Handle gdb core files better

Core files created by gdb can have sections missing that would be
present in OS created core files.

We work around this by first reading PT_LOAD entries from the exe and
then reading them from the core.

Fixes #1121

pkg/proc/core/linux_amd64_core.go

@@ -256,16 +256,23 @@ func readCore(corePath, exePath string) (*Core, error) {
 	if err != nil {
 		return nil, err
 	}
+	exeELF, err := elf.NewFile(exe)
+	if err != nil {
+		return nil, err
+	}
 
 	if coreFile.Type != elf.ET_CORE {
 		return nil, fmt.Errorf("%v is not a core file", coreFile)
 	}
+	if exeELF.Type != elf.ET_EXEC {
+		return nil, fmt.Errorf("%v is not an exe file", exeELF)
+	}
 
 	notes, err := readNotes(coreFile)
 	if err != nil {
 		return nil, err
 	}
-	memory := buildMemory(coreFile, exe, notes)
+	memory := buildMemory(coreFile, exeELF, exe, notes)
 
 	core := &Core{
 		MemoryReader: memory,
@@ -417,7 +424,7 @@ func skipPadding(r io.ReadSeeker, pad int64) error {
 	return nil
 }
 
-func buildMemory(core *elf.File, exe io.ReaderAt, notes []*Note) proc.MemoryReader {
+func buildMemory(core, exeELF *elf.File, exe io.ReaderAt, notes []*Note) proc.MemoryReader {
 	memory := &SplicedMemory{}
 
 	// For now, assume all file mappings are to the exe.
@@ -434,16 +441,21 @@ func buildMemory(core *elf.File, exe io.ReaderAt, notes []*Note) proc.MemoryRead
 
 		}
 	}
-	for _, prog := range core.Progs {
-		if prog.Type == elf.PT_LOAD {
-			if prog.Filesz == 0 {
-				continue
-			}
-			r := &OffsetReaderAt{
-				reader: prog.ReaderAt,
-				offset: uintptr(prog.Vaddr),
+
+	// Load memory segments from exe and then from the core file,
+	// allowing the corefile to overwrite previously loaded segments
+	for _, elfFile := range []*elf.File{exeELF, core} {
+		for _, prog := range elfFile.Progs {
+			if prog.Type == elf.PT_LOAD {
+				if prog.Filesz == 0 {
+					continue
+				}
+				r := &OffsetReaderAt{
+					reader: prog.ReaderAt,
+					offset: uintptr(prog.Vaddr),
+				}
+				memory.Add(r, uintptr(prog.Vaddr), uintptr(prog.Filesz))
 			}
-			memory.Add(r, uintptr(prog.Vaddr), uintptr(prog.Filesz))
 		}
 	}
 	return memory