+ // Determine the base address by reading ELF headers in process memory.
+ ElfW(Ehdr) ehdr;
+ // Skip non-readable maps.
+ if (flags_start[0] == 'r' &&
+ ReadFromOffsetExact(mem_fd, &ehdr, sizeof(ElfW(Ehdr)), start_address) &&
+ memcmp(ehdr.e_ident, ELFMAG, SELFMAG) == 0) {
+ switch (ehdr.e_type) {
+ case ET_EXEC:
+ base_address = 0;
+ break;
+ case ET_DYN:
+ // Find the segment containing file offset 0. This will correspond
+ // to the ELF header that we just read. Normally this will have
+ // virtual address 0, but this is not guaranteed. We must subtract
+ // the virtual address from the address where the ELF header was
+ // mapped to get the base address.
+ //
+ // If we fail to find a segment for file offset 0, use the address
+ // of the ELF header as the base address.
+ base_address = start_address;
+ for (unsigned i = 0; i != ehdr.e_phnum; ++i) {
+ ElfW(Phdr) phdr;
+ if (ReadFromOffsetExact(
+ mem_fd, &phdr, sizeof(phdr),
+ start_address + ehdr.e_phoff + i * sizeof(phdr)) &&
+ phdr.p_type == PT_LOAD && phdr.p_offset == 0) {
+ base_address = start_address - phdr.p_vaddr;
+ break;
+ }
+ }
+ break;
+ default:
+ // ET_REL or ET_CORE. These aren't directly executable, so they don't
+ // affect the base address.
+ break;
+ }
+ }
+
+ // Check start and end addresses.
+ if (!(start_address <= pc && pc < end_address)) {
+ continue; // We skip this map. PC isn't in this map.
+ }
+