/** Get the build-id (NT_GNU_BUILD_ID) from the ELF file
*
- * Return an empty vector is none is found.
+ * This build-id may is used to locate an external debug (DWARF) file
+ * for this ELF file.
+ *
+ * @param elf libelf handle for an ELF file
+ * @return build-id for this ELF file (or an empty vector if none is found)
*/
static
std::vector<char> get_build_id(Elf* elf)
size_t phnum;
if (elf_getphdrnum (elf, &phnum) != 0)
xbt_die("Could not read program headers");
+
+ // Iterate over the program headers and find the PT_NOTE ones:
for (size_t i = 0; i < phnum; ++i) {
GElf_Phdr phdr_temp;
GElf_Phdr *phdr = gelf_getphdr(elf, i, &phdr_temp);
if (phdr->p_type != PT_NOTE)
continue;
+
Elf_Data* data = elf_getdata_rawchunk(elf, phdr->p_offset, phdr->p_filesz, ELF_T_NHDR);
+
+ // Iterate over the notes and find the NT_GNU_BUILD_ID one:
size_t pos = 0;
while (1) {
GElf_Nhdr nhdr;
size_t name_pos;
size_t desc_pos;
pos = gelf_getnote(data, pos, &nhdr, &name_pos, &desc_pos);
+ // A note is identified by a name "GNU" and a integer type within
+ // the namespace defined by this name (here NT_GNU_BUILD_ID):
if (nhdr.n_type == NT_GNU_BUILD_ID
&& nhdr.n_namesz == sizeof("GNU")
&& memcmp((char*) data->d_buf + name_pos, "GNU", sizeof("GNU")) == 0) {
+
+ // Found the NT_GNU_BUILD_ID note:
char* start = (char*) data->d_buf + desc_pos;
char* end = (char*) start + nhdr.n_descsz;
return std::vector<char>(start, end);
+
}
}
+
}
return std::vector<char>();
}
'a', 'b', 'c', 'd', 'e', 'f'
};
+/** Binary data to hexadecimal */
static inline
std::array<char, 2> to_hex(std::uint8_t byte)
{
return { hexdigits[byte >> 4], hexdigits[byte & 0xF] };
}
-/** Hexadecimal representation of some binary data */
+/** Binary data to hexadecimal */
static
std::string to_hex(const char* data, std::size_t count)
{
return std::move(res);
}
+/** Binary data to hexadecimal */
static
std::string to_hex(std::vector<char> const& data)
{
return to_hex(data.data(), data.size());
}
+/** Base directories for external debug files */
const char* debug_paths[] = {
"/usr/lib/debug/",
- "/usr/local/lib/debug/"
+ "/usr/local/lib/debug/",
};
+/** Locate an external debug file from the NT_GNU_BUILD_ID
+ *
+ * This is one of the mechanisms used for
+ * [separate debug files](https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html).
+ */
static
std::string find_by_build_id(std::vector<char> id)
{
if (elf_version(EV_CURRENT) == EV_NONE)
xbt_die("libelf initialization error");
+ // Open the ELF file:
int fd = open(info->file_name.c_str(), O_RDONLY);
if (fd < 0)
xbt_die("Could not open file %s", info->file_name.c_str());
if (kind != ELF_K_ELF)
xbt_die("Not an ELF file 2");
+ // Remember if this is a `ET_EXEC` (fixed location) or `ET_DYN` (relocatable):
Elf64_Half type = get_type(elf);
if (type == ET_EXEC)
info->flags |= simgrid::mc::ObjectInformation::Executable;
+ // Read DWARF debug information in the file:
Dwarf* dwarf = dwarf_begin_elf (elf, DWARF_C_READ, nullptr);
if (dwarf != nullptr) {
- // This is the simple case where DWARF is located in the ELF file:
read_dwarf_info(info, dwarf);
dwarf_end(dwarf);
elf_end(elf);
}
dwarf_end(dwarf);
- // Try to find it with NT_GNU_BUILD_ID:
+ // If there was no DWARF in the file, try to find it in a separate file
+ // with NT_GNU_BUILD_ID:
std::vector<char> build_id = get_build_id(elf);
if (!build_id.empty()) {
elf_end(elf);
close(fd);
- // Find a debug file using the build id:
+ // Find the debug file using the build id:
std::string debug_file = find_by_build_id(build_id);
if (debug_file.empty()) {
std::string hex = to_hex(build_id);
}
// TODO, try to find DWARF info using debug-link.
- // Is this debug-link actually used anywhere?
+ // Is this method really used anywhere?
- // Everything failes, complain loudly:
xbt_die("Debugging information not found for %s\n"
"Try recompiling with -g\n",
info->file_name.c_str());