runtime: don't save G during VDSO if we're handling signal

On some platforms (currently ARM and ARM64), when calling into
VDSO we store the G to the gsignal stack, if there is one, so if
we receive a signal during VDSO we can find the G.

If we receive a signal during VDSO, and within the signal handler
we call nanotime again (e.g. when handling profiling signal),
we'll save/clear the G slot on the gsignal stack again, which
clobbers the original saved G. If we receive a second signal
during the same VDSO execution, we will fetch a nil G, which will
lead to bad things such as deadlock.

Don't save G if we're calling VDSO code from the gsignal stack.
Saving G is not necessary as we won't receive a nested signal.

Fixes #35473.

Change-Id: Ibfd8587a3c70c2f1533908b056e81b94d75d65a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/206397
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/runtime/sys_linux_arm.s

@@ -279,12 +279,16 @@ noswitch:
 	// 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.
 	MOVB	runtime·iscgo(SB), R6
 	CMP	$0, R6
 	BNE	nosaveg
 	MOVW	m_gsignal(R5), R6          // g.m.gsignal
 	CMP	$0, R6
 	BEQ	nosaveg
+	CMP	g, R6
+	BEQ	nosaveg
 	MOVW	(g_stack+stack_lo)(R6), R6 // g.m.gsignal.stack.lo
 	MOVW	g, (R6)
 
@@ -353,12 +357,16 @@ noswitch:
 	// 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.
 	MOVB	runtime·iscgo(SB), R6
 	CMP	$0, R6
 	BNE	nosaveg
 	MOVW	m_gsignal(R5), R6          // g.m.gsignal
 	CMP	$0, R6
 	BEQ	nosaveg
+	CMP	g, R6
+	BEQ	nosaveg
 	MOVW	(g_stack+stack_lo)(R6), R6 // g.m.gsignal.stack.lo
 	MOVW	g, (R6)
 

src/runtime/sys_linux_arm64.s

@@ -239,10 +239,14 @@ noswitch:
 	// 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.
 	MOVBU	runtime·iscgo(SB), R22
 	CBNZ	R22, nosaveg
 	MOVD	m_gsignal(R21), R22          // g.m.gsignal
 	CBZ	R22, nosaveg
+	CMP	g, R22
+	BEQ	nosaveg
 	MOVD	(g_stack+stack_lo)(R22), R22 // g.m.gsignal.stack.lo
 	MOVD	g, (R22)
 
@@ -303,10 +307,14 @@ noswitch:
 	// 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.
 	MOVBU	runtime·iscgo(SB), R22
 	CBNZ	R22, nosaveg
 	MOVD	m_gsignal(R21), R22          // g.m.gsignal
 	CBZ	R22, nosaveg
+	CMP	g, R22
+	BEQ	nosaveg
 	MOVD	(g_stack+stack_lo)(R22), R22 // g.m.gsignal.stack.lo
 	MOVD	g, (R22)