Support setting STATX_SIZE for kernfs.InodeAttrs.

Make setting STATX_SIZE a no-op, if it is valid for the given permissions and
file type.

Also update proc tests, which were overfitted before.

Fixes #3842.
Updates #1193.

PiperOrigin-RevId: 331861087

1 files changed, 11 insertions, 8 deletions

diff --git a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
index c0b863ba4..74408e322 100644
--- a/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
+++ b/pkg/sentry/fsimpl/kernfs/inode_impl_util.go
@@ -259,9 +259,19 @@ func (a *InodeAttrs) SetStat(ctx context.Context, fs *vfs.Filesystem, creds *aut
 	if opts.Stat.Mask == 0 {
 		return nil
 	}
-	if opts.Stat.Mask&^(linux.STATX_MODE|linux.STATX_UID|linux.STATX_GID) != 0 {
+
+	// Note that not all fields are modifiable. For example, the file type and
+	// inode numbers are immutable after node creation. Setting the size is often
+	// allowed by kernfs files but does not do anything. If some other behavior is
+	// needed, the embedder should consider extending SetStat.
+	//
+	// TODO(gvisor.dev/issue/1193): Implement other stat fields like timestamps.
+	if opts.Stat.Mask&^(linux.STATX_MODE|linux.STATX_UID|linux.STATX_GID|linux.STATX_SIZE) != 0 {
 		return syserror.EPERM
 	}
+	if opts.Stat.Mask&linux.STATX_SIZE != 0 && a.Mode().IsDir() {
+		return syserror.EISDIR
+	}
 	if err := vfs.CheckSetStat(ctx, creds, &opts, a.Mode(), auth.KUID(atomic.LoadUint32(&a.uid)), auth.KGID(atomic.LoadUint32(&a.gid))); err != nil {
 		return err
 	}
@@ -284,13 +294,6 @@ func (a *InodeAttrs) SetStat(ctx context.Context, fs *vfs.Filesystem, creds *aut
 		atomic.StoreUint32(&a.gid, stat.GID)
 	}
 
-	// Note that not all fields are modifiable. For example, the file type and
-	// inode numbers are immutable after node creation.
-
-	// TODO(gvisor.dev/issue/1193): Implement other stat fields like timestamps.
-	// Also, STATX_SIZE will need some special handling, because read-only static
-	// files should return EIO for truncate operations.
-
 	return nil
 }