cmd/asm: fix potential infinite loop in parser

For ARM machines, the assembler supports list of registers
operands such as [R1,R2].

A list missing a ']' results in the parser issuing many errors
and consuming all the tokens. At EOF (i.e. end of the line),
it still loops.

Normally, a counter is maintained to make sure the parser
stops after 10 errors. However, multiple errors occuring on the
same line are simply ignored. Only the first one is reported.
At most one error per line is accounted.

Missing ']' in a register list therefore results in an
infinite loop.

Fixed the parser by explicitly checking for ']' to interrupt
this loops

In the operand tests, also fixed a wrong entry which I think was
not set on purpose (but still led to a successful result).

Fixes #11764

Change-Id: Ie87773388ee0d21b3a2a4cb941d4d911d0230ba4
Reviewed-on: https://go-review.googlesource.com/13920Reviewed-by: Rob Pike <r@golang.org>

src/cmd/asm/internal/asm/operand_test.go

@@ -181,7 +181,7 @@ var amd64OperandTests = []operandTest{
 	{"x·y+8(SB)", "x.y+8(SB)"},
 	{"x·y+8(SP)", "x.y+8(SP)"},
 	{"y+56(FP)", "y+56(FP)"},
-	{"·AddUint32(SB", "\"\".AddUint32(SB)"},
+	{"·AddUint32(SB)", "\"\".AddUint32(SB)"},
 	{"·callReflect(SB)", "\"\".callReflect(SB)"},
 }
 
@@ -288,6 +288,7 @@ var armOperandTests = []operandTest{
 	{"runtime·_sfloat2(SB)", "runtime._sfloat2(SB)"},
 	{"·AddUint32(SB)", "\"\".AddUint32(SB)"},
 	{"(R1, R3)", "(R1, R3)"},
+	{"[R0,R1,g,R15", ""}, // Issue 11764 - previously asm just hung parsing ']' missing register lists
 }
 
 var ppc64OperandTests = []operandTest{

src/cmd/asm/internal/asm/parse.go

@@ -698,10 +698,15 @@ func (p *Parser) registerIndirect(a *obj.Addr, prefix rune) {
 func (p *Parser) registerList(a *obj.Addr) {
 	// One range per loop.
 	var bits uint16
+ListLoop:
 	for {
 		tok := p.next()
-		if tok.ScanToken == ']' {
-			break
+		switch tok.ScanToken {
+		case ']':
+			break ListLoop
+		case scanner.EOF:
+			p.errorf("missing ']' in register list")
+			return
 		}
 		lo := p.registerNumber(tok.String())
 		hi := lo