syscall, runtime/internal/syscall: zero r2 before mips linux syscalls

All mips variant perform syscalls similarly. R2 (v0) holds r1 and R3
(v1) holds r2 of a syscall. The latter is only used by 2-ret syscalls.
A 1-ret syscall would not touch R3 but keeps it as is, making r2 be a
random value. Always reset it to 0 before SYSCALL to fix the issue.

Fixes #56426

Change-Id: Ie49965c0c3c224c4a895703ac659205cd040ff56
Reviewed-on: https://go-review.googlesource.com/c/go/+/452975
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Meng Zhuo <mzh@golangcn.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Meng Zhuo <mzh@golangcn.org>

src/runtime/internal/syscall/asm_linux_mips64x.s

@@ -15,6 +15,7 @@ TEXT ·Syscall6(SB),NOSPLIT,$0-80
 	MOVV	a4+32(FP), R7
 	MOVV	a5+40(FP), R8
 	MOVV	a6+48(FP), R9
+	MOVV	R0, R3	// reset R3 to 0 as 1-ret SYSCALL keeps it
 	SYSCALL
 	BEQ	R7, ok
 	MOVV	$-1, R1

src/runtime/internal/syscall/asm_linux_mipsx.s

@@ -20,6 +20,7 @@ TEXT ·Syscall6(SB),NOSPLIT,$20-40
 	MOVW	a6+24(FP), R9
 	MOVW	R8, 16(R29)
 	MOVW	R9, 20(R29)
+	MOVW	R0, R3	// reset R3 to 0 as 1-ret SYSCALL keeps it
 	SYSCALL
 	BEQ	R7, ok
 	MOVW	$-1, R1

src/syscall/asm_linux_mips64x.s

@@ -38,7 +38,8 @@ TEXT ·rawSyscallNoError(SB),NOSPLIT,$0-48
 	MOVV	R0, R8
 	MOVV	R0, R9
 	MOVV	trap+0(FP), R2	// syscall entry
+	MOVV	R0, R3	// reset R3 to zero as 1-ret SYSCALL keeps it
 	SYSCALL
-	MOVV	R2, r1+32(FP)
-	MOVV	R3, r2+40(FP)
+	MOVV	R2, r1+32(FP)	// r1
+	MOVV	R3, r2+40(FP)	// r2
 	RET

src/syscall/asm_linux_mipsx.s

@@ -29,6 +29,7 @@ TEXT ·Syscall9(SB),NOSPLIT,$28-52
 	MOVW	R10, 24(R29)
 	MOVW	R11, 28(R29)
 	MOVW	trap+0(FP), R2	// syscall entry
+	MOVW	R0, R3	// reset R3 to zero as 1-ret SYSCALL keeps it
 	SYSCALL
 	BEQ	R7, ok9
 	MOVW	$-1, R1
@@ -66,6 +67,7 @@ TEXT ·rawSyscallNoError(SB),NOSPLIT,$20-24
 	MOVW	a2+8(FP), R5
 	MOVW	a3+12(FP), R6
 	MOVW	trap+0(FP), R2	// syscall entry
+	MOVW	R0, R3	// reset R3 to zero as 1-ret SYSCALL keeps it
 	SYSCALL
 	MOVW	R2, r1+16(FP)	// r1
 	MOVW	R3, r2+20(FP)	// r2