From 7da69fe9719b26f5d15e80b2764459c4cac80b0e Mon Sep 17 00:00:00 2001
From: Nicolas Lacasse <nlacasse@google.com>
Date: Wed, 3 Jun 2020 12:33:51 -0700
Subject: Fix data race on f.offset.

We must hold f.mu to write f.offset.

PiperOrigin-RevId: 314582968
---
 pkg/sentry/fs/file.go | 9 ++++-----
 pkg/sentry/fs/fs.go   | 3 ++-
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/pkg/sentry/fs/file.go b/pkg/sentry/fs/file.go
index 846252c89..2a278fbe3 100644
--- a/pkg/sentry/fs/file.go
+++ b/pkg/sentry/fs/file.go
@@ -310,7 +310,6 @@ func (f *File) Writev(ctx context.Context, src usermem.IOSequence) (int64, error
 	if !f.mu.Lock(ctx) {
 		return 0, syserror.ErrInterrupted
 	}
-
 	unlockAppendMu := f.Dirent.Inode.lockAppendMu(f.Flags().Append)
 	// Handle append mode.
 	if f.Flags().Append {
@@ -355,7 +354,6 @@ func (f *File) Pwritev(ctx context.Context, src usermem.IOSequence, offset int64
 	// offset."
 	unlockAppendMu := f.Dirent.Inode.lockAppendMu(f.Flags().Append)
 	defer unlockAppendMu()
-
 	if f.Flags().Append {
 		if err := f.offsetForAppend(ctx, &offset); err != nil {
 			return 0, err
@@ -374,9 +372,10 @@ func (f *File) Pwritev(ctx context.Context, src usermem.IOSequence, offset int64
 	return f.FileOperations.Write(ctx, f, src, offset)
 }
 
-// offsetForAppend sets the given offset to the end of the file.
+// offsetForAppend atomically sets the given offset to the end of the file.
 //
-// Precondition: the file.Dirent.Inode.appendMu mutex should be held for writing.
+// Precondition: the file.Dirent.Inode.appendMu mutex should be held for
+// writing.
 func (f *File) offsetForAppend(ctx context.Context, offset *int64) error {
 	uattr, err := f.Dirent.Inode.UnstableAttr(ctx)
 	if err != nil {
@@ -386,7 +385,7 @@ func (f *File) offsetForAppend(ctx context.Context, offset *int64) error {
 	}
 
 	// Update the offset.
-	*offset = uattr.Size
+	atomic.StoreInt64(offset, uattr.Size)
 
 	return nil
 }
diff --git a/pkg/sentry/fs/fs.go b/pkg/sentry/fs/fs.go
index bdba6efe5..d2dbff268 100644
--- a/pkg/sentry/fs/fs.go
+++ b/pkg/sentry/fs/fs.go
@@ -42,9 +42,10 @@
 //     Dirent.dirMu
 //       Dirent.mu
 //         DirentCache.mu
-//         Locks in InodeOperations implementations or overlayEntry
 //         Inode.Watches.mu (see `Inotify` for other lock ordering)
 //         MountSource.mu
+//         Inode.appendMu
+//           Locks in InodeOperations implementations or overlayEntry
 //
 // If multiple Dirent or MountSource locks must be taken, locks in the parent must be
 // taken before locks in their children.
-- 
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