executor, pkg/csource: make fd numbers consistent

Currently when executor creates fd's it gets: 0, 3, 4.
When tun is enabled: 3, 4, 5.
For C programs: 3, 4, 5.
When run is enabled: 4, 5, 6.

Theoretically it should not matter,
but these fd numbers are probably sometimes are used as data.
So make them consistent in all these cases (3, 4, 5).

3 files changed, 44 insertions, 26 deletions

diff --git a/executor/common_linux.h b/executor/common_linux.h
index 1411fe8e2..f74effbba 100644
--- a/executor/common_linux.h
+++ b/executor/common_linux.h
@@ -309,6 +309,13 @@ static void initialize_tun(int id)
 		return;
 #endif
 	}
+	// Remap tun onto higher fd number to hide it from fuzzer and to keep
+	// fd numbers stable regardless of whether tun is opened or not.
+	const int kTunFd = 252;
+	if (dup2(tunfd, kTunFd) < 0)
+		fail("dup2(tunfd, kTunFd) failed");
+	close(tunfd);
+	tunfd = kTunFd;
 
 	char iface[IFNAMSIZ];
 	snprintf_check(iface, sizeof(iface), "syz%d", id);
@@ -896,9 +903,12 @@ static int namespace_sandbox_proc(void* arg)
 		fail("mkdir failed");
 	// selinux mount used to be at /selinux, but then moved to /sys/fs/selinux.
 	const char* selinux_path = "./syz-tmp/newroot/selinux";
-	if (mount("/selinux", selinux_path, NULL, mount_flags, NULL) &&
-	    mount("/sys/fs/selinux", selinux_path, NULL, mount_flags, NULL))
-		fail("mount(selinuxfs) failed");
+	if (mount("/selinux", selinux_path, NULL, mount_flags, NULL)) {
+		if (errno != ENOENT)
+			fail("mount(/selinux) failed");
+		if (mount("/sys/fs/selinux", selinux_path, NULL, mount_flags, NULL) && errno != ENOENT)
+			fail("mount(/sys/fs/selinux) failed");
+	}
 	if (mkdir("./syz-tmp/pivot", 0777))
 		fail("mkdir failed");
 	if (syscall(SYS_pivot_root, "./syz-tmp", "./syz-tmp/pivot")) {
diff --git a/executor/executor.h b/executor/executor.h
index ee48981a9..063b61ff3 100644
--- a/executor/executor.h
+++ b/executor/executor.h
@@ -198,8 +198,10 @@ void setup_control_pipes()
 		fail("dup2(1, kOutPipeFd) failed");
 	if (dup2(2, 1) < 0)
 		fail("dup2(2, 1) failed");
-	if (close(0))
-		fail("close(0) failed");
+	// We used to close(0), but now we dup stderr to stdin to keep fd numbers
+	// stable across executor and C programs generated by pkg/csource.
+	if (dup2(2, 0) < 0)
+		fail("dup2(2, 0) failed");
 }
 
 void parse_env_flags(uint64 flags)
@@ -707,24 +709,22 @@ bool copyout(char* addr, uint64 size, uint64* res)
 {
 	bool ok = false;
 	NONFAILING(
-		switch (size) {
-		case 1:
-			*res = *(uint8*)addr;
-			break;
-		case 2:
-			*res = *(uint16*)addr;
-			break;
-		case 4:
-			*res = *(uint32*)addr;
-			break;
-		case 8:
-			*res = *(uint64*)addr;
-			break;
-		default:
-			fail("copyout: bad argument size %llu", size);
-		}
-		__atomic_store_n(&ok, true, __ATOMIC_RELEASE);
-	);
+	    switch (size) {
+		    case 1:
+			    *res = *(uint8*)addr;
+			    break;
+		    case 2:
+			    *res = *(uint16*)addr;
+			    break;
+		    case 4:
+			    *res = *(uint32*)addr;
+			    break;
+		    case 8:
+			    *res = *(uint64*)addr;
+			    break;
+		    default:
+			    fail("copyout: bad argument size %llu", size);
+	    } __atomic_store_n(&ok, true, __ATOMIC_RELEASE););
 	return ok;
 }
 
diff --git a/pkg/csource/linux_common.go b/pkg/csource/linux_common.go
index dc917fed2..fe380d9d4 100644
--- a/pkg/csource/linux_common.go
+++ b/pkg/csource/linux_common.go
@@ -451,6 +451,11 @@ static void initialize_tun(int id)
 		return;
 #endif
 	}
+	const int kTunFd = 252;
+	if (dup2(tunfd, kTunFd) < 0)
+		fail("dup2(tunfd, kTunFd) failed");
+	close(tunfd);
+	tunfd = kTunFd;
 
 	char iface[IFNAMSIZ];
 	snprintf_check(iface, sizeof(iface), "syz%d", id);
@@ -1938,9 +1943,12 @@ static int namespace_sandbox_proc(void* arg)
 	if (mkdir("./syz-tmp/newroot/selinux", 0700))
 		fail("mkdir failed");
 	const char* selinux_path = "./syz-tmp/newroot/selinux";
-	if (mount("/selinux", selinux_path, NULL, mount_flags, NULL) &&
-	    mount("/sys/fs/selinux", selinux_path, NULL, mount_flags, NULL))
-		fail("mount(selinuxfs) failed");
+	if (mount("/selinux", selinux_path, NULL, mount_flags, NULL)) {
+		if (errno != ENOENT)
+			fail("mount(/selinux) failed");
+		if (mount("/sys/fs/selinux", selinux_path, NULL, mount_flags, NULL) && errno != ENOENT)
+			fail("mount(/sys/fs/selinux) failed");
+	}
 	if (mkdir("./syz-tmp/pivot", 0777))
 		fail("mkdir failed");
 	if (syscall(SYS_pivot_root, "./syz-tmp", "./syz-tmp/pivot")) {