Allow open(O_TRUNC) and (f)truncate for proc files.

This allows writable proc and devices files to be opened with O_CREAT|O_TRUNC.
This is encountered most frequently when interacting with proc or devices files
via the command line.
e.g. $ echo 8192 1048576 4194304 > /proc/sys/net/ipv4/tcp_rmem

Also adds a test to test the behavior of open(O_TRUNC), truncate, and ftruncate
on named pipes.

Fixes #1116

PiperOrigin-RevId: 282677425

4 files changed, 34 insertions, 7 deletions

diff --git a/pkg/sentry/fs/proc/sys_net.go b/pkg/sentry/fs/proc/sys_net.go
index f3b63dfc2..bd93f83fa 100644
--- a/pkg/sentry/fs/proc/sys_net.go
+++ b/pkg/sentry/fs/proc/sys_net.go
@@ -64,7 +64,7 @@ var _ fs.InodeOperations = (*tcpMemInode)(nil)
 
 func newTCPMemInode(ctx context.Context, msrc *fs.MountSource, s inet.Stack, dir tcpMemDir) *fs.Inode {
 	tm := &tcpMemInode{
-		SimpleFileInode: *fsutil.NewSimpleFileInode(ctx, fs.RootOwner, fs.FilePermsFromMode(0444), linux.PROC_SUPER_MAGIC),
+		SimpleFileInode: *fsutil.NewSimpleFileInode(ctx, fs.RootOwner, fs.FilePermsFromMode(0644), linux.PROC_SUPER_MAGIC),
 		s:               s,
 		dir:             dir,
 	}
@@ -77,6 +77,11 @@ func newTCPMemInode(ctx context.Context, msrc *fs.MountSource, s inet.Stack, dir
 	return fs.NewInode(ctx, tm, msrc, sattr)
 }
 
+// Truncate implements fs.InodeOperations.Truncate.
+func (tcpMemInode) Truncate(context.Context, *fs.Inode, int64) error {
+	return nil
+}
+
 // GetFile implements fs.InodeOperations.GetFile.
 func (m *tcpMemInode) GetFile(ctx context.Context, dirent *fs.Dirent, flags fs.FileFlags) (*fs.File, error) {
 	flags.Pread = true
@@ -168,14 +173,15 @@ func writeSize(dirType tcpMemDir, s inet.Stack, size inet.TCPBufferSize) error {
 
 // +stateify savable
 type tcpSack struct {
+	fsutil.SimpleFileInode
+
 	stack   inet.Stack `state:"wait"`
 	enabled *bool
-	fsutil.SimpleFileInode
 }
 
 func newTCPSackInode(ctx context.Context, msrc *fs.MountSource, s inet.Stack) *fs.Inode {
 	ts := &tcpSack{
-		SimpleFileInode: *fsutil.NewSimpleFileInode(ctx, fs.RootOwner, fs.FilePermsFromMode(0444), linux.PROC_SUPER_MAGIC),
+		SimpleFileInode: *fsutil.NewSimpleFileInode(ctx, fs.RootOwner, fs.FilePermsFromMode(0644), linux.PROC_SUPER_MAGIC),
 		stack:           s,
 	}
 	sattr := fs.StableAttr{
@@ -187,6 +193,11 @@ func newTCPSackInode(ctx context.Context, msrc *fs.MountSource, s inet.Stack) *f
 	return fs.NewInode(ctx, ts, msrc, sattr)
 }
 
+// Truncate implements fs.InodeOperations.Truncate.
+func (tcpSack) Truncate(context.Context, *fs.Inode, int64) error {
+	return nil
+}
+
 // GetFile implements fs.InodeOperations.GetFile.
 func (s *tcpSack) GetFile(ctx context.Context, dirent *fs.Dirent, flags fs.FileFlags) (*fs.File, error) {
 	flags.Pread = true
diff --git a/pkg/sentry/fs/tty/master.go b/pkg/sentry/fs/tty/master.go
index bc56be696..934828c12 100644
--- a/pkg/sentry/fs/tty/master.go
+++ b/pkg/sentry/fs/tty/master.go
@@ -32,7 +32,6 @@ import (
 // +stateify savable
 type masterInodeOperations struct {
 	fsutil.SimpleFileInode
-	fsutil.InodeNoopTruncate
 
 	// d is the containing dir.
 	d *dirInodeOperations
@@ -77,6 +76,11 @@ func newMasterInode(ctx context.Context, d *dirInodeOperations, owner fs.FileOwn
 func (mi *masterInodeOperations) Release(ctx context.Context) {
 }
 
+// Truncate implements fs.InodeOperations.Truncate.
+func (masterInodeOperations) Truncate(context.Context, *fs.Inode, int64) error {
+	return nil
+}
+
 // GetFile implements fs.InodeOperations.GetFile.
 //
 // It allocates a new terminal.
diff --git a/pkg/sentry/fs/tty/slave.go b/pkg/sentry/fs/tty/slave.go
index 4cbea0367..2a51e6bab 100644
--- a/pkg/sentry/fs/tty/slave.go
+++ b/pkg/sentry/fs/tty/slave.go
@@ -31,7 +31,6 @@ import (
 // +stateify savable
 type slaveInodeOperations struct {
 	fsutil.SimpleFileInode
-	fsutil.InodeNoopTruncate
 
 	// d is the containing dir.
 	d *dirInodeOperations
@@ -73,6 +72,11 @@ func (si *slaveInodeOperations) Release(ctx context.Context) {
 	si.t.DecRef()
 }
 
+// Truncate implements fs.InodeOperations.Truncate.
+func (slaveInodeOperations) Truncate(context.Context, *fs.Inode, int64) error {
+	return nil
+}
+
 // GetFile implements fs.InodeOperations.GetFile.
 //
 // This may race with destruction of the terminal. If the terminal is gone, it
diff --git a/pkg/sentry/syscalls/linux/sys_file.go b/pkg/sentry/syscalls/linux/sys_file.go
index 167c2b60b..3b9181002 100644
--- a/pkg/sentry/syscalls/linux/sys_file.go
+++ b/pkg/sentry/syscalls/linux/sys_file.go
@@ -171,6 +171,9 @@ func openAt(t *kernel.Task, dirFD int32, addr usermem.Addr, flags uint) (fd uint
 			}
 		}
 
+		// Truncate is called when O_TRUNC is specified for any kind of
+		// existing Dirent. Behavior is delegated to the entry's Truncate
+		// implementation.
 		if flags&linux.O_TRUNC != 0 {
 			if err := d.Inode.Truncate(t, d, 0); err != nil {
 				return err
@@ -397,7 +400,9 @@ func createAt(t *kernel.Task, dirFD int32, addr usermem.Addr, flags uint, mode l
 				return err
 			}
 
-			// Should we truncate the file?
+			// Truncate is called when O_TRUNC is specified for any kind of
+			// existing Dirent. Behavior is delegated to the entry's Truncate
+			// implementation.
 			if flags&linux.O_TRUNC != 0 {
 				if err := found.Inode.Truncate(t, found, 0); err != nil {
 					return err
@@ -1484,6 +1489,8 @@ func Truncate(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sysc
 		if fs.IsDir(d.Inode.StableAttr) {
 			return syserror.EISDIR
 		}
+		// In contrast to open(O_TRUNC), truncate(2) is only valid for file
+		// types.
 		if !fs.IsFile(d.Inode.StableAttr) {
 			return syserror.EINVAL
 		}
@@ -1522,7 +1529,8 @@ func Ftruncate(t *kernel.Task, args arch.SyscallArguments) (uintptr, *kernel.Sys
 		return 0, nil, syserror.EINVAL
 	}
 
-	// Note that this is different from truncate(2) above, where a
+	// In contrast to open(O_TRUNC), truncate(2) is only valid for file
+	// types. Note that this is different from truncate(2) above, where a
 	// directory returns EISDIR.
 	if !fs.IsFile(file.Dirent.Inode.StableAttr) {
 		return 0, nil, syserror.EINVAL