runtime: fix line number for faulting instructions

Unlike function calls, when processing instructions that directly
fault we must not subtract 1 from the pc before looking up the
file/line information.

Since the file/line lookup unconditionally subtracts 1, add 1 to
the faulting instruction PCs to compensate.

Fixes #34123

Change-Id: Ie7361e3d2f84a0d4f48d97e5a9e74f6291ba7a8b
Reviewed-on: https://go-review.googlesource.com/c/go/+/196962
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/runtime/pprof/proto.go

@@ -359,13 +359,7 @@ func (b *profileBuilder) build() {
 			}
 		}
 
-		// Addresses from stack traces point to the next instruction after each call,
-		// except for the leaf, which points to where the signal occurred.
-		// appendLocsForStack expects return PCs so increment the leaf address to
-		// look like a return PC.
-		e.stk[0] += 1
 		locs = b.appendLocsForStack(locs[:0], e.stk)
-		e.stk[0] -= 1 // undo the adjustment on the leaf.
 
 		b.pbSample(values, locs, labels)
 	}

src/runtime/pprof/proto_test.go

@@ -116,9 +116,9 @@ func TestConvertCPUProfile(t *testing.T) {
 
 	b := []uint64{
 		3, 0, 500, // hz = 500
-		5, 0, 10, uint64(addr1), uint64(addr1 + 2), // 10 samples in addr1
-		5, 0, 40, uint64(addr2), uint64(addr2 + 2), // 40 samples in addr2
-		5, 0, 10, uint64(addr1), uint64(addr1 + 2), // 10 samples in addr1
+		5, 0, 10, uint64(addr1 + 1), uint64(addr1 + 2), // 10 samples in addr1
+		5, 0, 40, uint64(addr2 + 1), uint64(addr2 + 2), // 40 samples in addr2
+		5, 0, 10, uint64(addr1 + 1), uint64(addr1 + 2), // 10 samples in addr1
 	}
 	p, err := translateCPUProfile(b)
 	if err != nil {

src/runtime/traceback.go

@@ -340,7 +340,20 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 			pc := frame.pc
 			// backup to CALL instruction to read inlining info (same logic as below)
 			tracepc := pc
-			if (n > 0 || flags&_TraceTrap == 0) && frame.pc > f.entry && !waspanic {
+			// Normally, pc is a return address. In that case, we want to look up
+			// file/line information using pc-1, because that is the pc of the
+			// call instruction (more precisely, the last byte of the call instruction).
+			// Callers expect the pc buffer to contain return addresses and do the
+			// same -1 themselves, so we keep pc unchanged.
+			// When the pc is from a signal (e.g. profiler or segv) then we want
+			// to look up file/line information using pc, and we store pc+1 in the
+			// pc buffer so callers can unconditionally subtract 1 before looking up.
+			// See issue 34123.
+			// The pc can be at function entry when the frame is initialized without
+			// actually running code, like runtime.mstart.
+			if (n == 0 && flags&_TraceTrap != 0) || waspanic || pc == f.entry {
+				pc++
+			} else {
 				tracepc--
 			}
 

test/fixedbugs/issue34123.go

@@ -0,0 +1,43 @@
+// run
+
+// Copyright 2019 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.
+
+// Make sure that the line number is reported correctly
+// for faulting instructions.
+
+package main
+
+import (
+	"fmt"
+	"runtime"
+)
+
+var x byte
+var p *byte
+
+//go:noinline
+func f() {
+	q := p
+	x = 11  // line 23
+	*q = 12 // line 24
+}
+func main() {
+	defer func() {
+		recover()
+		var pcs [10]uintptr
+		n := runtime.Callers(1, pcs[:])
+		frames := runtime.CallersFrames(pcs[:n])
+		for {
+			f, more := frames.Next()
+			if f.Function == "main.f" && f.Line != 24 {
+				panic(fmt.Errorf("expected line 24, got line %d", f.Line))
+			}
+			if !more {
+				break
+			}
+		}
+	}()
+	f()
+}