Decouple file_handle_sharing and cache settings in the gofer client.

Earlier, both these settings were controlled by the cache= option. Only
cache_none was able to disable file handle sharing. But there can be cases
where we want the FS cache but disable file handle sharing. Instead of adding
yet another cache enum, decouple these settings so they can be set
independently. As a result, we don't need cache_none anymore, get rid of it.

PiperOrigin-RevId: 518780704

pkg/sentry/fsimpl/gofer/filesystem.go

@@ -1736,11 +1736,10 @@ func (fs *filesystem) MountOptions() string {
 	case InteropModeWritethrough:
 		optsKV = append(optsKV, mopt{moptCache, cacheFSCacheWritethrough})
 	case InteropModeShared:
-		if fs.opts.regularFilesUseSpecialFileFD {
-			optsKV = append(optsKV, mopt{moptCache, cacheNone})
-		} else {
-			optsKV = append(optsKV, mopt{moptCache, cacheRemoteRevalidating})
-		}
+		optsKV = append(optsKV, mopt{moptCache, cacheRemoteRevalidating})
+	}
+	if fs.opts.regularFilesUseSpecialFileFD {
+		optsKV = append(optsKV, mopt{moptDisableFileHandleSharing, nil})
 	}
 	if fs.opts.forcePageCache {
 		optsKV = append(optsKV, mopt{moptForcePageCache, nil})

pkg/sentry/fsimpl/gofer/gofer.go

@@ -73,16 +73,17 @@ const Name = "9p"
 
 // Mount option names for goferfs.
 const (
-	moptTransport              = "trans"
-	moptReadFD                 = "rfdno"
-	moptWriteFD                = "wfdno"
-	moptAname                  = "aname"
-	moptDfltUID                = "dfltuid"
-	moptDfltGID                = "dfltgid"
-	moptCache                  = "cache"
-	moptForcePageCache         = "force_page_cache"
-	moptLimitHostFDTranslation = "limit_host_fd_translation"
-	moptOverlayfsStaleRead     = "overlayfs_stale_read"
+	moptTransport                = "trans"
+	moptReadFD                   = "rfdno"
+	moptWriteFD                  = "wfdno"
+	moptAname                    = "aname"
+	moptDfltUID                  = "dfltuid"
+	moptDfltGID                  = "dfltgid"
+	moptCache                    = "cache"
+	moptForcePageCache           = "force_page_cache"
+	moptLimitHostFDTranslation   = "limit_host_fd_translation"
+	moptOverlayfsStaleRead       = "overlayfs_stale_read"
+	moptDisableFileHandleSharing = "disable_file_handle_sharing"
 
 	// Directfs options.
 	moptDirectfs = "directfs"
@@ -90,7 +91,6 @@ const (
 
 // Valid values for the "cache" mount option.
 const (
-	cacheNone                = "none"
 	cacheFSCache             = "fscache"
 	cacheFSCacheWritethrough = "fscache_writethrough"
 	cacheRemoteRevalidating  = "remote_revalidating"
@@ -422,9 +422,6 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 			fsopts.interop = InteropModeExclusive
 		case cacheFSCacheWritethrough:
 			fsopts.interop = InteropModeWritethrough
-		case cacheNone:
-			fsopts.regularFilesUseSpecialFileFD = true
-			fallthrough
 		case cacheRemoteRevalidating:
 			fsopts.interop = InteropModeShared
 		default:
@@ -459,6 +456,10 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 	}
 
 	// Handle simple flags.
+	if _, ok := mopts[moptDisableFileHandleSharing]; ok {
+		delete(mopts, moptDisableFileHandleSharing)
+		fsopts.regularFilesUseSpecialFileFD = true
+	}
 	if _, ok := mopts[moptForcePageCache]; ok {
 		delete(mopts, moptForcePageCache)
 		fsopts.forcePageCache = true