Implement copy-up-coherent mmap for VFS2 overlayfs.

This is very similar to copy-up-coherent mmap in the VFS1 overlay, with the
minor wrinkle that there is no fs.InodeOperations.Mappable().

Updates #1199

PiperOrigin-RevId: 331206314

1 files changed, 34 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/overlay/overlay.go b/pkg/sentry/fsimpl/overlay/overlay.go
index 9a8f7010e..b2efe5f80 100644
--- a/pkg/sentry/fsimpl/overlay/overlay.go
+++ b/pkg/sentry/fsimpl/overlay/overlay.go
@@ -22,6 +22,10 @@
 //   filesystem.renameMu
 //     dentry.dirMu
 //       dentry.copyMu
+//         *** "memmap.Mappable locks" below this point
+//         dentry.mapsMu
+//           *** "memmap.Mappable locks taken by Translate" below this point
+//           dentry.dataMu
 //
 // Locking dentry.dirMu in multiple dentries requires that parent dentries are
 // locked before child dentries, and that filesystem.renameMu is locked to
@@ -37,6 +41,7 @@ import (
 	"gvisor.dev/gvisor/pkg/fspath"
 	fslock "gvisor.dev/gvisor/pkg/sentry/fs/lock"
 	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/sync"
 	"gvisor.dev/gvisor/pkg/syserror"
@@ -419,6 +424,35 @@ type dentry struct {
 	devMinor uint32
 	ino      uint64
 
+	// If this dentry represents a regular file, then:
+	//
+	// - mapsMu is used to synchronize between copy-up and memmap.Mappable
+	// methods on dentry preceding mm.MemoryManager.activeMu in the lock order.
+	//
+	// - dataMu is used to synchronize between copy-up and
+	// dentry.(memmap.Mappable).Translate.
+	//
+	// - lowerMappings tracks memory mappings of the file. lowerMappings is
+	// used to invalidate mappings of the lower layer when the file is copied
+	// up to ensure that they remain coherent with subsequent writes to the
+	// file. (Note that, as of this writing, Linux overlayfs does not do this;
+	// this feature is a gVisor extension.) lowerMappings is protected by
+	// mapsMu.
+	//
+	// - If this dentry is copied-up, then wrappedMappable is the Mappable
+	// obtained from a call to the current top layer's
+	// FileDescription.ConfigureMMap(). Once wrappedMappable becomes non-nil
+	// (from a call to nonDirectoryFD.ensureMappable()), it cannot become nil.
+	// wrappedMappable is protected by mapsMu and dataMu.
+	//
+	// - isMappable is non-zero iff wrappedMappable is non-nil. isMappable is
+	// accessed using atomic memory operations.
+	mapsMu          sync.Mutex
+	lowerMappings   memmap.MappingSet
+	dataMu          sync.RWMutex
+	wrappedMappable memmap.Mappable
+	isMappable      uint32
+
 	locks vfs.FileLocks
 }