debug/dwarf: fix problem with DWARF 5 and Seek method

When clients use debug/dwarf to examine DWARF 5 binaries, we can run
into problems when the Seek() method is used to skip ahead from a DIE
in one compilation unit to a DIE in another unit. The problem here is
that it is common for DWARF 5 comp units to have attributes (ex:
DW_AT_addr_base) whose value must be applied as an offset when reading
certain forms (ex: DW_FORM_addrx) within that unit. The existing
implementation didn't have a good way to recover these attrs following
the Seek call, and had to essentially punt in this case, resulting in
incorrect attr values.

This patch adds new support for reading and caching the key comp unit
DIE attributes (DW_AT_addr_base, DW_AT_loclists_base, etc) prior to
visiting any of the DIE entries in a unit, storing the cache values of
these attrs the main table of units. This base attribute
reading/caching behavior also happens (where needed) after Seek calls.

Should resolve delve issue 3861.
Supercedes Go pull request 70400.

Updates #26379.
Fixes #57046.

Change-Id: I536a57e2ba4fc55132d91c7f36f67a91ac408dc3
Reviewed-on: https://go-review.googlesource.com/c/go/+/655976
Reviewed-by: Alessandro Arzilli <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: thanm

src/debug/dwarf/entry.go

@@ -426,16 +426,6 @@ func (b *buf) entry(cu *Entry, u *unit) *Entry {
 		Field:    make([]Field, len(a.field)),
 	}
 
-	// If we are currently parsing the compilation unit,
-	// we can't evaluate Addrx or Strx until we've seen the
-	// relevant base entry.
-	type delayed struct {
-		idx int
-		off uint64
-		fmt format
-	}
-	var delay []delayed
-
 	resolveStrx := func(strBase, off uint64) string {
 		off += strBase
 		if uint64(int(off)) != off {
@@ -532,18 +522,7 @@ func (b *buf) entry(cu *Entry, u *unit) *Entry {
 				return nil
 			}
 
-			// We have to adjust by the offset of the
-			// compilation unit. This won't work if the
-			// program uses Reader.Seek to skip over the
-			// unit. Not much we can do about that.
-			var addrBase int64
-			if cu != nil {
-				addrBase, _ = cu.Val(AttrAddrBase).(int64)
-			} else if a.tag == TagCompileUnit {
-				delay = append(delay, delayed{i, off, formAddrx})
-				break
-			}
-
+			addrBase := int64(u.addrBase())
 			var err error
 			val, err = b.dwarf.debugAddr(b.format, uint64(addrBase), off)
 			if err != nil {
@@ -683,18 +662,7 @@ func (b *buf) entry(cu *Entry, u *unit) *Entry {
 				off *= 4
 			}
 
-			// We have to adjust by the offset of the
-			// compilation unit. This won't work if the
-			// program uses Reader.Seek to skip over the
-			// unit. Not much we can do about that.
-			var strBase int64
-			if cu != nil {
-				strBase, _ = cu.Val(AttrStrOffsetsBase).(int64)
-			} else if a.tag == TagCompileUnit {
-				delay = append(delay, delayed{i, off, formStrx})
-				break
-			}
-
+			strBase := int64(u.strOffsetsBase())
 			val = resolveStrx(uint64(strBase), off)
 
 		case formStrpSup:
@@ -743,18 +711,7 @@ func (b *buf) entry(cu *Entry, u *unit) *Entry {
 		case formRnglistx:
 			off := b.uint()
 
-			// We have to adjust by the rnglists_base of
-			// the compilation unit. This won't work if
-			// the program uses Reader.Seek to skip over
-			// the unit. Not much we can do about that.
-			var rnglistsBase int64
-			if cu != nil {
-				rnglistsBase, _ = cu.Val(AttrRnglistsBase).(int64)
-			} else if a.tag == TagCompileUnit {
-				delay = append(delay, delayed{i, off, formRnglistx})
-				break
-			}
-
+			rnglistsBase := int64(u.rngListsBase())
 			val = resolveRnglistx(uint64(rnglistsBase), off)
 		}
 
@@ -763,32 +720,6 @@ func (b *buf) entry(cu *Entry, u *unit) *Entry {
 	if b.err != nil {
 		return nil
 	}
-
-	for _, del := range delay {
-		switch del.fmt {
-		case formAddrx:
-			addrBase, _ := e.Val(AttrAddrBase).(int64)
-			val, err := b.dwarf.debugAddr(b.format, uint64(addrBase), del.off)
-			if err != nil {
-				b.err = err
-				return nil
-			}
-			e.Field[del.idx].Val = val
-		case formStrx:
-			strBase, _ := e.Val(AttrStrOffsetsBase).(int64)
-			e.Field[del.idx].Val = resolveStrx(uint64(strBase), del.off)
-			if b.err != nil {
-				return nil
-			}
-		case formRnglistx:
-			rnglistsBase, _ := e.Val(AttrRnglistsBase).(int64)
-			e.Field[del.idx].Val = resolveRnglistx(uint64(rnglistsBase), del.off)
-			if b.err != nil {
-				return nil
-			}
-		}
-	}
-
 	return e
 }
 
@@ -840,6 +771,7 @@ func (r *Reader) Seek(off Offset) {
 		u := &d.unit[0]
 		r.unit = 0
 		r.b = makeBuf(r.d, u, "info", u.off, u.data)
+		r.collectDwarf5BaseOffsets(u)
 		r.cu = nil
 		return
 	}
@@ -855,6 +787,7 @@ func (r *Reader) Seek(off Offset) {
 	u := &d.unit[i]
 	r.unit = i
 	r.b = makeBuf(r.d, u, "info", off, u.data[off-u.off:])
+	r.collectDwarf5BaseOffsets(u)
 }
 
 // maybeNextUnit advances to the next unit if this one is finished.
@@ -870,6 +803,17 @@ func (r *Reader) nextUnit() {
 	u := &r.d.unit[r.unit]
 	r.b = makeBuf(r.d, u, "info", u.off, u.data)
 	r.cu = nil
+	r.collectDwarf5BaseOffsets(u)
+}
+
+func (r *Reader) collectDwarf5BaseOffsets(u *unit) {
+	if u.vers < 5 || u.unit5 != nil {
+		return
+	}
+	u.unit5 = new(unit5)
+	if err := r.d.collectDwarf5BaseOffsets(u); err != nil {
+		r.err = err
+	}
 }
 
 // Next reads the next entry from the encoded entry stream.

src/debug/dwarf/entry_test.go

@@ -6,6 +6,7 @@ package dwarf_test
 
 import (
 	. "debug/dwarf"
+	"debug/elf"
 	"encoding/binary"
 	"path/filepath"
 	"reflect"
@@ -457,3 +458,62 @@ func TestIssue52045(t *testing.T) {
 		t.Errorf("got non-nil entry0, wanted nil")
 	}
 }
+
+func TestIssue57046(t *testing.T) {
+	f, err := elf.Open("testdata/issue57046-clang.elf5")
+	if err != nil {
+		t.Fatalf("elf.Open returns err: %v", err)
+	}
+	d, err := f.DWARF()
+	if err != nil {
+		t.Fatalf("f.DWARF returns err: %v", err)
+	}
+	// Write down all the subprogram DIEs.
+	spdies := []Offset{}
+	lopcs := []uint64{}
+	r := d.Reader()
+	for {
+		e, err := r.Next()
+		if err != nil {
+			t.Fatalf("r.Next() returns err: %v", err)
+		}
+		if e == nil {
+			break
+		}
+		if e.Tag != TagSubprogram {
+			continue
+		}
+		var name string
+		var lopc uint64
+		if n, ok := e.Val(AttrName).(string); ok {
+			name = n
+		}
+		if lo, ok := e.Val(AttrLowpc).(uint64); ok {
+			lopc = lo
+		}
+		if name == "" || lopc == 0 {
+			continue
+		}
+		spdies = append(spdies, e.Offset)
+		lopcs = append(lopcs, lopc)
+	}
+
+	// Seek to the second entry in spdies (corresponding to mom() in
+	// issue57046_part2.c) and take a look at it.
+	r2 := d.Reader()
+	r2.Seek(spdies[1])
+	e, err := r2.Next()
+	if err != nil {
+		t.Fatalf("r2.Next() returns err: %v", err)
+	}
+	if e == nil {
+		t.Fatalf("r2.Next() returned nil")
+	}
+
+	// Verify that the lopc we see matches what we saw before.
+	got := e.Val(AttrLowpc).(uint64)
+	if got != lopcs[1] {
+		t.Errorf("bad lopc for fn2 following seek: want %x got %x\n",
+			lopcs[1], got)
+	}
+}

src/debug/dwarf/testdata/issue57046-clang.elf5

@@ -0,0 +1,42 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Part 1 of the sources for issue 57046 test case.
+
+// Build instructions:
+//
+// clang-16 -O -g -gdwarf-5 -c issue57046_part1.c
+// clang-16 -O -g -gdwarf-5 -c issue57046_part2.c
+// clang-16 -o issue57046-clang.elf5 issue57046_part1.o issue57046_part2.o
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+extern const char *mom();
+
+int gadgety() {
+  const char *ev = getenv("PATH");
+  int n = strlen(ev);
+  int s1 = (int)ev[0];
+  int s2 = (int)ev[1];
+  int s3 = (int)ev[2];
+  for (int i = 0; i < strlen(ev); i++) {
+    if (s1 == 101) {
+	int t = s1;
+	s1 = s3;
+	s3 = t;
+    }
+    if (ev[i] == 99) {
+      printf("%d\n", i);
+    }
+  }
+  s2 *= 2;
+  return n + s1 + s2;
+}
+
+int main(int argc, char **argv) {
+  printf("Hi %s %d\n", mom(), gadgety());
+  return 0;
+}

src/debug/dwarf/testdata/issue57046_part1.c

@@ -0,0 +1,10 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Part 2 of the sources for issue 57046 test case.
+// See part 1 for build instructions.
+
+const char *mom() {
+  return "mom";
+}

src/debug/dwarf/testdata/issue57046_part2.c

@@ -17,10 +17,18 @@ type unit struct {
 	off    Offset // byte offset of data within the aggregate info
 	data   []byte
 	atable abbrevTable
+	*unit5 // info specific to DWARF 5 units
 	asize  int
 	vers   int
-	utype  uint8 // DWARF 5 unit type
 	is64   bool  // True for 64-bit DWARF format
+	utype  uint8 // DWARF 5 unit type
+}
+
+type unit5 struct {
+	addrBase       uint64
+	strOffsetsBase uint64
+	rngListsBase   uint64
+	locListsBase   uint64
 }
 
 // Implement the dataFormat interface.
@@ -37,6 +45,34 @@ func (u *unit) addrsize() int {
 	return u.asize
 }
 
+func (u *unit) addrBase() uint64 {
+	if u.unit5 != nil {
+		return u.unit5.addrBase
+	}
+	return 0
+}
+
+func (u *unit) strOffsetsBase() uint64 {
+	if u.unit5 != nil {
+		return u.unit5.strOffsetsBase
+	}
+	return 0
+}
+
+func (u *unit) rngListsBase() uint64 {
+	if u.unit5 != nil {
+		return u.unit5.rngListsBase
+	}
+	return 0
+}
+
+func (u *unit) locListsBase() uint64 {
+	if u.unit5 != nil {
+		return u.unit5.locListsBase
+	}
+	return 0
+}
+
 func (d *Data) parseUnits() ([]unit, error) {
 	// Count units.
 	nunit := 0
@@ -135,3 +171,30 @@ func (d *Data) offsetToUnit(off Offset) int {
 	}
 	return -1
 }
+
+func (d *Data) collectDwarf5BaseOffsets(u *unit) error {
+	if u.unit5 == nil {
+		panic("expected unit5 to be set up already")
+	}
+	b := makeBuf(d, u, "info", u.off, u.data)
+	cu := b.entry(nil, u)
+	if cu == nil {
+		// Unknown abbreviation table entry or some other fatal
+		// problem; bail early on the assumption that this will be
+		// detected at some later point.
+		return b.err
+	}
+	if iAddrBase, ok := cu.Val(AttrAddrBase).(int64); ok {
+		u.unit5.addrBase = uint64(iAddrBase)
+	}
+	if iStrOffsetsBase, ok := cu.Val(AttrStrOffsetsBase).(int64); ok {
+		u.unit5.strOffsetsBase = uint64(iStrOffsetsBase)
+	}
+	if iRngListsBase, ok := cu.Val(AttrRnglistsBase).(int64); ok {
+		u.unit5.rngListsBase = uint64(iRngListsBase)
+	}
+	if iLocListsBase, ok := cu.Val(AttrLoclistsBase).(int64); ok {
+		u.unit5.locListsBase = uint64(iLocListsBase)
+	}
+	return nil
+}