[PATCH] D58102: Support X86 Control-flow Enforcement Technology (CET) in LLD

Fri Mar 8 17:57:38 PST 2019

xiangzhangllvm added a comment.

In D58102#1422698 <https://reviews.llvm.org/D58102#1422698>, @peter.smith wrote:

> I think  https://bugs.llvm.org/show_bug.cgi?id=41000 "PT_NOTE segment not packed correctly" has some relevance to this review. In particular the .note.gnu.property sections on 64-bit machines have 8-byte alignment, merging these with 4-byte aligned note sections such as build-id can cause problems for consumers of PT_NOTE (see also https://sourceware.org/ml/libc-alpha/2018-08/msg00340.html for a discussion on the glibc mailing list). My understanding is that the eventual solution was to collate 4 and 8 byte SHT_NOTE sections separately and output a PT_NOTE section for each. I think that we should follow that and have tests for it.

Good discovery！
I think we need change the .note.gnu.property section alignment to 4-byte in Clang-TableGen, because all its element is 4-byte required now. 
The old ABI required some of note.gnu.property' elements be 8-byte aligned in 64-bit machine, so the section required 8-byte alignment.
Last year, I found a bug about this alignment between GCC and CLANG, Then I discussed with the ABI designer and changed its element's alignment in https://reviews.llvm.org/D56080 .
I think there are already no ‘significance’ of 8 byte alignment for this section, we can change it now in LLVM.

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https://reviews.llvm.org/D58102