Add feature to launch Sentry from an open host FD.

Adds feature to launch from an open host FD instead of a binary_path.
The FD should point to a valid executable and most likely be statically
compiled. If the executable is not statically compiled, the loader will
search along the interpreter paths, which must be able to be resolved in
the Sandbox's file system or start will fail.

PiperOrigin-RevId: 260756825

2 files changed, 30 insertions, 7 deletions

diff --git a/pkg/sentry/kernel/kernel.go b/pkg/sentry/kernel/kernel.go
index cf8bf3ecd..55a9d3d29 100644
--- a/pkg/sentry/kernel/kernel.go
+++ b/pkg/sentry/kernel/kernel.go
@@ -590,11 +590,17 @@ func (k *Kernel) UniqueID() uint64 {
 
 // CreateProcessArgs holds arguments to kernel.CreateProcess.
 type CreateProcessArgs struct {
-	// Filename is the filename to load.
+	// Filename is the filename to load as the init binary.
 	//
-	// If this is provided as "", then the file will be guessed via Argv[0].
+	// If this is provided as "", File will be checked, then the file will be
+	// guessed via Argv[0].
 	Filename string
 
+	// File is a passed host FD pointing to a file to load as the init binary.
+	//
+	// This is checked if and only if Filename is "".
+	File *fs.File
+
 	// Argvv is a list of arguments.
 	Argv []string
 
@@ -779,8 +785,16 @@ func (k *Kernel) CreateProcess(args CreateProcessArgs) (*ThreadGroup, ThreadID,
 		defer wd.DecRef()
 	}
 
-	if args.Filename == "" {
-		// Was anything provided?
+	// Check which file to start from.
+	switch {
+	case args.Filename != "":
+		// If a filename is given, take that.
+		// Set File to nil so we resolve the path in LoadTaskImage.
+		args.File = nil
+	case args.File != nil:
+		// If File is set, take the File provided directly.
+	default:
+		// Otherwise look at Argv and see if the first argument is a valid path.
 		if len(args.Argv) == 0 {
 			return nil, 0, fmt.Errorf("no filename or command provided")
 		}
@@ -792,7 +806,9 @@ func (k *Kernel) CreateProcess(args CreateProcessArgs) (*ThreadGroup, ThreadID,
 
 	// Create a fresh task context.
 	remainingTraversals = uint(args.MaxSymlinkTraversals)
-	tc, se := k.LoadTaskImage(ctx, k.mounts, root, wd, &remainingTraversals, args.Filename, args.Argv, args.Envv, k.featureSet)
+
+	tc, se := k.LoadTaskImage(ctx, k.mounts, root, wd, &remainingTraversals, args.Filename, args.File, args.Argv, args.Envv, k.featureSet)
+
 	if se != nil {
 		return nil, 0, errors.New(se.String())
 	}
diff --git a/pkg/sentry/kernel/task_context.go b/pkg/sentry/kernel/task_context.go
index 54b1676b0..8639d379f 100644
--- a/pkg/sentry/kernel/task_context.go
+++ b/pkg/sentry/kernel/task_context.go
@@ -140,15 +140,22 @@ func (t *Task) Stack() *arch.Stack {
 //  * wd: Working directory to lookup filename under
 //  * maxTraversals: maximum number of symlinks to follow
 //  * filename: path to binary to load
+//  * file: an open fs.File object of the binary to load. If set,
+//  file will be loaded and not filename.
 //  * argv: Binary argv
 //  * envv: Binary envv
 //  * fs: Binary FeatureSet
-func (k *Kernel) LoadTaskImage(ctx context.Context, mounts *fs.MountNamespace, root, wd *fs.Dirent, maxTraversals *uint, filename string, argv, envv []string, fs *cpuid.FeatureSet) (*TaskContext, *syserr.Error) {
+func (k *Kernel) LoadTaskImage(ctx context.Context, mounts *fs.MountNamespace, root, wd *fs.Dirent, maxTraversals *uint, filename string, file *fs.File, argv, envv []string, fs *cpuid.FeatureSet) (*TaskContext, *syserr.Error) {
+	// If File is not nil, we should load that instead of resolving filename.
+	if file != nil {
+		filename = file.MappedName(ctx)
+	}
+
 	// Prepare a new user address space to load into.
 	m := mm.NewMemoryManager(k, k)
 	defer m.DecUsers(ctx)
 
-	os, ac, name, err := loader.Load(ctx, m, mounts, root, wd, maxTraversals, fs, filename, argv, envv, k.extraAuxv, k.vdso)
+	os, ac, name, err := loader.Load(ctx, m, mounts, root, wd, maxTraversals, fs, filename, file, argv, envv, k.extraAuxv, k.vdso)
 	if err != nil {
 		return nil, err
 	}