On 19 Feb 2021, at 15:18, Paul Floyd <pjfl...@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 <mask...@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
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