runtime: fix crash during VDSO calls on PowerPC

src/runtime/signal_unix.go

@@ -382,7 +382,7 @@ func preemptM(mp *m) {
 //go:nosplit
 func sigFetchG(c *sigctxt) *g {
 	switch GOARCH {
-	case "arm", "arm64":
+	case "arm", "arm64", "ppc64", "ppc64le":
 		if !iscgo && inVDSOPage(c.sigpc()) {
 			// When using cgo, we save the g on TLS and load it from there
 			// in sigtramp. Just use that.

src/runtime/sys_linux_ppc64x.s

@@ -216,15 +216,45 @@ TEXT runtime·walltime(SB),NOSPLIT,$16-12
 	MOVD	(g_sched+gobuf_sp)(R7), R1	// Set SP to g0 stack
 
 noswitch:
-	SUB	$16, R1			// Space for results
-	RLDICR	$0, R1, $59, R1		// Align for C code
+	SUB	$16, R1                 // Space for results
+	RLDICR	$0, R1, $59, R1         // Align for C code
 	MOVD	R12, CTR
 	MOVD	R1, R4
-	BL	(CTR)			// Call from VDSO
-	MOVD	$0, R0			// Restore R0
-	MOVD	0(R1), R3		// sec
-	MOVD	8(R1), R5		// nsec
-	MOVD	R15, R1			// Restore SP
+
+	// Store g on gsignal's stack, so if we receive a signal
+	// during VDSO code we can find the g.
+	// If we don't have a signal stack, we won't receive signal,
+	// so don't bother saving g.
+	// When using cgo, we already saved g on TLS, also don't save
+	// g here.
+	// Also don't save g if we are already on the signal stack.
+	// We won't get a nested signal.
+	MOVBZ	runtime·iscgo(SB), R22
+	CMP	R22, $0
+	BNE	nosaveg
+	MOVD	m_gsignal(R21), R22	// g.m.gsignal
+	CMP	R22, $0
+	BEQ	nosaveg
+
+	CMP	g, R22
+	BEQ	nosaveg
+	MOVD	(g_stack+stack_lo)(R22), R22 // g.m.gsignal.stack.lo
+	MOVD	g, (R22)
+
+	BL	(CTR)	// Call from VDSO
+
+	MOVD	$0, (R22)	// clear g slot, R22 is unchanged by C code
+
+	JMP	finish
+
+nosaveg:
+	BL	(CTR)	// Call from VDSO
+
+finish:
+	MOVD	$0, R0		// Restore R0
+	MOVD	0(R1), R3	// sec
+	MOVD	8(R1), R5	// nsec
+	MOVD	R15, R1		// Restore SP
 
 	// Restore vdsoPC, vdsoSP
 	// We don't worry about being signaled between the two stores.
@@ -236,7 +266,7 @@ noswitch:
 	MOVD	32(R1), R6
 	MOVD	R6, m_vdsoPC(R21)
 
-finish:
+return:
 	MOVD	R3, sec+0(FP)
 	MOVW	R5, nsec+8(FP)
 	RET
@@ -247,7 +277,7 @@ fallback:
 	SYSCALL $SYS_clock_gettime
 	MOVD	32(R1), R3
 	MOVD	40(R1), R5
-	JMP	finish
+	JMP	return
 
 TEXT runtime·nanotime1(SB),NOSPLIT,$16-8
 	MOVD	$1, R3		// CLOCK_MONOTONIC
@@ -283,7 +313,37 @@ noswitch:
 	RLDICR	$0, R1, $59, R1		// Align for C code
 	MOVD	R12, CTR
 	MOVD	R1, R4
-	BL	(CTR)			// Call from VDSO
+
+	// Store g on gsignal's stack, so if we receive a signal
+	// during VDSO code we can find the g.
+	// If we don't have a signal stack, we won't receive signal,
+	// so don't bother saving g.
+	// When using cgo, we already saved g on TLS, also don't save
+	// g here.
+	// Also don't save g if we are already on the signal stack.
+	// We won't get a nested signal.
+	MOVBZ	runtime·iscgo(SB), R22
+	CMP	R22, $0
+	BNE	nosaveg
+	MOVD	m_gsignal(R21), R22	// g.m.gsignal
+	CMP	R22, $0
+	BEQ	nosaveg
+
+	CMP	g, R22
+	BEQ	nosaveg
+	MOVD	(g_stack+stack_lo)(R22), R22 // g.m.gsignal.stack.lo
+	MOVD	g, (R22)
+
+	BL	(CTR)	// Call from VDSO
+
+	MOVD	$0, (R22)	// clear g slot, R22 is unchanged by C code
+
+	JMP	finish
+
+nosaveg:
+	BL	(CTR)	// Call from VDSO
+
+finish:
 	MOVD	$0, R0			// Restore R0
 	MOVD	0(R1), R3		// sec
 	MOVD	8(R1), R5		// nsec
@@ -299,7 +359,7 @@ noswitch:
 	MOVD	32(R1), R6
 	MOVD	R6, m_vdsoPC(R21)
 
-finish:
+return:
 	// sec is in R3, nsec in R5
 	// return nsec in R3
 	MOVD	$1000000000, R4
@@ -314,7 +374,7 @@ fallback:
 	SYSCALL $SYS_clock_gettime
 	MOVD	32(R1), R3
 	MOVD	40(R1), R5
-	JMP	finish
+	JMP	return
 
 TEXT runtime·rtsigprocmask(SB),NOSPLIT|NOFRAME,$0-28
 	MOVW	how+0(FP), R3
@@ -469,7 +529,7 @@ TEXT sigtramp<>(SB),NOSPLIT|NOFRAME,$0
 	// this might be called in external code context,
 	// where g is not set.
 	MOVBZ	runtime·iscgo(SB), R6
-	CMP 	R6, $0
+	CMP	R6, $0
 	BEQ	2(PC)
 	BL	runtime·load_g(SB)