[libunwind] Fix EH frame handling on illumos.

[jmcarp]

Illumos uses PT_SUNW_EH_FRAME and PT_SUNW_UNWIND program header types instead of PT_GNU_EH_FRAME for exception handling frames. This patch checks if the illumos headers are defined, and checks the appropriate fields in the ELF header if so.

FreeBSD defines PT_SUNW_UNWIND but not PT_SUNW_EH_FRAME for compatibility, so we check for both headers to avoid breaking FreeBSD builds.

Note: this is motivated by a project to add support for illumos to clickhouse. We initially sent this patch to clickhouse's fork of llvm, but would like to upstream the change first to prevent clickhouse's fork from further diverging from this repo.

cc @rschu1ze

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[llvmbot]

@llvm/pr-subscribers-libunwind

Author: Joshua Carp (jmcarp)

Changes

Illumos uses PT_SUNW_EH_FRAME and PT_SUNW_UNWIND program header types instead of PT_GNU_EH_FRAME for exception handling frames. This patch checks if the illumos headers are defined, and checks the appropriate fields in the ELF header if so.

FreeBSD defines PT_SUNW_UNWIND but not PT_SUNW_EH_FRAME for compatibility, so we check for both headers to avoid breaking FreeBSD builds.

Note: this is motivated by a project to add support for illumos to clickhouse. We initially sent this patch to clickhouse's fork of llvm, but would like to upstream the change first to prevent clickhouse's fork from further diverging from this repo.

cc @rschu1ze

---

Full diff: https://github.com/llvm/llvm-project/pull/176988.diff

1 Files Affected:

• (modified) libunwind/src/AddressSpace.hpp (+6)

diff --git a/libunwind/src/AddressSpace.hpp b/libunwind/src/AddressSpace.hpp
index 52477b16b355a..9b9e1d0a13073 100644
--- a/libunwind/src/AddressSpace.hpp
+++ b/libunwind/src/AddressSpace.hpp
@@ -424,7 +424,13 @@ static bool checkAddrInSegment(const Elf_Phdr *phdr, size_t image_base,
 static bool checkForUnwindInfoSegment(const Elf_Phdr *phdr, size_t image_base,
                                       dl_iterate_cb_data *cbdata) {
 #if defined(_LIBUNWIND_SUPPORT_DWARF_INDEX)
+#if defined(PT_SUNW_EH_FRAME) && defined(PT_SUNW_UNWIND)
+  // illumos/Solaris use PT_SUNW_EH_FRAME and PT_SUNW_UNWIND instead of PT_GNU_EH_FRAME.
+  // FreeBSD defines PT_SUNW_UNWIND but not PT_SUNW_EH_FRAME, so check for both.
+  if (phdr->p_type == PT_SUNW_EH_FRAME || phdr->p_type == PT_SUNW_UNWIND) {
+#else
   if (phdr->p_type == PT_GNU_EH_FRAME) {
+#endif
     EHHeaderParser<LocalAddressSpace>::EHHeaderInfo hdrInfo;
     uintptr_t eh_frame_hdr_start = image_base + phdr->p_vaddr;
     cbdata->sects->dwarf_index_section = eh_frame_hdr_start;

[MaskRay]

I believe major BSD, including FreeBSD, all use PT_GNU_EH_FRAME. The comment should be removed.

I think you just need to check one constant. PT_SUNW_EH_FRAME
(PT_SUNW_UNWIND, if defined, has the same value)

[MaskRay]

Actually, consider removing the source change and adding a cmake -D like Haiku did in https://reviews.llvm.org/D157866

[jmcarp]

Sorry if I'm misunderstanding, but I think PT_SUNW_EH_FRAME and PT_SUNW_UNWIND refer to different values. From the illumos source:

  PT_SUNW_UNWIND and PT_SUNW_EH_FRAME perform the same function,
  providing access to the .eh_frame_hdr section of the object.
  PT_SUNW_UNWIND is the original value, while PT_SUNW_EH_FRAME is
  required by the amd64 psABI. The Solaris link-editor (ld) tags output
  objects with PT_SUNW_UNWIND, but the Solaris runtime linker (ld.so.1)
  will accept and use either value.

  #define PT_SUNW_UNWIND    0x6464e550
  #define PT_SUNW_EH_FRAME  0x6474e550
  #define PT_GNU_EH_FRAME   PT_SUNW_EH_FRAME

So I think we need to check both values on illumos, and the cmake change wouldn't be enough. I updated the patch to fix the comment. Let me know if I'm missing something.

[jmcarp]

Ping, when you have time.