runtime: use getrandom(2) for readRandom on Linux

Add Getrandom to internal/runtime/syscall.

Use Getrandom instead of reading /dev/urandom to simplify the code.

Since Go 1.24 needs Linux kernel >= 3.2, and getrandom(2) was
introduced by Linux kernel 3.17, Getrandom could return ENOSYS.
This doesn't matter because the Linux kernel always sets startupRand,
so readRandom is usually not reachable. We also have a time-based
fallback even if readRandom fails.

Updates golang#51087
Fixes golang#68278

src/internal/runtime/syscall/defs_linux.go

@@ -16,4 +16,7 @@ const (
 	EPOLL_CTL_DEL = 0x2
 	EPOLL_CTL_MOD = 0x3
 	EFD_CLOEXEC   = 0x80000
+	GRND_NONBLOCK = 0x01
+	GRND_RANDOM   = 0x02
+	GRND_INSECURE = 0x04
 )

src/internal/runtime/syscall/defs_linux_386.go

@@ -12,6 +12,7 @@ const (
 	SYS_EPOLL_CREATE1 = 329
 	SYS_EPOLL_PWAIT2  = 441
 	SYS_EVENTFD2      = 328
+	SYS_GETRANDOM     = 355
 
 	EFD_NONBLOCK = 0x800
 )

src/internal/runtime/syscall/defs_linux_amd64.go

@@ -12,6 +12,7 @@ const (
 	SYS_EPOLL_CREATE1 = 291
 	SYS_EPOLL_PWAIT2  = 441
 	SYS_EVENTFD2      = 290
+	SYS_GETRANDOM     = 318
 
 	EFD_NONBLOCK = 0x800
 )

src/internal/runtime/syscall/defs_linux_arm.go

@@ -12,6 +12,7 @@ const (
 	SYS_EPOLL_CREATE1 = 357
 	SYS_EPOLL_PWAIT2  = 441
 	SYS_EVENTFD2      = 356
+	SYS_GETRANDOM     = 384
 
 	EFD_NONBLOCK = 0x800
 )

src/internal/runtime/syscall/defs_linux_arm64.go

@@ -12,6 +12,7 @@ const (
 	SYS_MPROTECT      = 226
 	SYS_EPOLL_PWAIT2  = 441
 	SYS_EVENTFD2      = 19
+	SYS_GETRANDOM     = 278
 
 	EFD_NONBLOCK = 0x800
 )

src/internal/runtime/syscall/defs_linux_loong64.go

@@ -12,6 +12,7 @@ const (
 	SYS_MPROTECT      = 226
 	SYS_EPOLL_PWAIT2  = 441
 	SYS_EVENTFD2      = 19
+	SYS_GETRANDOM     = 278
 
 	EFD_NONBLOCK = 0x800
 )

src/internal/runtime/syscall/defs_linux_mips64x.go

@@ -14,6 +14,7 @@ const (
 	SYS_EPOLL_CREATE1 = 5285
 	SYS_EPOLL_PWAIT2  = 5441
 	SYS_EVENTFD2      = 5284
+	SYS_GETRANDOM     = 5313
 
 	EFD_NONBLOCK = 0x80
 )

src/internal/runtime/syscall/defs_linux_mipsx.go

@@ -14,6 +14,7 @@ const (
 	SYS_EPOLL_CREATE1 = 4326
 	SYS_EPOLL_PWAIT2  = 4441
 	SYS_EVENTFD2      = 4325
+	SYS_GETRANDOM     = 4353
 
 	EFD_NONBLOCK = 0x80
 )

src/internal/runtime/syscall/defs_linux_ppc64x.go

@@ -14,6 +14,7 @@ const (
 	SYS_EPOLL_CREATE1 = 315
 	SYS_EPOLL_PWAIT2  = 441
 	SYS_EVENTFD2      = 314
+	SYS_GETRANDOM     = 359
 
 	EFD_NONBLOCK = 0x800
 )

src/internal/runtime/syscall/defs_linux_riscv64.go

@@ -12,6 +12,7 @@ const (
 	SYS_MPROTECT      = 226
 	SYS_EPOLL_PWAIT2  = 441
 	SYS_EVENTFD2      = 19
+	SYS_GETRANDOM     = 278
 
 	EFD_NONBLOCK = 0x800
 )

src/internal/runtime/syscall/defs_linux_s390x.go

@@ -12,6 +12,7 @@ const (
 	SYS_EPOLL_CREATE1 = 327
 	SYS_EPOLL_PWAIT2  = 441
 	SYS_EVENTFD2      = 323
+	SYS_GETRANDOM     = 349
 
 	EFD_NONBLOCK = 0x800
 )

src/internal/runtime/syscall/syscall_linux.go

@@ -42,3 +42,14 @@ func Eventfd(initval, flags int32) (fd int32, errno uintptr) {
 	r1, _, e := Syscall6(SYS_EVENTFD2, uintptr(initval), uintptr(flags), 0, 0, 0, 0)
 	return int32(r1), e
 }
+
+func Getrandom(buf []byte, flags int32) (ret int32, errno uintptr) {
+	var p unsafe.Pointer
+	if len(buf) > 0 {
+		p = unsafe.Pointer(&buf[0])
+	} else {
+		p = unsafe.Pointer(&_zero)
+	}
+	r, _, e := Syscall6(SYS_GETRANDOM, uintptr(p), uintptr(len(buf)), uintptr(flags), 0, 0, 0)
+	return int32(r), e
+}

src/runtime/os_linux.go

@@ -346,12 +346,8 @@ func osinit() {
 	osArchInit()
 }
 
-var urandom_dev = []byte("/dev/urandom\x00")
-
 func readRandom(r []byte) int {
-	fd := open(&urandom_dev[0], 0 /* O_RDONLY */, 0)
-	n := read(fd, unsafe.Pointer(&r[0]), int32(len(r)))
-	closefd(fd)
+	n, _ := syscall.Getrandom(r, 0)
 	return int(n)
 }