cmd/link: put padding between functions, not at the end of a function

Functions should be declared to end after the last real instruction, not
after the last padding byte. We achieve this by adding the padding while
assembling the text section in the linker instead of adding the padding
to the function symbol in the compiler. This change makes dtrace happy.

TODO: check that this works with external linking

Fixes #15969

Change-Id: I973e478d0cd34b61be1ddc55410552cbd645ad62
Reviewed-on: https://go-review.googlesource.com/24040
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/x86/asm6.go

@@ -1986,16 +1986,6 @@ func span6(ctxt *obj.Link, s *obj.LSym) {
 		c = naclpad(ctxt, s, c, -c&31)
 	}
 
-	// Pad functions with trap instruction, to catch invalid jumps
-	if c&(FuncAlign-1) != 0 {
-		v := -c & (FuncAlign - 1)
-		s.Grow(int64(c) + int64(v))
-		for i := c; i < c+v; i++ {
-			// 0xCC is INT $3 - breakpoint instruction
-			s.P[i] = uint8(0xCC)
-		}
-		c += v
-	}
 	s.Size = int64(c)
 
 	if false { /* debug['a'] > 1 */

src/cmd/link/internal/amd64/asm.go

@@ -615,7 +615,8 @@ func asmb() {
 
 	sect := ld.Segtext.Sect
 	ld.Cseek(int64(sect.Vaddr - ld.Segtext.Vaddr + ld.Segtext.Fileoff))
-	ld.Codeblk(int64(sect.Vaddr), int64(sect.Length))
+	// 0xCC is INT $3 - breakpoint instruction
+	ld.CodeblkPad(int64(sect.Vaddr), int64(sect.Length), []byte{0xCC})
 	for sect = sect.Next; sect != nil; sect = sect.Next {
 		ld.Cseek(int64(sect.Vaddr - ld.Segtext.Vaddr + ld.Segtext.Fileoff))
 		ld.Datblk(int64(sect.Vaddr), int64(sect.Length))

src/cmd/link/internal/ld/data.go

@@ -790,11 +790,14 @@ func blk(start *LSym, addr int64, size int64) {
 }
 
 func Codeblk(addr int64, size int64) {
+	CodeblkPad(addr, size, zeros[:])
+}
+func CodeblkPad(addr int64, size int64, pad []byte) {
 	if Debug['a'] != 0 {
 		fmt.Fprintf(Bso, "codeblk [%#x,%#x) at offset %#x\n", addr, addr+size, Cpos())
 	}
 
-	blkSlice(Ctxt.Textp, addr, size)
+	blkSlice(Ctxt.Textp, addr, size, pad)
 
 	/* again for printing */
 	if Debug['a'] == 0 {
@@ -858,7 +861,7 @@ func Codeblk(addr int64, size int64) {
 // blkSlice is a variant of blk that processes slices.
 // After text symbols are converted from a linked list to a slice,
 // delete blk and give this function its name.
-func blkSlice(syms []*LSym, addr, size int64) {
+func blkSlice(syms []*LSym, addr, size int64, pad []byte) {
 	for i, s := range syms {
 		if s.Type&obj.SSUB == 0 && s.Value >= addr {
 			syms = syms[i:]
@@ -880,13 +883,13 @@ func blkSlice(syms []*LSym, addr, size int64) {
 			errorexit()
 		}
 		if addr < s.Value {
-			strnput("", int(s.Value-addr))
+			strnputPad("", int(s.Value-addr), pad)
 			addr = s.Value
 		}
 		Cwrite(s.P)
 		addr += int64(len(s.P))
 		if addr < s.Value+s.Size {
-			strnput("", int(s.Value+s.Size-addr))
+			strnputPad("", int(s.Value+s.Size-addr), pad)
 			addr = s.Value + s.Size
 		}
 		if addr != s.Value+s.Size {
@@ -899,7 +902,7 @@ func blkSlice(syms []*LSym, addr, size int64) {
 	}
 
 	if addr < eaddr {
-		strnput("", int(eaddr-addr))
+		strnputPad("", int(eaddr-addr), pad)
 	}
 	Cflush()
 }
@@ -909,7 +912,7 @@ func Datblk(addr int64, size int64) {
 		fmt.Fprintf(Bso, "datblk [%#x,%#x) at offset %#x\n", addr, addr+size, Cpos())
 	}
 
-	blkSlice(datap, addr, size)
+	blkSlice(datap, addr, size, zeros[:])
 
 	/* again for printing */
 	if Debug['a'] == 0 {
@@ -986,23 +989,27 @@ func Dwarfblk(addr int64, size int64) {
 var zeros [512]byte
 
 // strnput writes the first n bytes of s.
-// If n is larger then len(s),
+// If n is larger than len(s),
 // it is padded with NUL bytes.
 func strnput(s string, n int) {
+	strnputPad(s, n, zeros[:])
+}
+
+// strnput writes the first n bytes of s.
+// If n is larger than len(s),
+// it is padded with the bytes in pad (repeated as needed).
+func strnputPad(s string, n int, pad []byte) {
 	if len(s) >= n {
 		Cwritestring(s[:n])
 	} else {
 		Cwritestring(s)
 		n -= len(s)
-		for n > 0 {
-			if len(zeros) >= n {
-				Cwrite(zeros[:n])
-				return
-			} else {
-				Cwrite(zeros[:])
-				n -= len(zeros)
-			}
+		for n > len(pad) {
+			Cwrite(pad)
+			n -= len(pad)
+
 		}
+		Cwrite(pad[:n])
 	}
 }
 

src/cmd/link/internal/x86/asm.go

@@ -609,7 +609,8 @@ func asmb() {
 
 	sect := ld.Segtext.Sect
 	ld.Cseek(int64(sect.Vaddr - ld.Segtext.Vaddr + ld.Segtext.Fileoff))
-	ld.Codeblk(int64(sect.Vaddr), int64(sect.Length))
+	// 0xCC is INT $3 - breakpoint instruction
+	ld.CodeblkPad(int64(sect.Vaddr), int64(sect.Length), []byte{0xCC})
 	for sect = sect.Next; sect != nil; sect = sect.Next {
 		ld.Cseek(int64(sect.Vaddr - ld.Segtext.Vaddr + ld.Segtext.Fileoff))
 		ld.Datblk(int64(sect.Vaddr), int64(sect.Length))