syscall: eliminate aliasing of syscall error strings in Plan 9

To avoid allocation between entersyscall and exitsyscall in Plan 9,
syscall error strings retrieved from the OS were being stored in
a shared buffer for each M, leading to overwriting of error strings
by subsequent syscalls, and potential confusion if exitsyscall
switched to a different M.  Instead, the error string is now
retrieved to the G stack and then copied to a new allocated array
after exitsyscall.

A new test TestPlan9Syserr is provided to confirm the correction.

Fixes #13770
Fixes #24921

Change-Id: I013c4a42baae80d03a5b61d828396527189f5551
Reviewed-on: https://go-review.googlesource.com/111195Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: David du Colombier <0intro@gmail.com>

src/syscall/asm_plan9_386.s

@@ -14,13 +14,16 @@
 //func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
 //func RawSyscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
 
+#define SYS_ERRSTR 41	/* from zsysnum_plan9.go */
+
 // Trap # in AX, args on stack above caller pc.
-TEXT	·Syscall(SB),NOSPLIT,$0-32
+TEXT	·Syscall(SB),NOSPLIT,$148-32
+	NO_LOCAL_POINTERS
 	CALL	runtime·entersyscall(SB)
 	MOVL	trap+0(FP), AX	// syscall entry
-	// slide args down on top of system call number
+	// copy args down
 	LEAL	a1+4(FP), SI
-	LEAL	trap+0(FP), DI
+	LEAL	sysargs-144(SP), DI
 	CLD
 	MOVSL
 	MOVSL
@@ -31,13 +34,20 @@ TEXT	·Syscall(SB),NOSPLIT,$0-32
 	CMPL	AX, $-1
 	JNE	ok3
 
-	SUBL	$8, SP
-	CALL	runtime·errstr(SB)
-	MOVL	SP, SI
-	ADDL	$8, SP
+	LEAL	errbuf-128(SP), AX
+	MOVL	AX, sysargs-144(SP)
+	MOVL	$128, sysargs1-140(SP)
+	MOVL	$SYS_ERRSTR, AX
+	INT	$64
+	CALL	runtime·exitsyscall(SB)
+	MOVL	sysargs-144(SP), AX
+	MOVL	AX, errbuf-148(SP)
+	CALL	runtime·gostring(SB)
+	LEAL	str-144(SP), SI
 	JMP	copyresult3
 	
 ok3:
+	CALL	runtime·exitsyscall(SB)
 	LEAL	·emptystring(SB), SI
 	
 copyresult3:
@@ -47,15 +57,15 @@ copyresult3:
 	MOVSL
 	MOVSL
 
-	CALL	runtime·exitsyscall(SB)
 	RET
 
-TEXT	·Syscall6(SB),NOSPLIT,$0-44
+TEXT	·Syscall6(SB),NOSPLIT,$148-44
+	NO_LOCAL_POINTERS
 	CALL	runtime·entersyscall(SB)
 	MOVL	trap+0(FP), AX	// syscall entry
-	// slide args down on top of system call number
+	// copy args down
 	LEAL	a1+4(FP), SI
-	LEAL	trap+0(FP), DI
+	LEAL	sysargs-144(SP), DI
 	CLD
 	MOVSL
 	MOVSL
@@ -69,13 +79,20 @@ TEXT	·Syscall6(SB),NOSPLIT,$0-44
 	CMPL	AX, $-1
 	JNE	ok4
 	
-	SUBL	$8, SP
-	CALL	runtime·errstr(SB)
-	MOVL	SP, SI
-	ADDL	$8, SP
+	LEAL	errbuf-128(SP), AX
+	MOVL	AX, sysargs-144(SP)
+	MOVL	$128, sysargs1-140(SP)
+	MOVL	$SYS_ERRSTR, AX
+	INT	$64
+	CALL	runtime·exitsyscall(SB)
+	MOVL	sysargs-144(SP), AX
+	MOVL	AX, errbuf-148(SP)
+	CALL	runtime·gostring(SB)
+	LEAL	str-144(SP), SI
 	JMP	copyresult4
 	
 ok4:
+	CALL	runtime·exitsyscall(SB)
 	LEAL	·emptystring(SB), SI
 	
 copyresult4:
@@ -85,7 +102,6 @@ copyresult4:
 	MOVSL
 	MOVSL
 
-	CALL	runtime·exitsyscall(SB)
 	RET
 
 TEXT ·RawSyscall(SB),NOSPLIT,$0-28
@@ -121,13 +137,23 @@ TEXT	·RawSyscall6(SB),NOSPLIT,$0-40
 	MOVL	AX, err+36(FP)
 	RET
 
-#define SYS_SEEK 39	/* from zsysnum_plan9_386.go */
+#define SYS_SEEK 39	/* from zsysnum_plan9.go */
 
 //func seek(placeholder uintptr, fd int, offset int64, whence int) (newoffset int64, err string)
-TEXT ·seek(SB),NOSPLIT,$0-36
+TEXT ·seek(SB),NOSPLIT,$24-36
+	NO_LOCAL_POINTERS
 	LEAL	newoffset+20(FP), AX
 	MOVL	AX, placeholder+0(FP)
 	
+	// copy args down
+	LEAL	placeholder+0(FP), SI
+	LEAL	sysargs-20(SP), DI
+	CLD
+	MOVSL
+	MOVSL
+	MOVSL
+	MOVSL
+	MOVSL
 	MOVL	$SYS_SEEK, AX	// syscall entry
 	INT	$64
 	
@@ -136,10 +162,8 @@ TEXT ·seek(SB),NOSPLIT,$0-36
 	MOVL	AX, newoffset_lo+20(FP)
 	MOVL	AX, newoffset_hi+24(FP)
 	
-	SUBL	$8, SP
 	CALL	syscall·errstr(SB)
 	MOVL	SP, SI
-	ADDL	$8, SP	
 	JMP	copyresult6
 	
 ok6:

src/syscall/asm_plan9_amd64.s

@@ -14,12 +14,15 @@
 //func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
 //func RawSyscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
 
-TEXT	·Syscall(SB),NOSPLIT,$0-64
+#define SYS_ERRSTR 41	/* from zsysnum_plan9.go */
+
+TEXT	·Syscall(SB),NOSPLIT,$168-64
+	NO_LOCAL_POINTERS
 	CALL	runtime·entersyscall(SB)
 	MOVQ	trap+0(FP), BP	// syscall entry
-	// slide args down on top of system call number
+	// copy args down
 	LEAQ	a1+8(FP), SI
-	LEAQ	trap+0(FP), DI
+	LEAQ	sysargs-160(SP), DI
 	CLD
 	MOVSQ
 	MOVSQ
@@ -30,13 +33,20 @@ TEXT	·Syscall(SB),NOSPLIT,$0-64
 	CMPL	AX, $-1
 	JNE	ok3
 
-	SUBQ	$16, SP
-	CALL	runtime·errstr(SB)
-	MOVQ	SP, SI
-	ADDQ	$16, SP
+	LEAQ	errbuf-128(SP), AX
+	MOVQ	AX, sysargs-160(SP)
+	MOVQ	$128, sysargs1-152(SP)
+	MOVQ	$SYS_ERRSTR, BP
+	SYSCALL
+	CALL	runtime·exitsyscall(SB)
+	MOVQ	sysargs-160(SP), AX
+	MOVQ	AX, errbuf-168(SP)
+	CALL	runtime·gostring(SB)
+	LEAQ	str-160(SP), SI
 	JMP	copyresult3
 	
 ok3:
+	CALL	runtime·exitsyscall(SB)
 	LEAQ	·emptystring(SB), SI
 	
 copyresult3:
@@ -46,15 +56,15 @@ copyresult3:
 	MOVSQ
 	MOVSQ
 
-	CALL	runtime·exitsyscall(SB)
 	RET
 
-TEXT	·Syscall6(SB),NOSPLIT,$0-88
+TEXT	·Syscall6(SB),NOSPLIT,$168-88
+	NO_LOCAL_POINTERS
 	CALL	runtime·entersyscall(SB)
 	MOVQ	trap+0(FP), BP	// syscall entry
-	// slide args down on top of system call number
+	// copy args down
 	LEAQ	a1+8(FP), SI
-	LEAQ	trap+0(FP), DI
+	LEAQ	sysargs-160(SP), DI
 	CLD
 	MOVSQ
 	MOVSQ
@@ -68,13 +78,20 @@ TEXT	·Syscall6(SB),NOSPLIT,$0-88
 	CMPL	AX, $-1
 	JNE	ok4
 	
-	SUBQ	$16, SP
-	CALL	runtime·errstr(SB)
-	MOVQ	SP, SI
-	ADDQ	$16, SP
+	LEAQ	errbuf-128(SP), AX
+	MOVQ	AX, sysargs-160(SP)
+	MOVQ	$128, sysargs1-152(SP)
+	MOVQ	$SYS_ERRSTR, BP
+	SYSCALL
+	CALL	runtime·exitsyscall(SB)
+	MOVQ	sysargs-160(SP), AX
+	MOVQ	AX, errbuf-168(SP)
+	CALL	runtime·gostring(SB)
+	LEAQ	str-160(SP), SI
 	JMP	copyresult4
 	
 ok4:
+	CALL	runtime·exitsyscall(SB)
 	LEAQ	·emptystring(SB), SI
 	
 copyresult4:
@@ -84,7 +101,6 @@ copyresult4:
 	MOVSQ
 	MOVSQ
 
-	CALL	runtime·exitsyscall(SB)
 	RET
 
 TEXT ·RawSyscall(SB),NOSPLIT,$0-56
@@ -120,24 +136,32 @@ TEXT	·RawSyscall6(SB),NOSPLIT,$0-80
 	MOVQ	AX, err+72(FP)
 	RET
 
-#define SYS_SEEK 39	/* from zsysnum_plan9_amd64.go */
+#define SYS_SEEK 39	/* from zsysnum_plan9.go */
 
 //func seek(placeholder uintptr, fd int, offset int64, whence int) (newoffset int64, err string)
-TEXT ·seek(SB),NOSPLIT,$0-56
+TEXT ·seek(SB),NOSPLIT,$48-56
+	NO_LOCAL_POINTERS
 	LEAQ	newoffset+32(FP), AX
 	MOVQ	AX, placeholder+0(FP)
 	
+	// copy args down
+	LEAQ	placeholder+0(FP), SI
+	LEAQ	sysargs-40(SP), DI
+	CLD
+	MOVSQ
+	MOVSQ
+	MOVSQ
+	MOVSQ
+	MOVSQ
 	MOVQ	$SYS_SEEK, BP	// syscall entry
 	SYSCALL
 	
 	CMPL	AX, $-1
 	JNE	ok6
-	MOVQ	$-1, newoffset+32(FP)
+	MOVQ	AX, newoffset+32(FP)
 	
-	SUBQ	$16, SP
 	CALL	syscall·errstr(SB)
 	MOVQ	SP, SI
-	ADDQ	$16, SP	
 	JMP	copyresult6
 	
 ok6:

src/syscall/asm_plan9_arm.s

@@ -5,58 +5,87 @@
 #include "textflag.h"
 #include "funcdata.h"
 
+#define SYS_ERRSTR 41	/* from zsysnum_plan9.go */
 #define SYS_SEEK 39	/* from zsysnum_plan9.go */
 
 // System call support for plan9 on arm
 
-TEXT	sysresult<>(SB),NOSPLIT,$12
-	MOVW	$·emptystring+0(SB), R2
-	CMP		$-1, R0
-	B.NE	ok
-	MOVW	R1, save-4(SP)
-	BL		runtime·errstr(SB)
-	MOVW	save-4(SP), R1
-	MOVW	$err-12(SP), R2
-ok:
-	MOVM.IA	(R2), [R3-R4]
-	MOVM.IA	[R3-R4], (R1)
-	RET
-	
 //func Syscall(trap, a1, a2, a3 uintptr) (r1, r2 uintptr, err ErrorString)
-TEXT	·Syscall(SB),NOSPLIT,$0-32
+TEXT	·Syscall(SB),NOSPLIT,$144-32
+	NO_LOCAL_POINTERS
 	BL		runtime·entersyscall(SB)
+	MOVW	$a1+4(FP), R0	// move syscall args
+	MOVW	$sysargs-144(SP), R1
+	MOVM.IA	(R0), [R2-R4]
+	MOVM.IA	[R2-R4], (R1)
 	MOVW	trap+0(FP), R0	// syscall num
-	MOVM.IA.W	(R13),[R1-R2]	// pop LR and caller's LR
 	SWI		$0
-	MOVM.DB.W	[R1-R2],(R13)	// push LR and caller's LR
 	MOVW	$0, R2
-	MOVW	$r1+16(FP), R1
-	MOVM.IA.W	[R0,R2], (R1)
-	BL		sysresult<>(SB)
+	MOVW	$r1+16(FP), R3
+	MOVM.IA	[R0,R2], (R3)
+	CMP		$-1, R0
+	B.EQ	syscallerr
 	BL		runtime·exitsyscall(SB)
+	MOVW	$·emptystring+0(SB), R2
+	B		syscallok
+syscallerr:
+	MOVW	$errbuf-128(SP), R2
+	MOVW	$128, R3
+	MOVM.IA	[R2,R3], (R1)
+	MOVW	$SYS_ERRSTR, R0
+	SWI		$0
+	BL		runtime·exitsyscall(SB)
+	BL		runtime·gostring(SB)
+	MOVW	$str-140(SP), R2
+syscallok:
+	MOVW	$err+24(FP), R1
+	MOVM.IA	(R2), [R3-R4]
+	MOVM.IA	[R3-R4], (R1)
 	RET
 
+
 //func Syscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2 uintptr, err ErrorString)
 // Actually Syscall5 but the rest of the code expects it to be named Syscall6.
-TEXT	·Syscall6(SB),NOSPLIT,$0-44
+TEXT	·Syscall6(SB),NOSPLIT,$144-44
+	NO_LOCAL_POINTERS
 	BL		runtime·entersyscall(SB)
+	MOVW	$a1+4(FP), R0	// move syscall args
+	MOVW	$sysargs-144(SP), R1
+	MOVM.IA	(R0), [R2-R6]
+	MOVM.IA	[R2-R6], (R1)
 	MOVW	trap+0(FP), R0	// syscall num
-	MOVM.IA.W	(R13),[R1-R2]	// pop LR and caller's LR
 	SWI		$0
-	MOVM.DB.W	[R1-R2],(R13)	// push LR and caller's LR
-	MOVW	$0, R1
-	MOVW	$r1+28(FP), R1
-	MOVM.IA.W	[R0,R2], (R1)
-	BL		sysresult<>(SB)
+	MOVW	$0, R2
+	MOVW	$r1+28(FP), R3
+	MOVM.IA.W	[R0,R2], (R3)
+	CMP		$-1, R0
+	B.EQ	syscall6err
+	BL		runtime·exitsyscall(SB)
+	MOVW	$·emptystring+0(SB), R2
+	B		syscall6ok
+syscall6err:
+	MOVW	$errbuf-128(SP), R2
+	MOVW	$128, R3
+	MOVM.IA	[R2,R3], (R1)
+	MOVW	$SYS_ERRSTR, R0
+	SWI		$0
 	BL		runtime·exitsyscall(SB)
+	BL		runtime·gostring(SB)
+	MOVW	$str-140(SP), R2
+syscall6ok:
+	MOVW	$err+36(FP), R1
+	MOVM.IA	(R2), [R3-R4]
+	MOVM.IA	[R3-R4], (R1)
 	RET
 
 //func RawSyscall(trap, a1, a2, a3 uintptr) (r1, r2, err uintptr)
-TEXT ·RawSyscall(SB),NOSPLIT,$0-28
+TEXT ·RawSyscall(SB),NOSPLIT,$12-28
+	MOVW	$a1+4(FP), R0	// move syscall args
+	MOVW	$sysargs-12(SP), R1
+	MOVM.IA	(R0), [R2-R4]
+	MOVM.IA	[R2-R4], (R1)
 	MOVW	trap+0(FP), R0	// syscall num
-	MOVM.IA.W	(R13),[R1]		// pop caller's LR
 	SWI		$0
-	MOVM.DB.W	[R1],(R13)		// push caller's LR
 	MOVW	R0, r1+16(FP)
 	MOVW	R0, r2+20(FP)
 	MOVW	R0, err+24(FP)
@@ -64,27 +93,40 @@ TEXT ·RawSyscall(SB),NOSPLIT,$0-28
 
 //func RawSyscall6(trap, a1, a2, a3, a4, a5, a6 uintptr) (r1, r2, err uintptr)
 // Actually RawSyscall5 but the rest of the code expects it to be named RawSyscall6.
-TEXT	·RawSyscall6(SB),NOSPLIT,$0-40
+TEXT	·RawSyscall6(SB),NOSPLIT,$20-40
+	MOVW	$a1+4(FP), R0	// move syscall args
+	MOVW	$sysargs-20(SP), R1
+	MOVM.IA	(R0), [R2-R6]
+	MOVM.IA	[R2-R6], (R1)
 	MOVW	trap+0(FP), R0	// syscall num
-	MOVM.IA.W	(R13),[R1]		// pop caller's LR
 	SWI		$0
-	MOVM.DB.W	[R1],(R13)		// push caller's LR
 	MOVW	R0, r1+28(FP)
 	MOVW	R0, r2+32(FP)
 	MOVW	R0, err+36(FP)
 	RET
 
 //func seek(placeholder uintptr, fd int, offset int64, whence int) (newoffset int64, err string)
-TEXT ·seek(SB),NOSPLIT,$0-36
-	MOVW	$newoffset_lo+20(FP), R5
-	MOVW	R5, placeholder+0(FP)	//placeholder = dest for return value
+TEXT ·seek(SB),NOSPLIT,$20-36
+	NO_LOCAL_POINTERS
+	MOVW	$newoffset_lo+20(FP), R6
+	MOVW	R6, sysargs-20(SP)	// dest for return value
+	MOVW	$fd+4(FP), R0		// move syscall args
+	MOVW	$sysarg1-16(SP), R1
+	MOVM.IA	(R0), [R2-R5]
+	MOVM.IA	[R2-R5], (R1)
 	MOVW	$SYS_SEEK, R0		// syscall num
-	MOVM.IA.W	(R13),[R1]		// pop LR
 	SWI		$0
-	MOVM.DB.W	[R1],(R13)		// push LR
 	CMP		$-1, R0
-	MOVW.EQ	R0, 0(R5)
-	MOVW.EQ	R0, 4(R5)
+	B.EQ	seekerr
+	MOVW	$·emptystring+0(SB), R2
+	B		seekok
+seekerr:
+	MOVW	R0, 0(R6)
+	MOVW	R0, 4(R6)
+	BL		·errstr(SB)
+	MOVW	$ret-20(SP), R2
+seekok:
 	MOVW	$err+28(FP), R1
-	BL		sysresult<>(SB)
+	MOVM.IA	(R2), [R3-R4]
+	MOVM.IA	[R3-R4], (R1)
 	RET

src/syscall/syscall_plan9_test.go

+// Copyright 2018 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.
+
+package syscall_test
+
+import (
+	"syscall"
+	"testing"
+)
+
+// testalias checks for aliasing of error strings returned by sys1 and sys2,
+// which both call the function named fn in package syscall
+func testalias(t *testing.T, fn string, sys1, sys2 func() error) {
+	err := sys1().Error()
+	errcopy := string([]byte(err))
+	sys2()
+	if err != errcopy {
+		t.Errorf("syscall.%s error string changed from %q to %q\n", fn, errcopy, err)
+	}
+}
+
+// issue 13770: errors cannot be nested in Plan 9
+
+func TestPlan9Syserr(t *testing.T) {
+	testalias(t,
+		"Syscall",
+		func() error {
+			return syscall.Mkdir("/", 0)
+		},
+		func() error {
+			return syscall.Mkdir("#", 0)
+		})
+	testalias(t,
+		"Syscall6",
+		func() error {
+			return syscall.Mount(0, 0, "", 0, "")
+		},
+		func() error {
+			return syscall.Mount(-1, 0, "", 0, "")
+		})
+	// originally failed only on plan9_arm
+	testalias(t,
+		"seek",
+		func() error {
+			_, err := syscall.Seek(0, 0, -1)
+			return err
+		},
+		func() error {
+			_, err := syscall.Seek(-1, 0, 0)
+			return err
+		})
+}