Implement /proc/cmdline

This change implements /proc/cmdline with a basic faux command line
"BOOT_IMAGE=/vmlinuz-[version]-gvisor quiet" so apps that may expect
it do not receive errors.

Also tests for the existence of /proc/cmdline as part of the system
call test suite

PiperOrigin-RevId: 372462070

pkg/sentry/fsimpl/proc/tasks.go

@@ -65,6 +65,7 @@ var _ kernfs.Inode = (*tasksInode)(nil)
 func (fs *filesystem) newTasksInode(ctx context.Context, k *kernel.Kernel, pidns *kernel.PIDNamespace, fakeCgroupControllers map[string]string) *tasksInode {
 	root := auth.NewRootCredentials(pidns.UserNamespace())
 	contents := map[string]kernfs.Inode{
+		"cmdline":     fs.newInode(ctx, root, 0444, &cmdLineData{}),
 		"cpuinfo":     fs.newInode(ctx, root, 0444, newStaticFileSetStat(cpuInfoData(k))),
 		"filesystems": fs.newInode(ctx, root, 0444, &filesystemsData{}),
 		"loadavg":     fs.newInode(ctx, root, 0444, &loadavgData{}),

pkg/sentry/fsimpl/proc/tasks_files.go

@@ -336,15 +336,6 @@ var _ dynamicInode = (*versionData)(nil)
 
 // Generate implements vfs.DynamicBytesSource.Generate.
 func (*versionData) Generate(ctx context.Context, buf *bytes.Buffer) error {
-	k := kernel.KernelFromContext(ctx)
-	init := k.GlobalInit()
-	if init == nil {
-		// Attempted to read before the init Task is created. This can
-		// only occur during startup, which should never need to read
-		// this file.
-		panic("Attempted to read version before initial Task is available")
-	}
-
 	// /proc/version takes the form:
 	//
 	// "SYSNAME version RELEASE (COMPILE_USER@COMPILE_HOST)
@@ -364,7 +355,7 @@ func (*versionData) Generate(ctx context.Context, buf *bytes.Buffer) error {
 	// FIXME(mpratt): Using Version from the init task SyscallTable
 	// disregards the different version a task may have (e.g., in a uts
 	// namespace).
-	ver := init.Leader().SyscallTable().Version
+	ver := kernelVersion(ctx)
 	fmt.Fprintf(buf, "%s version %s %s\n", ver.Sysname, ver.Release, ver.Version)
 	return nil
 }
@@ -400,3 +391,31 @@ func (*cgroupsData) Generate(ctx context.Context, buf *bytes.Buffer) error {
 	r.GenerateProcCgroups(buf)
 	return nil
 }
+
+// cmdLineData backs /proc/cmdline.
+//
+// +stateify savable
+type cmdLineData struct {
+	dynamicBytesFileSetAttr
+}
+
+var _ dynamicInode = (*cmdLineData)(nil)
+
+// Generate implements vfs.DynamicByteSource.Generate.
+func (*cmdLineData) Generate(ctx context.Context, buf *bytes.Buffer) error {
+	fmt.Fprintf(buf, "BOOT_IMAGE=/vmlinuz-%s-gvisor quiet\n", kernelVersion(ctx).Release)
+	return nil
+}
+
+// kernelVersion returns the kernel version.
+func kernelVersion(ctx context.Context) kernel.Version {
+	k := kernel.KernelFromContext(ctx)
+	init := k.GlobalInit()
+	if init == nil {
+		// Attempted to read before the init Task is created. This can
+		// only occur during startup, which should never need to read
+		// this file.
+		panic("Attempted to read version before initial Task is available")
+	}
+	return init.Leader().SyscallTable().Version
+}

pkg/sentry/fsimpl/proc/tasks_test.go

@@ -47,6 +47,7 @@ var (
 
 var (
 	tasksStaticFiles = map[string]testutil.DirentType{
+		"cmdline":     linux.DT_REG,
 		"cpuinfo":     linux.DT_REG,
 		"filesystems": linux.DT_REG,
 		"loadavg":     linux.DT_REG,

test/syscalls/linux/proc.cc

@@ -1201,6 +1201,15 @@ TEST(ProcSelfCwd, Absolute) {
   EXPECT_EQ(exe[0], '/');
 }
 
+// Sanity check that /proc/cmdline is present.
+TEST(ProcCmdline, IsPresent) {
+  SKIP_IF(IsRunningWithVFS1());
+
+  std::string proc_cmdline =
+      ASSERT_NO_ERRNO_AND_VALUE(GetContents("/proc/cmdline"));
+  ASSERT_FALSE(proc_cmdline.empty());
+}
+
 // Sanity check for /proc/cpuinfo fields that must be present.
 TEST(ProcCpuinfo, RequiredFieldsArePresent) {
   std::string proc_cpuinfo =