Rework findUnwindSectionsByPhdr to be more optimal.

[saugustine]

Summary

• Executable segment is usually segment 3. Look there for the address first.
• GNU_EH_FRAME_HEADER segment is usually near the end. Iterate from the end.
• Exit early if both phdrs have been found.

A subsequent patch will add a cache of found phdrs.

[jgorbe]

libunwind/src/AddressSpace.hpp

463–464

Is this early check no longer necessary/beneficial?

[saugustine]

libunwind/src/AddressSpace.hpp

463–464

I think it is still needed by the reverse-iteration below, which assumes there is one or more phdrs.

This is likely always true--Would dl_iterate_phdr ever pass a phdr-less pinfo? But I don't think dl_iterate_phdr actually guarantees that, at least not in any documentation I have read.

[jgorbe]

libunwind/src/AddressSpace.hpp

502–503

The old code only touches dwarf_section_length if both phdrs are found, whereas this patch always resets it to 0 after the loop if it didn't find both. Is this intentional?

[saugustine]

libunwind/src/AddressSpace.hpp

463–464

Actually, I responded to the wrong question. You are right that this code is still useful. Restored.

502–503

It is. I believe the code is equivalent if dwarf_section_length is always zero on entry (which it is). The new code avoids carrying object_length as a separate variable, and trying to figure out what it means.

[saugustine]

Update for upstream comments

[jgorbe]

Thanks!

libunwind/src/AddressSpace.hpp

struct _LIBUNWIND_HIDDEN dl_iterate_cb_data {
   uintptr_t targetAddr;
 };
 
 #if defined(_LIBUNWIND_SUPPORT_DWARF_UNWIND)
   #if !defined(_LIBUNWIND_SUPPORT_DWARF_INDEX)
     #error "_LIBUNWIND_SUPPORT_DWARF_UNWIND requires _LIBUNWIND_SUPPORT_DWARF_INDEX on this platform."
   #endif
 
-int findUnwindSectionsByPhdr(struct dl_phdr_info *pinfo, size_t, void *data) {
-  auto *cbdata = static_cast<dl_iterate_cb_data *>(data);
-  bool found_obj = false;
-  bool found_hdr = false;
-
-  assert(cbdata);
-  assert(cbdata->sects);
-
-  if (cbdata->targetAddr < pinfo->dlpi_addr)
-    return 0;
-
-  Elf_Addr image_base = calculateImageBase(pinfo);
-  size_t object_length;
-
-  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 = image_base + 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;
-        found_obj = true;
-      }
-    } else if (phdr->p_type == PT_GNU_EH_FRAME) {
+static bool checkAddrInSegment(const Elf_Phdr *phdr, size_t image_base,
+                               dl_iterate_cb_data *cbdata) {
+      cbdata->sects->dwarf_section_length = phdr->p_memsz;
+      return true;
+    }
+  return false;
+}
+
+int findUnwindSectionsByPhdr(struct dl_phdr_info *pinfo, size_t, void *data) {
+  auto cbdata = static_cast<dl_iterate_cb_data *>(data);
+  if (pinfo->dlpi_phnum == 0 || cbdata->targetAddr < pinfo->dlpi_addr)
+    return 0;
+
+  Elf_Addr image_base = calculateImageBase(pinfo);
+  bool found_obj = false;
+  bool found_hdr = false;
+
+  // Third phdr is usually the executable phdr.
+  if (pinfo->dlpi_phnum > 2)
+    found_obj = checkAddrInSegment(&pinfo->dlpi_phdr[2], image_base, cbdata);
+
+  // PT_GNU_EH_FRAME is usually near the end. Iterate backward. We already know
+  // that there is one or more phdrs.
+  for (Elf_Half i = pinfo->dlpi_phnum; i > 0; i--) {
+    const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i - 1];
+    if (!found_hdr && phdr->p_type == PT_GNU_EH_FRAME) {
       EHHeaderParser<LocalAddressSpace>::EHHeaderInfo hdrInfo;
       uintptr_t eh_frame_hdr_start = image_base + phdr->p_vaddr;
       cbdata->sects->dwarf_index_section = eh_frame_hdr_start;
       cbdata->sects->dwarf_index_section_length = phdr->p_memsz;
       found_hdr = EHHeaderParser<LocalAddressSpace>::decodeEHHdr(
           *cbdata->addressSpace, eh_frame_hdr_start, phdr->p_memsz,
           hdrInfo);
       if (found_hdr)
         cbdata->sects->dwarf_section = hdrInfo.eh_frame_ptr;
+    } else if (!found_obj) {
+      found_obj = checkAddrInSegment(phdr, image_base, cbdata);
     }
+    if (found_obj && found_hdr)
+      return 1;
   }
-
-  if (found_obj && found_hdr) {
-    cbdata->sects->dwarf_section_length = object_length;
-    return true;
-  } else {
-    return false;
-  }
+  cbdata->sects->dwarf_section_length = 0;
+  return 0;
 }
 
 #else  // defined(LIBUNWIND_SUPPORT_DWARF_UNWIND)
 // Given all the #ifdef's above, the code here is for
 // defined(LIBUNWIND_ARM_EHABI)
 
 int findUnwindSectionsByPhdr(struct dl_phdr_info *pinfo, size_t, void *data) {
   auto *cbdata = static_cast<dl_iterate_cb_data *>(data);