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Re: ld corrupting .cfi_label uses
- From: Alan Modra <amodra at gmail dot com>
- To: Jan Beulich <JBeulich at suse dot com>
- Cc: binutils at sourceware dot org
- Date: Fri, 3 Mar 2017 08:04:44 +1030
- Subject: Re: ld corrupting .cfi_label uses
- Authentication-results: sourceware.org; auth=none
- References: <58B04B47020000780013D936@prv-mh.provo.novell.com> <20170226234208.GA12547@bubble.grove.modra.org> <58B3E32902000078000F9844@prv-mh.provo.novell.com> <20170301024246.GG29416@bubble.grove.modra.org> <58B856CC020000780013F70D@prv-mh.provo.novell.com>
On Thu, Mar 02, 2017 at 09:30:52AM -0700, Jan Beulich wrote:
> >>> On 01.03.17 at 03:42, <email@example.com> wrote:
> > Yeah, I've run into the padding issue myself. It is a pain to deal
> > with. If one input .eh_frame has larger alignment than other
> > .eh_frame sections for whatever reason, then padding placed before
> > that section will be seen as a zero terminator.
> > I suspect that is why ld ensures FDEs are a multiple of eight in size
> > (it would be better to look at the output section alignment) because
> > gcc emits .eh_frame aligned to eight bytes on 64-bit targets. You can
> > pad with NOPs *inside* an FDE or CIE, not outside.
> > The following implements my suggestion re. output section alignment.
> > That may well cure your complaint.
> For .eh_frame with 4-byte alignment this indeed helps at least for the
> ld -r case. I haven't got to the final link part yet, as I have to fiddle
> with the object files in order to make the alignment such - even gas
> emits 8-byte alignment by default. And I can't seem to spot an
> objcopy option to alter the alignment of a section. I'll see if I can spot
> why gas behaves this way, and if I may possibly change it.
Good. I'm not going to commit the patch as is, because it breaks
powerpc64. From elf64-ppc.c:
/* Note that the glink_eh_frame check here is not only testing that
the generated size matched the calculated size but also that
bfd_elf_discard_info didn't make any changes to the section. */
So I have some further patches for powerpc64, and am modifying
elf-eh-frame.c to align CIEs and FDEs ideally. Current patch looks at
the FDE encoding to choose 4 or 8 byte alignment in order to make FDE
range start and size naturally aligned.
Australia Development Lab, IBM