Add equivalents to FMODE_PREAD/PWRITE to VFS2.

This is mostly required for PipeTest_OffsetCalls.

The options are DenyPRead/PWrite rather than AllowPRead/PWrite since, in Linux
terms, fs/open.c:do_dentry_open sets FMODE_PREAD|FMODE_PWRITE unconditionally
(although it allows filesystem implementations of open to unset these flags),
so they're set for most FDs; it's usually FDs created outside of open(2) that
don't get them, e.g.:

- Syscall-created pipes (fs/pipe.c:create_pipe_files =>
  fs/file_table.c:alloc_file_pseudo)

- Epoll instances (fs/eventpoll.c:do_epoll_create =>
  fs/anon_inodes.c:anon_inode_getfile => alloc_file_pseudo)

- Sockets (net/socket.c:sock_alloc_file => alloc_file_pseudo)

This CL adds the flags to epoll instances; a subsequent CL reworks the VFS2
implementation of pipe FDs to be filesystem-independent and adds the flags
there, and sockets aren't implemented yet.

Updates #1035

PiperOrigin-RevId: 304506434

1 files changed, 2 insertions, 0 deletions

diff --git a/pkg/sentry/vfs/epoll.go b/pkg/sentry/vfs/epoll.go
index 3da45d744..8e0b40841 100644
--- a/pkg/sentry/vfs/epoll.go
+++ b/pkg/sentry/vfs/epoll.go
@@ -99,6 +99,8 @@ func (vfs *VirtualFilesystem) NewEpollInstanceFD() (*FileDescription, error) {
 		interest: make(map[epollInterestKey]*epollInterest),
 	}
 	if err := ep.vfsfd.Init(ep, linux.O_RDWR, vd.Mount(), vd.Dentry(), &FileDescriptionOptions{
+		DenyPRead:         true,
+		DenyPWrite:        true,
 		UseDentryMetadata: true,
 	}); err != nil {
 		return nil, err