runtime: add padding to Linux kernel structures

Go exchanges siginfo and sigevent structures with the kernel. They
contain unions, but Go's use is limited to the first few fields. Pad out
the rest so the size Go sees is the same as what the Linux kernel sees.

This is a follow-up to CL 342052 which added the sigevent struct without
padding, and to CL 353136 which added the padding but with an assertion
that confused several type-checkers. It updates the siginfo struct as
well so there are no bad examples in the defs_linux_*.go files.

Reviewed-on: https://go-review.googlesource.com/c/go/+/353136

Change-Id: I9610632ff0ec43eba91f560536f5441fa907b36f
Reviewed-on: https://go-review.googlesource.com/c/go/+/360094
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

src/runtime/defs_linux_386.go

@@ -3,6 +3,8 @@
 
 package runtime
 
+import "unsafe"
+
 const (
 	_EINTR  = 0x4
 	_EAGAIN = 0xb
@@ -166,7 +168,7 @@ type sigactiont struct {
 	sa_mask     uint64
 }
 
-type siginfo struct {
+type siginfoFields struct {
 	si_signo int32
 	si_errno int32
 	si_code  int32
@@ -174,6 +176,13 @@ type siginfo struct {
 	si_addr uint32
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type stackt struct {
 	ss_sp    *byte
 	ss_flags int32
@@ -229,7 +238,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -237,6 +246,13 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
 type epollevent struct {
 	events uint32
 	data   [8]byte // to match amd64

src/runtime/defs_linux_amd64.go

@@ -3,6 +3,8 @@
 
 package runtime
 
+import "unsafe"
+
 const (
 	_EINTR  = 0x4
 	_EAGAIN = 0xb
@@ -128,7 +130,7 @@ type sigactiont struct {
 	sa_mask     uint64
 }
 
-type siginfo struct {
+type siginfoFields struct {
 	si_signo int32
 	si_errno int32
 	si_code  int32
@@ -136,6 +138,13 @@ type siginfo struct {
 	si_addr uint64
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type itimerspec struct {
 	it_interval timespec
 	it_value    timespec
@@ -146,7 +155,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -154,6 +163,13 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
 type epollevent struct {
 	events uint32
 	data   [8]byte // unaligned uintptr

src/runtime/defs_linux_arm.go

@@ -4,6 +4,8 @@
 
 package runtime
 
+import "unsafe"
+
 // Constants
 const (
 	_EINTR  = 0x4
@@ -169,7 +171,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -177,7 +179,14 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
-type siginfo struct {
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
+type siginfoFields struct {
 	si_signo int32
 	si_errno int32
 	si_code  int32
@@ -185,6 +194,13 @@ type siginfo struct {
 	si_addr uint32
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type sigactiont struct {
 	sa_handler  uintptr
 	sa_flags    uint32

src/runtime/defs_linux_arm64.go

@@ -3,6 +3,8 @@
 
 package runtime
 
+import "unsafe"
+
 const (
 	_EINTR  = 0x4
 	_EAGAIN = 0xb
@@ -128,7 +130,7 @@ type sigactiont struct {
 	sa_mask     uint64
 }
 
-type siginfo struct {
+type siginfoFields struct {
 	si_signo int32
 	si_errno int32
 	si_code  int32
@@ -136,6 +138,13 @@ type siginfo struct {
 	si_addr uint64
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type itimerspec struct {
 	it_interval timespec
 	it_value    timespec
@@ -146,7 +155,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -154,6 +163,13 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
 type epollevent struct {
 	events uint32
 	_pad   uint32

src/runtime/defs_linux_mips64x.go

@@ -6,6 +6,8 @@
 
 package runtime
 
+import "unsafe"
+
 const (
 	_EINTR  = 0x4
 	_EAGAIN = 0xb
@@ -134,7 +136,7 @@ type sigactiont struct {
 	sa_restorer uintptr
 }
 
-type siginfo struct {
+type siginfoFields struct {
 	si_signo int32
 	si_code  int32
 	si_errno int32
@@ -143,6 +145,13 @@ type siginfo struct {
 	si_addr uint64
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type itimerspec struct {
 	it_interval timespec
 	it_value    timespec
@@ -153,7 +162,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -161,6 +170,13 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
 type epollevent struct {
 	events    uint32
 	pad_cgo_0 [4]byte

src/runtime/defs_linux_mipsx.go

@@ -6,6 +6,8 @@
 
 package runtime
 
+import "unsafe"
+
 const (
 	_EINTR  = 0x4
 	_EAGAIN = 0xb
@@ -129,7 +131,7 @@ type sigactiont struct {
 	sa_restorer uintptr
 }
 
-type siginfo struct {
+type siginfoFields struct {
 	si_signo int32
 	si_code  int32
 	si_errno int32
@@ -137,6 +139,13 @@ type siginfo struct {
 	si_addr uint32
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type itimerspec struct {
 	it_interval timespec
 	it_value    timespec
@@ -147,7 +156,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -155,6 +164,13 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
 type epollevent struct {
 	events    uint32
 	pad_cgo_0 [4]byte

src/runtime/defs_linux_ppc64.go

@@ -3,6 +3,8 @@
 
 package runtime
 
+import "unsafe"
+
 const (
 	_EINTR  = 0x4
 	_EAGAIN = 0xb
@@ -129,7 +131,7 @@ type sigactiont struct {
 	sa_mask     uint64
 }
 
-type siginfo struct {
+type siginfoFields struct {
 	si_signo int32
 	si_errno int32
 	si_code  int32
@@ -137,6 +139,13 @@ type siginfo struct {
 	si_addr uint64
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type itimerspec struct {
 	it_interval timespec
 	it_value    timespec
@@ -147,7 +156,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -155,6 +164,13 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
 type epollevent struct {
 	events    uint32
 	pad_cgo_0 [4]byte

src/runtime/defs_linux_ppc64le.go

@@ -3,6 +3,8 @@
 
 package runtime
 
+import "unsafe"
+
 const (
 	_EINTR  = 0x4
 	_EAGAIN = 0xb
@@ -129,7 +131,7 @@ type sigactiont struct {
 	sa_mask     uint64
 }
 
-type siginfo struct {
+type siginfoFields struct {
 	si_signo int32
 	si_errno int32
 	si_code  int32
@@ -137,6 +139,13 @@ type siginfo struct {
 	si_addr uint64
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type itimerspec struct {
 	it_interval timespec
 	it_value    timespec
@@ -147,7 +156,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -155,6 +164,13 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
 type epollevent struct {
 	events    uint32
 	pad_cgo_0 [4]byte

src/runtime/defs_linux_riscv64.go

@@ -4,6 +4,8 @@
 
 package runtime
 
+import "unsafe"
+
 const (
 	_EINTR  = 0x4
 	_EAGAIN = 0xb
@@ -126,7 +128,7 @@ type sigactiont struct {
 	sa_mask     uint64
 }
 
-type siginfo struct {
+type siginfoFields struct {
 	si_signo int32
 	si_errno int32
 	si_code  int32
@@ -134,6 +136,13 @@ type siginfo struct {
 	si_addr uint64
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type itimerspec struct {
 	it_interval timespec
 	it_value    timespec
@@ -144,7 +153,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -152,6 +161,13 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
 type epollevent struct {
 	events    uint32
 	pad_cgo_0 [4]byte

src/runtime/defs_linux_s390x.go

@@ -4,6 +4,8 @@
 
 package runtime
 
+import "unsafe"
+
 const (
 	_EINTR  = 0x4
 	_EAGAIN = 0xb
@@ -125,7 +127,7 @@ type sigactiont struct {
 	sa_mask     uint64
 }
 
-type siginfo struct {
+type siginfoFields struct {
 	si_signo int32
 	si_errno int32
 	si_code  int32
@@ -133,6 +135,13 @@ type siginfo struct {
 	si_addr uint64
 }
 
+type siginfo struct {
+	siginfoFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_si_max_size - unsafe.Sizeof(siginfoFields{})]byte
+}
+
 type itimerspec struct {
 	it_interval timespec
 	it_value    timespec
@@ -143,7 +152,7 @@ type itimerval struct {
 	it_value    timeval
 }
 
-type sigevent struct {
+type sigeventFields struct {
 	value  uintptr
 	signo  int32
 	notify int32
@@ -151,6 +160,13 @@ type sigevent struct {
 	sigev_notify_thread_id int32
 }
 
+type sigevent struct {
+	sigeventFields
+
+	// Pad struct to the max size in the kernel.
+	_ [_sigev_max_size - unsafe.Sizeof(sigeventFields{})]byte
+}
+
 type epollevent struct {
 	events    uint32
 	pad_cgo_0 [4]byte

src/runtime/export_linux_test.go

@@ -8,11 +8,16 @@ package runtime
 
 import "unsafe"
 
+const SiginfoMaxSize = _si_max_size
+const SigeventMaxSize = _sigev_max_size
+
 var NewOSProc0 = newosproc0
 var Mincore = mincore
 var Add = add
 
 type EpollEvent epollevent
+type Siginfo siginfo
+type Sigevent sigevent
 
 func Epollctl(epfd, op, fd int32, ev unsafe.Pointer) int32 {
 	return epollctl(epfd, op, fd, (*epollevent)(ev))

src/runtime/os_linux.go

@@ -440,6 +440,11 @@ func pipe() (r, w int32, errno int32)
 func pipe2(flags int32) (r, w int32, errno int32)
 func setNonblock(fd int32)
 
+const (
+	_si_max_size    = 128
+	_sigev_max_size = 64
+)
+
 //go:nosplit
 //go:nowritebarrierrec
 func setsig(i uint32, fn uintptr) {
@@ -636,12 +641,11 @@ func setThreadCPUProfiler(hz int32) {
 	spec.it_interval.setNsec(1e9 / int64(hz))
 
 	var timerid int32
-	sevp := &sigevent{
-		notify:                 _SIGEV_THREAD_ID,
-		signo:                  _SIGPROF,
-		sigev_notify_thread_id: int32(mp.procid),
-	}
-	ret := timer_create(_CLOCK_THREAD_CPUTIME_ID, sevp, &timerid)
+	var sevp sigevent
+	sevp.notify = _SIGEV_THREAD_ID
+	sevp.signo = _SIGPROF
+	sevp.sigev_notify_thread_id = int32(mp.procid)
+	ret := timer_create(_CLOCK_THREAD_CPUTIME_ID, &sevp, &timerid)
 	if ret != 0 {
 		// If we cannot create a timer for this M, leave profileTimerValid false
 		// to fall back to the process-wide setitimer profiler.

src/runtime/runtime_linux_test.go

@@ -61,3 +61,14 @@ func TestEpollctlErrorSign(t *testing.T) {
 		t.Errorf("epollctl = %v, want %v", v, -EBADF)
 	}
 }
+
+func TestKernelStructSize(t *testing.T) {
+	// Check that the Go definitions of structures exchanged with the kernel are
+	// the same size as what the kernel defines.
+	if have, want := unsafe.Sizeof(Siginfo{}), uintptr(SiginfoMaxSize); have != want {
+		t.Errorf("Go's siginfo struct is %d bytes long; kernel expects %d", have, want)
+	}
+	if have, want := unsafe.Sizeof(Sigevent{}), uintptr(SigeventMaxSize); have != want {
+		t.Errorf("Go's sigevent struct is %d bytes long; kernel expects %d", have, want)
+	}
+}