[libc] Add dl_iterate_phdr to Linux x86-64 entrypoints

[boomanaiden154]

This is needed for building libunwind.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Aiden Grossman (boomanaiden154)

Changes

This is needed for building libunwind.

---

Full diff: https://github.com/llvm/llvm-project/pull/193601.diff

1 Files Affected:

• (modified) libc/config/linux/x86_64/entrypoints.txt (+3)

diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
index f67d658d0b176..51da06cec588d 100644
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -1454,6 +1454,9 @@ if(LLVM_LIBC_FULL_BUILD)
     libc.src.nl_types.catopen 
     libc.src.nl_types.catclose
     libc.src.nl_types.catgets
+
+    # link.h entrypoints
+    libc.src.link.dl_iterate_phdr
   )
 endif()
 

[frobtech]

This is needed for building libunwind.

AFAICT libunwind when using dl_iterate_phdr will use only dl_iterate_phdr even if statically linked. So it won't find anything at all if dl_iterate_phdr makes no callbacks. Hence I don't think the current no-op stub is sufficient for libunwind to actually function. The simplest implementation would probably be to just fill out one dl_phdr_info once in linux/do_start.cpp and use a copy of that to make a single callback for the executable in dl_iterate_phdr.

[boomanaiden154]

Hence I don't think the current no-op stub is sufficient for libunwind to actually function.

Ah. I was missing that this was only a stub implementation. For some reason I was under the impression that this was a full implementation.

Do we need to fill out a dl_phdr_info on startup and keep that around? I was under the impression that we could implement it by just sending over info for the main executable and vDSO without needing to keep anything around.