Drop the dependency on dl_unwind_find_exidx().

[ed]

Summary

While porting libunwind over to CloudABI for ARMv6, I observed that this
source file doesn't build, as it depends on dl_unwind_find_exidx(),
which CloudABI's C library was lacking. After I added that function,
I still needed to patch up libunwind to define _Unwind_Ptr.

Taking a step back, I wonder why we need to make use of this function
anyway. The unwinder already has some nice code to use dl_iterate_phdr()
to scan for a PT_GNU_EH_FRAME header. The dl_unwind_find_exidx() does
the same thing, except matching PT_ARM_EXIDX instead. We could also do
that ourselves.

This change gets rid of the dl_unwind_find_exidx() call and extends the
dl_iterate_phdr() loop. In addition to making the code a bit shorter, it
has the advantage of getting rid of some of those OS-specific #ifdefs.

This now means that if an operating system only provides
dl_iterate_phdr(), it gets support for unwinding on all architectures.
There is no need to add more stuff, just to get ARMv6 support.

[ed]

Interesting observation: dwarf_index_section_length stores the length of the section, whereas arm_section_length stores the number of entries in the section. This feels a bit unnatural in my opinion. Should this be tackled in this change as well, or do we want to do this separately?

[emaste]

This seems reasonable to me.

IMO I would propose the dwarf_index_section_length vs arm_section_length change in a separate review.

src/AddressSpace.hpp

430 ↗

(On Diff #82422)

perhaps a comment on the #else indicating which case it's for?

[ed]

Add a comment after the 'else' to indicate this code applies to ARM.

[ed]

Also an interesting observation: _LIBUNWIND_IS_BAREMETAL seems to be used only in this single source file and only provides an implementation for ARMv6.

Given that we're not implementing support for this for any other hardware architectures (x86, ARM64), is running on bare metal configurations even something we're aiming at? Is anyone using this? If not, should we consider removing it?

[ed]

Friendly ping! :-)

[ed]

Sync to latest codebase.

[ed]

Restore comment that was lost during sync to latest code.

[ed]

Any more feedback before I commit this change to the trunk?

libunwind/trunk/src/AddressSpace.hpp

 #endif
 
 #include "libunwind.h"
 #include "config.h"
 #include "dwarf2.h"
 #include "Registers.hpp"
 
 #if _LIBUNWIND_ARM_EHABI
-#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__OpenBSD__)
-
-typedef void *_Unwind_Ptr;
-
-#elif defined(__linux__)
-
-typedef long unsigned int *_Unwind_Ptr;
-extern "C" _Unwind_Ptr __gnu_Unwind_Find_exidx(_Unwind_Ptr addr, int *len);
-
-// Emulate the BSD dl_unwind_find_exidx API when on a GNU libdl system.
-#define dl_unwind_find_exidx __gnu_Unwind_Find_exidx
-
-#elif !defined(_LIBUNWIND_IS_BAREMETAL)
-#include <link.h>
-#else // !defined(_LIBUNWIND_IS_BAREMETAL)
-// When statically linked on bare-metal, the symbols for the EH table are looked
-// up without going through the dynamic loader.
 struct EHTEntry {
   uint32_t functionOffset;
   uint32_t unwindOpcodes;
 };
+#if defined(_LIBUNWIND_IS_BAREMETAL)
+// When statically linked on bare-metal, the symbols for the EH table are looked
+// up without going through the dynamic loader.
 extern EHTEntry __exidx_start;
 extern EHTEntry __exidx_end;
+#else
+#include <link.h>
 #endif // !defined(_LIBUNWIND_IS_BAREMETAL)
 #endif // _LIBUNWIND_ARM_EHABI
 
 #if defined(__CloudABI__) || defined(__FreeBSD__) || defined(__Fuchsia__) ||  \
     defined(__linux__) || defined(__NetBSD__)
 #if _LIBUNWIND_SUPPORT_DWARF_UNWIND && _LIBUNWIND_SUPPORT_DWARF_INDEX
 #include <link.h>
 // Macro for machine-independent access to the ELF program headers. This

  #if _LIBUNWIND_SUPPORT_DWARF_UNWIND
     info.dwarf_section                 = (uintptr_t)dyldInfo.dwarf_section;
     info.dwarf_section_length          = dyldInfo.dwarf_section_length;
  #endif
     info.compact_unwind_section        = (uintptr_t)dyldInfo.compact_unwind_section;
     info.compact_unwind_section_length = dyldInfo.compact_unwind_section_length;
     return true;
   }
-#elif _LIBUNWIND_ARM_EHABI
- #ifdef _LIBUNWIND_IS_BAREMETAL
+#elif _LIBUNWIND_ARM_EHABI && defined(_LIBUNWIND_IS_BAREMETAL)
   // Bare metal is statically linked, so no need to ask the dynamic loader
   info.arm_section =        (uintptr_t)(&__exidx_start);
   info.arm_section_length = (uintptr_t)(&__exidx_end - &__exidx_start);
- #else
-  int length = 0;
-  info.arm_section = (uintptr_t) dl_unwind_find_exidx(
-      (_Unwind_Ptr) targetAddr, &length);
-  info.arm_section_length = (uintptr_t)length;
- #endif
   _LIBUNWIND_TRACE_UNWINDING("findUnwindSections: section %X length %x",
                              info.arm_section, info.arm_section_length);
   if (info.arm_section && info.arm_section_length)
     return true;
-#elif _LIBUNWIND_SUPPORT_DWARF_UNWIND
-#if _LIBUNWIND_SUPPORT_DWARF_INDEX
+#elif _LIBUNWIND_ARM_EHABI || _LIBUNWIND_SUPPORT_DWARF_UNWIND
   struct dl_iterate_cb_data {
     LocalAddressSpace *addressSpace;
     UnwindInfoSections *sects;
     uintptr_t targetAddr;
   };
 
   dl_iterate_cb_data cb_data = {this, &info, targetAddr};
   int found = dl_iterate_phdr(
       [](struct dl_phdr_info *pinfo, size_t, void *data) -> int {
         auto cbdata = static_cast<dl_iterate_cb_data *>(data);
-        size_t object_length;
-        bool found_obj = false;
-        bool found_hdr = false;
 
         assert(cbdata);
         assert(cbdata->sects);
 
         if (cbdata->targetAddr < pinfo->dlpi_addr) {
           return false;
         }
 
 #if !defined(Elf_Half)
         typedef ElfW(Half) Elf_Half;
 #endif
 #if !defined(Elf_Phdr)
         typedef ElfW(Phdr) Elf_Phdr;
 #endif
 
+ #if _LIBUNWIND_SUPPORT_DWARF_UNWIND
+  #if !_LIBUNWIND_SUPPORT_DWARF_INDEX
+   #error "_LIBUNWIND_SUPPORT_DWARF_UNWIND requires _LIBUNWIND_SUPPORT_DWARF_INDEX on this platform."
+  #endif
+        size_t object_length;
+        bool found_obj = false;
+        bool found_hdr = false;
+
         for (Elf_Half i = 0; i < pinfo->dlpi_phnum; i++) {
           const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i];
           if (phdr->p_type == PT_LOAD) {
             uintptr_t begin = pinfo->dlpi_addr + phdr->p_vaddr;
             uintptr_t end = begin + phdr->p_memsz;
             if (cbdata->targetAddr >= begin && cbdata->targetAddr < end) {
               cbdata->sects->dso_base = begin;
               object_length = phdr->p_memsz;
#endif
         }
 
         if (found_obj && found_hdr) {
           cbdata->sects->dwarf_section_length = object_length;
           return true;
         } else {
           return false;
         }
+ #else // _LIBUNWIND_ARM_EHABI
+        for (Elf_Half i = 0; i < pinfo->dlpi_phnum; i++) {
+          const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i];
+          if (phdr->p_type == PT_ARM_EXIDX) {
+            uintptr_t exidx_start = pinfo->dlpi_addr + phdr->p_vaddr;
+            cbdata->sects->arm_section = exidx_start;
+            cbdata->sects->arm_section_length = phdr->p_memsz /
+                                                sizeof(EHTEntry);
+            return true;
+          }
+        }
+        return false;
+ #endif
       },
       &cb_data);
   return static_cast<bool>(found);
-#else
-#error "_LIBUNWIND_SUPPORT_DWARF_UNWIND requires _LIBUNWIND_SUPPORT_DWARF_INDEX on this platform."
-#endif
 #endif
 
   return false;
 }
 
 
 inline bool LocalAddressSpace::findOtherFDE(pint_t targetAddr, pint_t &fde) {
 #ifdef __APPLE__