debug/elf: handle sections with non-zero addresses when applying relo…

…cations

commit 72ec930 added basic support for
relocations, but assumed that the symbol value would be 0, likely because
.debug_info always has address == 0 in the ELF section headers.

commit df855da added further support
for relocations, but explicitly encoded the original assumption that
section addresses would be 0.

This commit removes that assumption, and adds support for relocations
relative to sections that have non-zero addresses.
Typically, sections that are part of a LOAD program segment are such
sections. For example, .debug_ranges relocations could be relative to
.text, which usually has an address > 0.

This addresses golang#40879, causing
relocations for .debug_ranges to be incorrectly applied.

src/debug/elf/file.go

@@ -630,17 +630,14 @@ func (f *File) applyRelocations(dst []byte, rels []byte) error {
 
 // relocSymbolTargetOK decides whether we should try to apply a
 // relocation to a DWARF data section, given a pointer to the symbol
-// targeted by the relocation. Most relocations in DWARF data tend to
-// be section-relative, but some target non-section symbols (for
-// example, low_PC attrs on subprogram or compilation unit DIEs that
-// target function symbols), and we need to include these as well.
+// targeted by the relocation.
+// Note: most relocations in DWARF data tend to be section-relative,
+// but some target non-section symbols (for example, low_PC attrs on
+// subprogram or compilation unit DIEs that target function symbols).
 // Return value is a pair (X,Y) where X is a boolean indicating
 // whether the relocation is needed, and Y is the symbol value in the
 // case of a non-section relocation that needs to be applied.
 func relocSymbolTargetOK(sym *Symbol) (bool, uint64) {
-	if ST_TYPE(sym.Info) == STT_SECTION {
-		return true, 0
-	}
 	if sym.Section != SHN_UNDEF && sym.Section < SHN_LORESERVE {
 		return true, sym.Value
 	}

src/debug/elf/file_test.go

@@ -293,6 +293,7 @@ func decompress(gz string) (io.ReaderAt, error) {
 type relocationTestEntry struct {
 	entryNumber int
 	entry       *dwarf.Entry
+	pcRanges    [][2]uint64
 }
 
 type relocationTest struct {
@@ -317,7 +318,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: uint64(0x6), Class: dwarf.ClassAddress},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -336,7 +337,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: uint64(0x5), Class: dwarf.ClassAddress},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -355,7 +356,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: uint64(0x6), Class: dwarf.ClassAddress},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -374,7 +375,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: int64(0x24), Class: dwarf.ClassConstant},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -393,7 +394,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: int64(0x28), Class: dwarf.ClassConstant},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -412,7 +413,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrLowpc, Val: uint64(0x0), Class: dwarf.ClassAddress},
 					{Attr: dwarf.AttrHighpc, Val: int64(48), Class: dwarf.ClassConstant},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -431,7 +432,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: int64(0x44), Class: dwarf.ClassConstant},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -450,7 +451,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: uint64(0x24), Class: dwarf.ClassAddress},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -469,7 +470,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: int64(100), Class: dwarf.ClassConstant},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -488,7 +489,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: int64(58), Class: dwarf.ClassConstant},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -507,7 +508,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: int64(0x2c), Class: dwarf.ClassConstant},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -526,7 +527,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: int64(0x58), Class: dwarf.ClassConstant},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -545,7 +546,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: uint64(0x5c), Class: dwarf.ClassAddress},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -564,7 +565,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: int64(100), Class: dwarf.ClassConstant},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -583,7 +584,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrHighpc, Val: uint64(0x2c), Class: dwarf.ClassAddress},
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -600,7 +601,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
 					{Attr: dwarf.AttrCompDir, Val: "/tmp", Class: dwarf.ClassString},
 				},
-			}},
+			}, nil},
 		},
 	},
 	{
@@ -617,7 +618,7 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrType, Val: dwarf.Offset(0xb7f), Class: dwarf.ClassReference},
 					{Attr: dwarf.AttrDataMemberLoc, Val: []byte{0x23, 0x0}, Class: dwarf.ClassExprLoc},
 				},
-			}},
+			}, nil},
 			{204, &dwarf.Entry{
 				Offset:   0xc70,
 				Tag:      dwarf.TagMember,
@@ -629,6 +630,50 @@ var relocationTests = []relocationTest{
 					{Attr: dwarf.AttrType, Val: dwarf.Offset(0xb7f), Class: dwarf.ClassReference},
 					{Attr: dwarf.AttrDataMemberLoc, Val: []byte{0x23, 0x10}, Class: dwarf.ClassExprLoc},
 				},
+			}, nil},
+		},
+	},
+	{
+		"testdata/go-relocation-test-gcc930-ranges-no-rela-x86-64",
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{
+				Offset:   0xb,
+				Tag:      dwarf.TagCompileUnit,
+				Children: true,
+				Field: []dwarf.Field{
+					{Attr: dwarf.AttrProducer, Val: "GNU C17 9.3.0 -mtune=generic -march=x86-64 -g -fasynchronous-unwind-tables", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrLanguage, Val: int64(12), Class: dwarf.ClassConstant},
+					{Attr: dwarf.AttrName, Val: "multiple-code-sections.c", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrCompDir, Val: "/tmp", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrRanges, Val: int64(0), Class: dwarf.ClassRangeListPtr},
+					{Attr: dwarf.AttrLowpc, Val: uint64(0), Class: dwarf.ClassAddress},
+					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
+				},
+			}, [][2]uint64{
+				{0x1125, 0x1137},
+				{0xc0ffee, 0xc0fff9},
+			}},
+		},
+	},
+	{
+		"testdata/go-relocation-test-gcc930-ranges-with-rela-x86-64",
+		[]relocationTestEntry{
+			{0, &dwarf.Entry{
+				Offset:   0xb,
+				Tag:      dwarf.TagCompileUnit,
+				Children: true,
+				Field: []dwarf.Field{
+					{Attr: dwarf.AttrProducer, Val: "GNU C17 9.3.0 -mtune=generic -march=x86-64 -g -fasynchronous-unwind-tables", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrLanguage, Val: int64(12), Class: dwarf.ClassConstant},
+					{Attr: dwarf.AttrName, Val: "multiple-code-sections.c", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrCompDir, Val: "/tmp", Class: dwarf.ClassString},
+					{Attr: dwarf.AttrRanges, Val: int64(0), Class: dwarf.ClassRangeListPtr},
+					{Attr: dwarf.AttrLowpc, Val: uint64(0), Class: dwarf.ClassAddress},
+					{Attr: dwarf.AttrStmtList, Val: int64(0), Class: dwarf.ClassLinePtr},
+				},
+			}, [][2]uint64{
+				{0x1125, 0x1137},
+				{0xc0ffee, 0xc0fff9},
 			}},
 		},
 	},
@@ -664,6 +709,17 @@ func TestDWARFRelocations(t *testing.T) {
 				t.Errorf("#%d/%d: mismatch: got:%#v want:%#v", i, testEntry.entryNumber, entry, testEntry.entry)
 				continue
 			}
+			if testEntry.pcRanges != nil {
+				pcRanges, err := dwarf.Ranges(entry)
+				if err != nil {
+					t.Error(err)
+					continue
+				}
+				if !reflect.DeepEqual(testEntry.pcRanges, pcRanges) {
+					t.Errorf("#%d/%d: PC range mismatch: got:%#v want:%#v", i, testEntry.entryNumber, pcRanges, testEntry.pcRanges)
+					continue
+				}
+			}
 		}
 	}
 }

src/debug/elf/testdata/multiple-code-sections.c

@@ -0,0 +1,11 @@
+// Build with:
+// gcc -g multiple-code-sections.c multiple-code-sections.ld -Wl,--emit-relocs -Wl,--discard-none -o go-relocation-test-gcc930-ranges-with-rela-x86-64
+// gcc -g multiple-code-sections.c multiple-code-sections.ld -o go-relocation-test-gcc930-ranges-no-rela-x86-64
+__attribute__((section(".coffee_section")))
+int coffee(void) {
+    return 0;
+}
+
+int main(int argc, char *argv[]) {
+	return 0;
+}

src/debug/elf/testdata/multiple-code-sections.ld

@@ -0,0 +1,5 @@
+SECTIONS
+{
+    . = 0xC0FFEE;
+    .coffee_section : {}
+}