Re: RFC: Objcopy --only-keep-debug update

[Mark Wielaard <mjw at redhat dot com>]

On Wed, 2015-07-29 at 12:09 -0700, H.J. Lu wrote:
> On Wed, Jul 29, 2015 at 12:00 PM, Mark Wielaard <mjw@redhat.com> wrote:
> > On Wed, 2015-07-29 at 14:21 +0100, Nicholas Clifton wrote:
> >>  One problem I did have with this part of the patch was in determining
> >>  which sections should have their sh_link fields preserved.  In the
> >>  end I had to encode the section names directly, but I feel that their
> >>  ought to be a better way.
> >
> > I should prefix this with noting that eu-unstrip only got confused
> > because the sh_flags weren't preserved. In particular the missing
> > SHF_INFO_LINK on .plt.rela sections caused it to not match those in the
> > main and debug file. I have a workaround for that now:
> > https://lists.fedorahosted.org/pipermail/elfutils-devel/2015-July/005076.html
> >
> 
> Can you try binutils master branch?  I think I have fixed it.

It doesn't seem so. This is how I tried:

$ gcc --version | head -1
gcc (GCC) 5.1.1 20150618 (Red Hat 5.1.1-4)
$ ./objcopy --version | head -1
GNU objcopy (GNU Binutils) 2.25.51.20150729
$ cat > testprog.c <<EOF
int main() 
{
    return 0;
}
EOF
$ gcc -o testprog -g testprog.c
$ ./objcopy --only-keep-debug testprog testprog.debuginfo
$ eu-readelf -S testprog testprog.debuginfo | grep rela.plt
[10] .rela.plt            RELA         0000000000400378 00000378 00000030 24 AI     5  12  8
[10] .rela.plt            NOBITS       0000000000400378 00000298 00000030 24 A      0   0  8

Note how the SHF_INFO_LINK flag, sh_info and sh_link are missing in the
debuginfo file.

elfutils doesn't have a eu-objcopy, but when using eu-strip you'll get:

$ eu-strip --strip-debug -f testprog.debuginfo.eu testprog
$ eu-readelf -S testprog.debuginfo.eu | grep rela.plt
[10] .rela.plt            NOBITS       0000000000400378 00000280 00000030 24 AI     5  12  8

So only the sh_type and sh_offset are different. Of course elfutils only
deals with ELF files in the first place, so it is easier. The elfutils
patch referenced above contains a fuller testcase (testfiles generated
with gcc5 and objcopy, eu-unstrip them and compare with eu-elfcmp to the
original).

Cheers,

Mark