runtime: when using cgo on 386, call C sigaction function

On 386 the C sigaction function assumes that the caller does not set
the SA_RESTORER flag. It does not copy the C sa_restorer field to
the kernel sa_restorer field. The effect is that the kernel sees
the SA_RESTORER flag but a NULL sa_restorer field, and the program
crashes when returning from a signal handler.

On the other hand, the C sigaction function will return the SA_RESTORER
flag and the sa_restorer field stored in the kernel.

This means that if the Go runtime installs a signal handler,
with SA_RESTORER as is required when calling the kernel,
and the Go program calls C code that calls the C sigaction function
to query the current signal handler, that C code will get a result
that it can't pass back to sigaction.

This CL fixes the problem by using the C sigaction function
for 386 programs that use cgo. This reuses the functionality
used on amd64 and other GOARCHs to support the race detector.

See #75253, or runtime/testdata/testprogcgo/eintr.go, for sample
code that used to fail on 386. No new test case is required,
we just remove the skip we used to have for eintr.go.

Fixes #75253

Change-Id: I803059b1fb9e09e9fbb43f68eccb6a59a92c2991
Reviewed-on: https://go-review.googlesource.com/c/go/+/701375
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

src/runtime/cgo/gcc_sigaction.c

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build linux && (amd64 || arm64 || loong64 || ppc64le)
+//go:build linux && (386 || amd64 || arm64 || loong64 || ppc64le)
 
 #include <errno.h>
 #include <stddef.h>
@@ -17,7 +17,7 @@
 // to and from struct sigaction — are specific to ${goos}/${goarch}.
 typedef struct {
 	uintptr_t handler;
-	uint64_t flags;
+	unsigned long flags;
 #ifdef __loongarch__
 	uint64_t mask;
 	uintptr_t restorer;
@@ -57,7 +57,7 @@ x_cgo_sigaction(intptr_t signum, const go_sigaction_t *goact, go_sigaction_t *ol
 				sigaddset(&act.sa_mask, (int)(i+1));
 			}
 		}
-		act.sa_flags = (int)(goact->flags & ~(uint64_t)SA_RESTORER);
+		act.sa_flags = (int)(goact->flags & ~(unsigned long)SA_RESTORER);
 	}
 
 	ret = sigaction((int)signum, goact ? &act : NULL, oldgoact ? &oldact : NULL);
@@ -79,7 +79,7 @@ x_cgo_sigaction(intptr_t signum, const go_sigaction_t *goact, go_sigaction_t *ol
 				oldgoact->mask |= (uint64_t)(1)<<i;
 			}
 		}
-		oldgoact->flags = (uint64_t)oldact.sa_flags;
+		oldgoact->flags = (unsigned long)oldact.sa_flags;
 	}
 
 	_cgo_tsan_release();

src/runtime/cgo/sigaction.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (linux && (amd64 || arm64 || loong64 || ppc64le)) || (freebsd && amd64)
+//go:build (linux && (386 || amd64 || arm64 || loong64 || ppc64le)) || (freebsd && amd64)
 
 package cgo
 

src/runtime/cgo_sigaction.go

@@ -3,8 +3,10 @@
 // license that can be found in the LICENSE file.
 
 // Support for sanitizers. See runtime/cgo/sigaction.go.
+// Also used on linux/386 to clear the SA_RESTORER flag
+// when using cgo; see issue #75253.
 
-//go:build (linux && (amd64 || arm64 || loong64 || ppc64le)) || (freebsd && amd64)
+//go:build (linux && (386 || amd64 || arm64 || loong64 || ppc64le)) || (freebsd && amd64)
 
 package runtime
 
@@ -42,6 +44,8 @@ func sigaction(sig uint32, new, old *sigactiont) {
 
 		var ret int32
 
+		fixSigactionForCgo(new)
+
 		var g *g
 		if mainStarted {
 			g = getg()

src/runtime/crash_cgo_test.go

@@ -842,17 +842,6 @@ func TestEINTR(t *testing.T) {
 	switch runtime.GOOS {
 	case "plan9", "windows":
 		t.Skipf("no EINTR on %s", runtime.GOOS)
-	case "linux":
-		if runtime.GOARCH == "386" {
-			// On linux-386 the Go signal handler sets
-			// a restorer function that is not preserved
-			// by the C sigaction call in the test,
-			// causing the signal handler to crash when
-			// returning the normal code. The test is not
-			// architecture-specific, so just skip on 386
-			// rather than doing a complicated workaround.
-			t.Skip("skipping on linux-386; C sigaction does not preserve Go restorer")
-		}
 	}
 	if runtime.GOOS == "freebsd" && race.Enabled {
 		t.Skipf("race + cgo freebsd not supported. See https://go.dev/issue/73788.")

src/runtime/os_freebsd.go

@@ -457,6 +457,12 @@ func sysSigaction(sig uint32, new, old *sigactiont) {
 	}
 }
 
+// fixSigactionForCgo is needed for Linux.
+//
+//go:nosplit
+func fixSigactionForCgo(new *sigactiont) {
+}
+
 // asmSigaction is implemented in assembly.
 //
 //go:noescape

src/runtime/os_linux.go

@@ -486,7 +486,8 @@ func setsig(i uint32, fn uintptr) {
 	sigfillset(&sa.sa_mask)
 	// Although Linux manpage says "sa_restorer element is obsolete and
 	// should not be used". x86_64 kernel requires it. Only use it on
-	// x86.
+	// x86. Note that on 386 this is cleared when using the C sigaction
+	// function via cgo; see fixSigactionForCgo.
 	if GOARCH == "386" || GOARCH == "amd64" {
 		sa.sa_restorer = abi.FuncPCABI0(sigreturn__sigaction)
 	}
@@ -562,6 +563,21 @@ func sysSigaction(sig uint32, new, old *sigactiont) {
 //go:noescape
 func rt_sigaction(sig uintptr, new, old *sigactiont, size uintptr) int32
 
+// fixSigactionForCgo is called when we are using cgo to call the
+// C sigaction function. On 386 the C function does not expect the
+// SA_RESTORER flag to be set, and in some cases will fail if it is set:
+// it will pass the SA_RESTORER flag to the kernel without passing
+// the sa_restorer field. Since the C function will handle SA_RESTORER
+// for us, we need not pass it. See issue #75253.
+//
+//go:nosplit
+func fixSigactionForCgo(new *sigactiont) {
+	if GOARCH == "386" && new != nil {
+		new.sa_flags &^= _SA_RESTORER
+		new.sa_restorer = 0
+	}
+}
+
 func getpid() int
 func tgkill(tgid, tid, sig int)
 

src/runtime/sigaction.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build (linux && !amd64 && !arm64 && !loong64 && !ppc64le) || (freebsd && !amd64)
+//go:build (linux && !386 && !amd64 && !arm64 && !loong64 && !ppc64le) || (freebsd && !amd64)
 
 package runtime
 

src/runtime/sys_linux_386.s

@@ -410,6 +410,25 @@ TEXT runtime·rt_sigaction(SB),NOSPLIT,$0
 	MOVL	AX, ret+16(FP)
 	RET
 
+// Call the function stored in _cgo_sigaction using the GCC calling convention.
+TEXT runtime·callCgoSigaction(SB),NOSPLIT,$0-16
+	MOVL	_cgo_sigaction(SB), AX
+	MOVL	sig+0(FP), BX
+	MOVL	new+4(FP), CX
+	MOVL	old+8(FP), DX
+	MOVL	SP, SI // align stack to call C function
+	SUBL	$32, SP
+	ANDL	$~15, SP
+	MOVL	BX, 0(SP)
+	MOVL	CX, 4(SP)
+	MOVL	DX, 8(SP)
+	MOVL	SI, 12(SP)
+	CALL	AX
+	MOVL	12(SP), BX
+	MOVL	BX, SP
+	MOVL	AX, ret+12(FP)
+	RET
+
 TEXT runtime·sigfwd(SB),NOSPLIT,$12-16
 	MOVL	fn+0(FP), AX
 	MOVL	sig+4(FP), BX