[vfs2][gofer] Update file size to 0 on O_TRUNC

Some Open:TruncateXxx syscall tests were failing because the file size was
not being updated when the file was opened with O_TRUNC.

Fixes Truncate tests in test/syscalls:open_test_runsc_ptrace_vfs2.

Updates #2923

PiperOrigin-RevId: 319340127

4 files changed, 76 insertions, 42 deletions

diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index 51082359d..7bcc99b29 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -869,11 +869,22 @@ func (d *dentry) openLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vf
 	if err := d.checkPermissions(rp.Credentials(), ats); err != nil {
 		return nil, err
 	}
+
+	trunc := opts.Flags&linux.O_TRUNC != 0 && d.fileType() == linux.S_IFREG
+	if trunc {
+		// Lock metadataMu *while* we open a regular file with O_TRUNC because
+		// open(2) will change the file size on server.
+		d.metadataMu.Lock()
+		defer d.metadataMu.Unlock()
+	}
+
+	var vfd *vfs.FileDescription
+	var err error
 	mnt := rp.Mount()
 	switch d.fileType() {
 	case linux.S_IFREG:
 		if !d.fs.opts.regularFilesUseSpecialFileFD {
-			if err := d.ensureSharedHandle(ctx, ats&vfs.MayRead != 0, ats&vfs.MayWrite != 0, opts.Flags&linux.O_TRUNC != 0); err != nil {
+			if err := d.ensureSharedHandle(ctx, ats&vfs.MayRead != 0, ats&vfs.MayWrite != 0, trunc); err != nil {
 				return nil, err
 			}
 			fd := &regularFileFD{}
@@ -883,7 +894,7 @@ func (d *dentry) openLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vf
 			}); err != nil {
 				return nil, err
 			}
-			return &fd.vfsfd, nil
+			vfd = &fd.vfsfd
 		}
 	case linux.S_IFDIR:
 		// Can't open directories with O_CREAT.
@@ -923,7 +934,25 @@ func (d *dentry) openLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vf
 			return d.pipe.Open(ctx, mnt, &d.vfsd, opts.Flags, &d.locks)
 		}
 	}
-	return d.openSpecialFileLocked(ctx, mnt, opts)
+
+	if vfd == nil {
+		if vfd, err = d.openSpecialFileLocked(ctx, mnt, opts); err != nil {
+			return nil, err
+		}
+	}
+
+	if trunc {
+		// If no errors occured so far then update file size in memory. This
+		// step is required even if !d.cachedMetadataAuthoritative() because
+		// d.mappings has to be updated.
+		// d.metadataMu has already been acquired if trunc == true.
+		d.updateFileSizeLocked(0)
+
+		if d.cachedMetadataAuthoritative() {
+			d.touchCMtimeLocked()
+		}
+	}
+	return vfd, err
 }
 
 func (d *dentry) connectSocketLocked(ctx context.Context, opts *vfs.OpenOptions) (*vfs.FileDescription, error) {
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 71c8d3ae1..709b5e496 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -794,9 +794,7 @@ func (d *dentry) updateFromP9Attrs(mask p9.AttrMask, attr *p9.Attr) {
 		atomic.StoreUint32(&d.nlink, uint32(attr.NLink))
 	}
 	if mask.Size {
-		d.dataMu.Lock()
-		atomic.StoreUint64(&d.size, attr.Size)
-		d.dataMu.Unlock()
+		d.updateFileSizeLocked(attr.Size)
 	}
 	d.metadataMu.Unlock()
 }
@@ -964,39 +962,44 @@ func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, stat *lin
 	}
 	atomic.StoreInt64(&d.ctime, now)
 	if stat.Mask&linux.STATX_SIZE != 0 {
+		d.updateFileSizeLocked(stat.Size)
+	}
+	return nil
+}
+
+// Preconditions: d.metadataMu must be locked.
+func (d *dentry) updateFileSizeLocked(newSize uint64) {
+	d.dataMu.Lock()
+	oldSize := d.size
+	d.size = newSize
+	// d.dataMu must be unlocked to lock d.mapsMu and invalidate mappings
+	// below. This allows concurrent calls to Read/Translate/etc. These
+	// functions synchronize with truncation by refusing to use cache
+	// contents beyond the new d.size. (We are still holding d.metadataMu,
+	// so we can't race with Write or another truncate.)
+	d.dataMu.Unlock()
+	if d.size < oldSize {
+		oldpgend, _ := usermem.PageRoundUp(oldSize)
+		newpgend, _ := usermem.PageRoundUp(d.size)
+		if oldpgend != newpgend {
+			d.mapsMu.Lock()
+			d.mappings.Invalidate(memmap.MappableRange{newpgend, oldpgend}, memmap.InvalidateOpts{
+				// Compare Linux's mm/truncate.c:truncate_setsize() =>
+				// truncate_pagecache() =>
+				// mm/memory.c:unmap_mapping_range(evencows=1).
+				InvalidatePrivate: true,
+			})
+			d.mapsMu.Unlock()
+		}
+		// We are now guaranteed that there are no translations of
+		// truncated pages, and can remove them from the cache. Since
+		// truncated pages have been removed from the remote file, they
+		// should be dropped without being written back.
 		d.dataMu.Lock()
-		oldSize := d.size
-		d.size = stat.Size
-		// d.dataMu must be unlocked to lock d.mapsMu and invalidate mappings
-		// below. This allows concurrent calls to Read/Translate/etc. These
-		// functions synchronize with truncation by refusing to use cache
-		// contents beyond the new d.size. (We are still holding d.metadataMu,
-		// so we can't race with Write or another truncate.)
+		d.cache.Truncate(d.size, d.fs.mfp.MemoryFile())
+		d.dirty.KeepClean(memmap.MappableRange{d.size, oldpgend})
 		d.dataMu.Unlock()
-		if d.size < oldSize {
-			oldpgend, _ := usermem.PageRoundUp(oldSize)
-			newpgend, _ := usermem.PageRoundUp(d.size)
-			if oldpgend != newpgend {
-				d.mapsMu.Lock()
-				d.mappings.Invalidate(memmap.MappableRange{newpgend, oldpgend}, memmap.InvalidateOpts{
-					// Compare Linux's mm/truncate.c:truncate_setsize() =>
-					// truncate_pagecache() =>
-					// mm/memory.c:unmap_mapping_range(evencows=1).
-					InvalidatePrivate: true,
-				})
-				d.mapsMu.Unlock()
-			}
-			// We are now guaranteed that there are no translations of
-			// truncated pages, and can remove them from the cache. Since
-			// truncated pages have been removed from the remote file, they
-			// should be dropped without being written back.
-			d.dataMu.Lock()
-			d.cache.Truncate(d.size, d.fs.mfp.MemoryFile())
-			d.dirty.KeepClean(memmap.MappableRange{d.size, oldpgend})
-			d.dataMu.Unlock()
-		}
 	}
-	return nil
 }
 
 func (d *dentry) checkPermissions(creds *auth.Credentials, ats vfs.AccessTypes) error {
diff --git a/pkg/sentry/fsimpl/gofer/time.go b/pkg/sentry/fsimpl/gofer/time.go
index 1d5aa82dc..0eef4e16e 100644
--- a/pkg/sentry/fsimpl/gofer/time.go
+++ b/pkg/sentry/fsimpl/gofer/time.go
@@ -36,7 +36,7 @@ func statxTimestampFromDentry(ns int64) linux.StatxTimestamp {
 	}
 }
 
-// Preconditions: fs.interop != InteropModeShared.
+// Preconditions: d.cachedMetadataAuthoritative() == true.
 func (d *dentry) touchAtime(mnt *vfs.Mount) {
 	if mnt.Flags.NoATime {
 		return
@@ -51,8 +51,8 @@ func (d *dentry) touchAtime(mnt *vfs.Mount) {
 	mnt.EndWrite()
 }
 
-// Preconditions: fs.interop != InteropModeShared. The caller has successfully
-// called vfs.Mount.CheckBeginWrite().
+// Preconditions: d.cachedMetadataAuthoritative() == true. The caller has
+// successfully called vfs.Mount.CheckBeginWrite().
 func (d *dentry) touchCtime() {
 	now := d.fs.clock.Now().Nanoseconds()
 	d.metadataMu.Lock()
@@ -60,8 +60,8 @@ func (d *dentry) touchCtime() {
 	d.metadataMu.Unlock()
 }
 
-// Preconditions: fs.interop != InteropModeShared. The caller has successfully
-// called vfs.Mount.CheckBeginWrite().
+// Preconditions: d.cachedMetadataAuthoritative() == true. The caller has
+// successfully called vfs.Mount.CheckBeginWrite().
 func (d *dentry) touchCMtime() {
 	now := d.fs.clock.Now().Nanoseconds()
 	d.metadataMu.Lock()
@@ -70,6 +70,8 @@ func (d *dentry) touchCMtime() {
 	d.metadataMu.Unlock()
 }
 
+// Preconditions: d.cachedMetadataAuthoritative() == true. The caller has
+// locked d.metadataMu.
 func (d *dentry) touchCMtimeLocked() {
 	now := d.fs.clock.Now().Nanoseconds()
 	atomic.StoreInt64(&d.mtime, now)
diff --git a/pkg/sentry/fsimpl/tmpfs/filesystem.go b/pkg/sentry/fsimpl/tmpfs/filesystem.go
index ce0dbb154..ed40f6b52 100644
--- a/pkg/sentry/fsimpl/tmpfs/filesystem.go
+++ b/pkg/sentry/fsimpl/tmpfs/filesystem.go
@@ -410,7 +410,7 @@ func (d *dentry) open(ctx context.Context, rp *vfs.ResolvingPath, opts *vfs.Open
 		if err := fd.vfsfd.Init(&fd, opts.Flags, rp.Mount(), &d.vfsd, &vfs.FileDescriptionOptions{AllowDirectIO: true}); err != nil {
 			return nil, err
 		}
-		if opts.Flags&linux.O_TRUNC != 0 {
+		if !afterCreate && opts.Flags&linux.O_TRUNC != 0 {
 			if _, err := impl.truncate(0); err != nil {
 				return nil, err
 			}