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From: Benjamin Barenblat <bbaren@google.com>
Subject: Fix symbolization on PowerPC ELF v1
Origin: backport, https://github.com/abseil/abseil-cpp/commit/372bfc86105728732fc115af46223d7a4e49f8d9
The big-endian PowerPC ELF ABI (ppc64 in Debian) relies on function descriptors
mapped in a non-executable segment. Make sure that segment is scanned during
symbolization. Also correct bounds computation for that segment.
--- a/absl/debugging/symbolize_elf.inc
+++ b/absl/debugging/symbolize_elf.inc
@@ -648,8 +648,10 @@
}
// Return true if an address is inside a section.
-static bool InSection(const void *address, const ElfW(Shdr) * section) {
- const char *start = reinterpret_cast<const char *>(section->sh_addr);
+static bool InSection(const void *address, ptrdiff_t relocation,
+ const ElfW(Shdr) * section) {
+ const char *start = reinterpret_cast<const char *>(
+ section->sh_addr + static_cast<ElfW(Addr)>(relocation));
size_t size = static_cast<size_t>(section->sh_size);
return start <= address && address < (start + size);
}
@@ -689,8 +691,8 @@
// starting address. However, we do not always want to use the real
// starting address because we sometimes want to symbolize a function
// pointer into the .opd section, e.g. FindSymbol(&foo,...).
- const bool pc_in_opd =
- kPlatformUsesOPDSections && opd != nullptr && InSection(pc, opd);
+ const bool pc_in_opd = kPlatformUsesOPDSections && opd != nullptr &&
+ InSection(pc, relocation, opd);
const bool deref_function_descriptor_pointer =
kPlatformUsesOPDSections && opd != nullptr && !pc_in_opd;
@@ -730,7 +732,7 @@
#endif
if (deref_function_descriptor_pointer &&
- InSection(original_start_address, opd)) {
+ InSection(original_start_address, /*relocation=*/0, opd)) {
// The opd section is mapped into memory. Just dereference
// start_address to get the first double word, which points to the
// function entry.
@@ -1326,7 +1328,7 @@
const int phnum = obj->elf_header.e_phnum;
const int phentsize = obj->elf_header.e_phentsize;
auto phoff = static_cast<off_t>(obj->elf_header.e_phoff);
- size_t num_executable_load_segments = 0;
+ size_t num_interesting_load_segments = 0;
for (int j = 0; j < phnum; j++) {
ElfW(Phdr) phdr;
if (!ReadFromOffsetExact(obj->fd, &phdr, sizeof(phdr), phoff)) {
@@ -1335,23 +1337,35 @@
return false;
}
phoff += phentsize;
- constexpr int rx = PF_X | PF_R;
- if (phdr.p_type != PT_LOAD || (phdr.p_flags & rx) != rx) {
- // Not a LOAD segment, or not executable code.
+
+#if defined(__powerpc__) && !(_CALL_ELF > 1)
+ // On the PowerPC ELF v1 ABI, function pointers actually point to function
+ // descriptors. These descriptors are stored in an .opd section, which is
+ // mapped read-only. We thus need to look at all readable segments, not
+ // just the executable ones.
+ constexpr int interesting = PF_R;
+#else
+ constexpr int interesting = PF_X | PF_R;
+#endif
+
+ if (phdr.p_type != PT_LOAD
+ || (phdr.p_flags & interesting) != interesting) {
+ // Not a LOAD segment, not executable code, and not a function
+ // descriptor.
continue;
}
- if (num_executable_load_segments < obj->phdr.size()) {
- memcpy(&obj->phdr[num_executable_load_segments++], &phdr, sizeof(phdr));
+ if (num_interesting_load_segments < obj->phdr.size()) {
+ memcpy(&obj->phdr[num_interesting_load_segments++], &phdr, sizeof(phdr));
} else {
ABSL_RAW_LOG(
- WARNING, "%s: too many executable LOAD segments: %zu >= %zu",
- obj->filename, num_executable_load_segments, obj->phdr.size());
+ WARNING, "%s: too many interesting LOAD segments: %zu >= %zu",
+ obj->filename, num_interesting_load_segments, obj->phdr.size());
break;
}
}
- if (num_executable_load_segments == 0) {
- // This object has no "r-x" LOAD segments. That's unexpected.
- ABSL_RAW_LOG(WARNING, "%s: no executable LOAD segments", obj->filename);
+ if (num_interesting_load_segments == 0) {
+ // This object has no interesting LOAD segments. That's unexpected.
+ ABSL_RAW_LOG(WARNING, "%s: no interesting LOAD segments", obj->filename);
return false;
}
}
@@ -1379,8 +1393,8 @@
// X in the file will have a start address of [true relocation]+X.
relocation = static_cast<ptrdiff_t>(start_addr - obj->offset);
- // Note: some binaries have multiple "rx" LOAD segments. We must
- // find the right one.
+ // Note: some binaries have multiple LOAD segments that can contain
+ // function pointers. We must find the right one.
ElfW(Phdr) *phdr = nullptr;
for (size_t j = 0; j < obj->phdr.size(); j++) {
ElfW(Phdr) &p = obj->phdr[j];
@@ -1390,7 +1404,7 @@
ABSL_RAW_CHECK(p.p_type == PT_NULL, "unexpected p_type");
break;
}
- if (pc < reinterpret_cast<void *>(start_addr + p.p_memsz)) {
+ if (pc < reinterpret_cast<void *>(start_addr + p.p_vaddr + p.p_memsz)) {
phdr = &p;
break;
}
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