Plumb getting/setting xattrs through InodeOperations and 9p gofer interfaces.

There was a very bare get/setxattr in the InodeOperations interface. Add
context.Context to both, size to getxattr, and flags to setxattr.
Note that extended attributes are passed around as strings in this
implementation, so size is automatically encoded into the value. Size is
added in getxattr so that implementations can return ERANGE if a value is larger
than can fit in the user-allocated buffer. This prevents us from unnecessarily
passing around an arbitrarily large xattr when the user buffer is actually too
small.

Don't use the existing xattrwalk and xattrcreate messages and define our
own, mainly for the sake of simplicity.

Extended attributes will be implemented in future commits.

PiperOrigin-RevId: 290121300

1 files changed, 16 insertions, 9 deletions

diff --git a/pkg/sentry/fs/inode_operations.go b/pkg/sentry/fs/inode_operations.go
index 5cde9d215..13261cb81 100644
--- a/pkg/sentry/fs/inode_operations.go
+++ b/pkg/sentry/fs/inode_operations.go
@@ -170,20 +170,27 @@ type InodeOperations interface {
 	// file system events.
 	UnstableAttr(ctx context.Context, inode *Inode) (UnstableAttr, error)
 
-	// Getxattr retrieves the value of extended attribute name. Inodes that
-	// do not support extended attributes return EOPNOTSUPP. Inodes that
-	// support extended attributes but don't have a value at name return
+	// GetXattr retrieves the value of extended attribute specified by name.
+	// Inodes that do not support extended attributes return EOPNOTSUPP. Inodes
+	// that support extended attributes but don't have a value at name return
 	// ENODATA.
-	Getxattr(inode *Inode, name string) (string, error)
+	//
+	// If this is called through the getxattr(2) syscall, size indicates the
+	// size of the buffer that the application has allocated to hold the
+	// attribute value. If the value is larger than size, implementations may
+	// return ERANGE to indicate that the buffer is too small, but they are also
+	// free to ignore the hint entirely (i.e. the value returned may be larger
+	// than size). All size checking is done independently at the syscall layer.
+	GetXattr(ctx context.Context, inode *Inode, name string, size uint64) (string, error)
 
-	// Setxattr sets the value of extended attribute name. Inodes that
-	// do not support extended attributes return EOPNOTSUPP.
-	Setxattr(inode *Inode, name, value string) error
+	// SetXattr sets the value of extended attribute specified by name. Inodes
+	// that do not support extended attributes return EOPNOTSUPP.
+	SetXattr(ctx context.Context, inode *Inode, name, value string, flags uint32) error
 
-	// Listxattr returns the set of all extended attributes names that
+	// ListXattr returns the set of all extended attributes names that
 	// have values. Inodes that do not support extended attributes return
 	// EOPNOTSUPP.
-	Listxattr(inode *Inode) (map[string]struct{}, error)
+	ListXattr(ctx context.Context, inode *Inode) (map[string]struct{}, error)
 
 	// Check determines whether an Inode can be accessed with the
 	// requested permission mask using the context (which gives access