[lldb] Correct address calculation for reading segment data (#120655)

This commit addresses a bug introduced in commit bcf654c, which
prevented LLDB from parsing the GNU build ID for the main executable
from a core file. The fix finds the `p_vaddr` of the first `PT_LOAD`
segment as the `base_addr` and subtract this `base_addr` from the
virtual address being read.

Co-authored-by: George Hu <[email protected]>

lldb/source/Plugins/Process/elf-core/ProcessElfCore.cpp

@@ -1031,6 +1031,8 @@ UUID ProcessElfCore::FindBuidIdInCoreMemory(lldb::addr_t address) {
 
   std::vector<uint8_t> ph_bytes;
   ph_bytes.resize(elf_header.e_phentsize);
+  lldb::addr_t base_addr = 0;
+  bool found_first_load_segment = false;
   for (unsigned int i = 0; i < elf_header.e_phnum; ++i) {
     byte_read = ReadMemory(ph_addr + i * elf_header.e_phentsize,
                            ph_bytes.data(), elf_header.e_phentsize, error);
@@ -1041,6 +1043,11 @@ UUID ProcessElfCore::FindBuidIdInCoreMemory(lldb::addr_t address) {
     offset = 0;
     elf::ELFProgramHeader program_header;
     program_header.Parse(program_header_data, &offset);
+    if (program_header.p_type == llvm::ELF::PT_LOAD &&
+        !found_first_load_segment) {
+      base_addr = program_header.p_vaddr;
+      found_first_load_segment = true;
+    }
     if (program_header.p_type != llvm::ELF::PT_NOTE)
       continue;
 
@@ -1049,7 +1056,7 @@ UUID ProcessElfCore::FindBuidIdInCoreMemory(lldb::addr_t address) {
 
     // We need to slide the address of the p_vaddr as these values don't get
     // relocated in memory.
-    const lldb::addr_t vaddr = program_header.p_vaddr + address;
+    const lldb::addr_t vaddr = program_header.p_vaddr + address - base_addr;
     byte_read =
         ReadMemory(vaddr, note_bytes.data(), program_header.p_memsz, error);
     if (byte_read != program_header.p_memsz)