clang options for load segments

[Dimitry Andric]

On 19 Feb 2021, at 15:18, Paul Floyd <pjfloyd at wanadoo.fr> wrote:
> 
> A while back when I upgraded to FreeBSD 12.2 (and thus to clang 10) I got quite a new category of errors with Valgrind.
> 
> The problem is that the clang 10 toolchain produces two RW LOAD segments, for instance see below. Valgrind assumes
> that there is only one, and ignores the second one which results in false positives when reading PLTs. I've added a hack
> to make it seem like there is just one such segment, but it isn't 100% reliable - there's at least one issue when loading
> shared libraries.

This changed in lld 9.0.0, with upstream r356226 (aka
https://github.com/llvm/llvm-project/commit/e8710ef1fbe8109eaa36143654f325dd345f8a0133 )

commit e8710ef1fbe8109eaa36143654f325dd345f8a01
Author: Fangrui Song <maskray at google.com>
Date:   Fri Mar 15 01:29:57 2019 +0000

    [ELF] Split RW PT_LOAD on the PT_GNU_RELRO boundary

    Summary:
    Based on Peter Collingbourne's suggestion in D56828.

    Before D56828: PT_LOAD(.data PT_GNU_RELRO(.data.rel.ro .bss.rel.ro) .bss)
    Old:           PT_LOAD(PT_GNU_RELRO(.data.rel.ro .bss.rel.ro) .data .bss)
    New:           PT_LOAD(PT_GNU_RELRO(.data.rel.ro .bss.rel.ro)) PT_LOAD(.data. .bss)

    The new layout reflects the runtime memory mappings.
    By having two PT_LOAD segments, we can utilize the NOBITS part of the
    first PT_LOAD and save bytes for .bss.rel.ro.

    .bss.rel.ro is currently small and only used by copy relocations of
    symbols in read-only segments, but it can be used for other purposes in
    the future, e.g. if a relro section's statically relocated data is all
    zeros, we can move it to .bss.rel.ro.

    Reviewers: espindola, ruiu, pcc

    Reviewed By: ruiu

    Subscribers: nemanjai, jvesely, nhaehnle, javed.absar, kbarton, emaste, arichardson, llvm-commits

    Tags: #llvm

    Differential Revision: https://reviews.llvm.org/D58892

    llvm-svn: 356226

In other words, this is shuffling the segments around a bit to achieve a
more optimal layout for relro.


> I will carry on looking for a proper solution. In the meantime, are there any flags to revert to the previous behaviour
> and only generate a single RW LOAD segment?

I think valgrind should be fixed to able to cope with additional
segments, but I haven't seen valgrind working on FreeBSD for years now,
so I am not going to hold my breath. :)

That said, you can attempt to link your executables with -z norelro (or
-Wl,-z,norelro via the compiler driver). If there is no PT_GNU_RELRO
header, lld will not split the segments.

And of course, you can link with lld 8.0 if all else fails.

-Dimitry

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 223 bytes
Desc: Message signed with OpenPGP
URL: <http://lists.freebsd.org/pipermail/freebsd-toolchain/attachments/20210302/8c253e10/attachment.sig>