Send SETATTR request after open when atomic_o_trunc is not set.

In Linux, the VFS is responsible for implementing O_TRUNC. It does
this by calling setattr with a zero size on the underlying inode.
The sentry VFS does not have the same inode abstraction, so we need
to implement the SETATTR behavior inside the fusefs implementation.

PiperOrigin-RevId: 501385452

pkg/sentry/fsimpl/fuse/connection_control.go

@@ -141,6 +141,7 @@ func (conn *connection) initProcessReply(out *linux.FUSEInitOut, hasSysAdminCap
 		conn.bigWrites = out.Flags&linux.FUSE_BIG_WRITES != 0
 		conn.dontMask = out.Flags&linux.FUSE_DONT_MASK != 0
 		conn.writebackCache = out.Flags&linux.FUSE_WRITEBACK_CACHE != 0
+		conn.atomicOTrunc = out.Flags&linux.FUSE_ATOMIC_O_TRUNC != 0
 
 		// TODO(gvisor.dev/issue/3195): figure out how to use TimeGran (0 < TimeGran <= fuseMaxTimeGranNs).
 

pkg/sentry/fsimpl/fuse/fusefs.go

@@ -476,15 +476,20 @@ func (i *inode) Open(ctx context.Context, rp *vfs.ResolvingPath, d *kernfs.Dentr
 		fd.Nonseekable = true
 	}
 
-	// If we don't send SETATTR before open (which is indicated by atomicOTrunc)
-	// and O_TRUNC is set, update the inode's version number and clean existing data
-	// by setting the file size to 0.
-	if i.fs.conn.atomicOTrunc && opts.Flags&linux.O_TRUNC != 0 {
-		i.fs.conn.mu.Lock()
-		i.attributeVersion.Store(i.fs.conn.attributeVersion.Add(1))
-		i.size.Store(0)
-		i.fs.conn.mu.Unlock()
-		i.attributeTime = 0
+	// If atomicOTrunc and O_TRUNC are set, just update the inode's version number
+	// and set its size to 0 since the truncation is handled by the FUSE daemon.
+	// Otherwise send a separate SETATTR to truncate the file size.
+	if opts.Flags&linux.O_TRUNC != 0 {
+		if i.fs.conn.atomicOTrunc {
+			i.fs.conn.mu.Lock()
+			i.attributeVersion.Store(i.fs.conn.attributeVersion.Add(1))
+			i.size.Store(0)
+			i.fs.conn.mu.Unlock()
+			i.attributeTime = 0
+		} else {
+			opts := vfs.SetStatOptions{Stat: linux.Statx{Size: 0, Mask: linux.STATX_SIZE}}
+			i.setAttr(ctx, i.fs.VFSFilesystem(), auth.CredentialsFromContext(ctx), opts, true, i.newFhData.fh)
+		}
 	}
 
 	if err := fd.vfsfd.Init(fdImpl, opts.Flags, rp.Mount(), d.VFSDentry(), fdOptions); err != nil {

test/fuse/linux/fuse_base.h

@@ -35,7 +35,8 @@ namespace testing {
 
 constexpr char kMountOpts[] = "rootmode=755,user_id=0,group_id=0";
 
-constexpr struct fuse_init_out kDefaultFUSEInitOutPayload = {.major = 7};
+constexpr struct fuse_init_out kDefaultFUSEInitOutPayload = {
+    .major = 7, .minor = 14, .flags = FUSE_ATOMIC_O_TRUNC};
 
 // Internal commands used to communicate between testing thread and the FUSE
 // server. See test/fuse/README.md for further detail.

test/syscalls/BUILD

@@ -128,6 +128,7 @@ syscall_test(
 syscall_test(
     add_overlay = True,
     test = "//test/syscalls/linux:creat_test",
+    use_fusefs = True,
 )
 
 syscall_test(