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

Tue Mar 12 18:01:18 PDT 2019

xiangzhangllvm added a comment.

In D58102#1426422 <https://reviews.llvm.org/D58102#1426422>, @peter.smith wrote:

> As an earlier comment mentioned, there is quite a lot of code that depends on the details of the .note.gnu.property section added very early to Driver.c which is not where we'd normally expect to see it. It also looks like you using quite a lot of code to get an InputSection to represent the output .note.gnu.property section. Given that there will be multiple Input .note.gnu.property sections but only one Output .note.gnu.property section, I think it would be cleaner to use a SyntheticSection to represent the .note.gnu.property section. SyntheticSections can implement all sorts of custom functionality that could move the parsing and merging code out of Driver.cpp. My suggestion is to:
>
> - Create a new SyntheticSection (see SyntheticSections.h), something like NotePropertySyntheticSection. This will represent our single output .note.gnu.property section. It can store the status of the FEATUREAND bits.
> - Add an entry to InStruct for a single instance of it that we will use for the output.
> - After InStruct instance is created in Writer.c but before the relocations are scanned to create PLT entries iterate through InputSections and pass each .note.gnu.property to the NotePropertySyntheticSection. Then erase these InputSections from the InputSections. The combineEhFrameSections<ELFT> function in Writer.cpp is one way of doing something like that.
> - When you add the .note.gnu.property InputSection to the NotePropertySyntheticSection you can update the FEATUREAND bits
> - The writeTo() member function of the NotePropertySyntheticSection can output the data needed to make up the output.
>
>   I think that this should allow you to integrate the .note.gnu.property in a similar way to other "merge" features such as EHFrame, and SHF_MERGE sections.
>
>   The ld.bfd AArch64 BTI patches have just gone up on https://www.sourceware.org/ml/binutils/2019-03/msg00021.html so I'll hopefully be starting work on another feature that uses .note.gnu.property soon.

Thanks very much for your suggestion, I was not much familiar to the function of  SyntheticSection before,  I'll carefully dig this class, It seems really not good to do the "calculate bits" work in Driver.cpp now. Thank you! peter

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https://reviews.llvm.org/D58102