runtime: print hexdump on traceback failure

Currently, if anything goes wrong when printing a traceback, we simply
cut off the traceback without any further diagnostics. Unfortunately,
right now, we have a few issues that are difficult to debug because
the traceback simply cuts off (#21431, #23484).

This is an attempt to improve the debuggability of traceback failure
by printing a diagnostic message plus a hex dump around the failed
traceback frame when something goes wrong.

The failures look like:

goroutine 5 [running]:
runtime: unexpected return pc for main.badLR2 called from 0xbad
stack: frame={sp:0xc42004dfa8, fp:0xc42004dfc8} stack=[0xc42004d800,0xc42004e000)
000000c42004dea8:  0000000000000001  0000000000000001
000000c42004deb8:  000000c42004ded8  000000c42004ded8
000000c42004dec8:  0000000000427eea <runtime.dopanic+74>  000000c42004ded8
000000c42004ded8:  000000000044df70 <runtime.dopanic.func1+0>  000000c420001080
000000c42004dee8:  0000000000427b21 <runtime.gopanic+961>  000000c42004df08
000000c42004def8:  000000c42004df98  0000000000427b21 <runtime.gopanic+961>
000000c42004df08:  0000000000000000  0000000000000000
000000c42004df18:  0000000000000000  0000000000000000
000000c42004df28:  0000000000000000  0000000000000000
000000c42004df38:  0000000000000000  000000c420001080
000000c42004df48:  0000000000000000  0000000000000000
000000c42004df58:  0000000000000000  0000000000000000
000000c42004df68:  000000c4200010a0  0000000000000000
000000c42004df78:  00000000004c6400  00000000005031d0
000000c42004df88:  0000000000000000  0000000000000000
000000c42004df98:  000000c42004dfb8  00000000004ae7d9 <main.badLR2+73>
000000c42004dfa8: <00000000004c6400  00000000005031d0
000000c42004dfb8:  000000c42004dfd0 !0000000000000bad
000000c42004dfc8: >0000000000000000  0000000000000000
000000c42004dfd8:  0000000000451821 <runtime.goexit+1>  0000000000000000
000000c42004dfe8:  0000000000000000  0000000000000000
000000c42004dff8:  0000000000000000
main.badLR2(0x0)
	/go/src/runtime/testdata/testprog/badtraceback.go:42 +0x49

For #21431, #23484.

Change-Id: I8718fc76ced81adb0b4b0b4f2293f3219ca80786
Reviewed-on: https://go-review.googlesource.com/89016
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

1 files changed, 53 insertions, 0 deletions

diff --git a/src/runtime/print.go b/src/runtime/print.go
index a698fcb0e0..7b2e4f40ff 100644
--- a/src/runtime/print.go
+++ b/src/runtime/print.go
@@ -6,6 +6,7 @@ package runtime
 
 import (
 	"runtime/internal/atomic"
+	"runtime/internal/sys"
 	"unsafe"
 )
 
@@ -249,3 +250,55 @@ func printeface(e eface) {
 func printiface(i iface) {
 	print("(", i.tab, ",", i.data, ")")
 }
+
+// hexdumpWords prints a word-oriented hex dump of [p, end).
+//
+// If mark != nil, it will be called with each printed word's address
+// and should return a character mark to appear just before that
+// word's value. It can return 0 to indicate no mark.
+func hexdumpWords(p, end uintptr, mark func(uintptr) byte) {
+	p1 := func(x uintptr) {
+		var buf [2 * sys.PtrSize]byte
+		for i := len(buf) - 1; i >= 0; i-- {
+			if x&0xF < 10 {
+				buf[i] = byte(x&0xF) + '0'
+			} else {
+				buf[i] = byte(x&0xF) - 10 + 'a'
+			}
+			x >>= 4
+		}
+		gwrite(buf[:])
+	}
+
+	printlock()
+	var markbuf [1]byte
+	markbuf[0] = ' '
+	for i := uintptr(0); p+i < end; i += sys.PtrSize {
+		if i%16 == 0 {
+			if i != 0 {
+				println()
+			}
+			p1(p + i)
+			print(": ")
+		}
+
+		if mark != nil {
+			markbuf[0] = mark(p + i)
+			if markbuf[0] == 0 {
+				markbuf[0] = ' '
+			}
+		}
+		gwrite(markbuf[:])
+		val := *(*uintptr)(unsafe.Pointer(p + i))
+		p1(val)
+		print(" ")
+
+		// Can we symbolize val?
+		fn := findfunc(val)
+		if fn.valid() {
+			print("<", funcname(fn), "+", val-fn.entry, "> ")
+		}
+	}
+	println()
+	printunlock()
+}