fs: Add new cache policy "remote_revalidate".

This CL adds a new cache-policy for gofer filesystems that uses the host page
cache, but causes dirents to be reloaded on each Walk, and does not cache
readdir results.

This policy is useful when the remote filesystem may change out from underneath
us, as any remote changes will be reflected on the next Walk.

Importantly, this cache policy is only consistent if we do not use gVisor's
internal page cache, since that page cache is tied to the Inode and may be
thrown away upon Revalidation.

This cache policy should only be used when the gofer supports donating host
FDs, since then gVisor will make use of the host kernel page cache, which will
be consistent for all open files in the gofer. In fact, a panic will be raised
if a file is opened without a donated FD.

PiperOrigin-RevId: 207752937
Change-Id: I233cb78b4695bbe00a4605ae64080a47629329b8

1 files changed, 12 insertions, 1 deletions

diff --git a/pkg/sentry/fs/gofer/file.go b/pkg/sentry/fs/gofer/file.go
index 46a6bbd5d..c4a210656 100644
--- a/pkg/sentry/fs/gofer/file.go
+++ b/pkg/sentry/fs/gofer/file.go
@@ -15,6 +15,7 @@
 package gofer
 
 import (
+	"fmt"
 	"syscall"
 
 	"gvisor.googlesource.com/gvisor/pkg/log"
@@ -72,6 +73,17 @@ func NewFile(ctx context.Context, dirent *fs.Dirent, name string, flags fs.FileF
 	flags.Pread = true
 	flags.Pwrite = true
 
+	if fs.IsFile(dirent.Inode.StableAttr) {
+		// If cache policy is "remote revalidating", then we must
+		// ensure that we have a host FD. Otherwise, the
+		// sentry-internal page cache will be used, and we can end up
+		// in an inconsistent state if the remote file changes.
+		cp := dirent.Inode.InodeOperations.(*inodeOperations).session().cachePolicy
+		if cp == cacheRemoteRevalidating && handles.Host == nil {
+			panic(fmt.Sprintf("remote-revalidating cache policy requires gofer to donate host FD, but file %q did not have host FD", name))
+		}
+	}
+
 	f := &fileOperations{
 		inodeOperations: i,
 		handles:         handles,
@@ -202,7 +214,6 @@ func (f *fileOperations) Write(ctx context.Context, file *fs.File, src usermem.I
 			err = f.inodeOperations.cachingInodeOps.WriteOut(ctx, file.Dirent.Inode)
 		}
 		return n, err
-
 	}
 	return src.CopyInTo(ctx, f.handles.readWriterAt(ctx, offset))
 }