[PATCH] D130877: [MCObjectFileInfo] Add getPCSection() helper

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added inline comments.


================
Comment at: llvm/lib/MC/MCObjectFileInfo.cpp:1189
+  // SHF_WRITE for relocations, and let user post-process data in-place.
+  unsigned Flags = ELF::SHF_WRITE | ELF::SHF_ALLOC | ELF::SHF_LINK_ORDER;
+
----------------
dvyukov wrote:
> melver wrote:
> > dvyukov wrote:
> > > Do we actually plan to write to these sections? Does setting SHF_WRITE prevent sharing of pages across multiple processes? Of SHF_WRITE pages still mapped as shared/write-protected first and then subject to copy-on-write?
> > This was needed for relocations (because linker needs section writable if it contains relocations).
> > glibc seems to map sections as MAP_COPY (AFAIK), which is MAP_PRIVATE on Linux, so it'll COW.
> > 
> > Now that we have relative relocations, maybe we don't need it anymore, but I'd like to reserve the option to modify the data in-place (whether we do it, or someone else finds a use for it).
> > Changing it again in future is too painful, and we're likely stuck with whatever we decide now. I don't see a huge downside to it being writable (there's no security concern here).
> > 
> > Cc @MaskRay 
> If it's shared across processes and COW, then it should be fine.
Note: Relocate relocations usually refer to `R_*_RELATIVE`. PC-relative static relocations are very different and should not be conflated.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D130877/new/

https://reviews.llvm.org/D130877