld\n", i, j, k, l, m, n, o);
      exit(0);
}
EOF
$ gcc -o conftest.exe -ggdb -O2 -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2 -fstack-protector-strong --param=ssp-buffer-size=4 -fdebug-prefix-map=/cygwin/src/openssh/openssh-7.7p1/openssh-7.7p1-1.x86_64/build=/usr/src/debug/openssh-7.7p1-1 -fdebug-prefix-map=/cygwin/src/openssh/openssh-7.7p1/openssh-7.7p1-1.x86_64/src/openssh-7.7p1=/usr/src/debug/openssh-7.7p1-1 -pipe -Wall -Wpointer-arith -Wuninitialized -Wsign-compare -Wformat-security -Wsizeof-pointer-memaccess -Wno-pointer-sign -Wno-unused-result -fno-strict-aliasing -Werror -mfunction-return=thunk conftest.c
$

Yes, no error.  The reason is that the code never returns due to calling
exit(0), rather than return 0.  Given that exit is a noreturn function,
the thunking doesn't take place in the testcase.

I check with OpenSSH upstream if this cqan be fixed from their side
as well somehow.


Thanks,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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