Return non-zero size for tmpfs statfs(2).

This does not implement accepting or enforcing any size limit, which will be
more complex and has performance implications; it just returns a fixed non-zero
size.

Updates #1936

PiperOrigin-RevId: 328428588

pkg/abi/linux/fs.go

@@ -44,17 +44,18 @@ type Statfs struct {
 	// Type is one of the filesystem magic values, defined above.
 	Type uint64
 
-	// BlockSize is the data block size.
+	// BlockSize is the optimal transfer block size in bytes.
 	BlockSize int64
 
-	// Blocks is the number of data blocks in use.
+	// Blocks is the maximum number of data blocks the filesystem may store, in
+	// units of BlockSize.
 	Blocks uint64
 
-	// BlocksFree is the number of free blocks.
+	// BlocksFree is the number of free data blocks, in units of BlockSize.
 	BlocksFree uint64
 
-	// BlocksAvailable is the number of blocks free for use by
-	// unprivileged users.
+	// BlocksAvailable is the number of data blocks free for use by
+	// unprivileged users, in units of BlockSize.
 	BlocksAvailable uint64
 
 	// Files is the number of used file nodes on the filesystem.

pkg/sentry/fs/tmpfs/tmpfs.go

@@ -16,6 +16,8 @@
 package tmpfs
 
 import (
+	"math"
+
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
@@ -32,9 +34,15 @@ import (
 var fsInfo = fs.Info{
 	Type: linux.TMPFS_MAGIC,
 
+	// tmpfs currently does not support configurable size limits. In Linux,
+	// such a tmpfs mount will return f_blocks == f_bfree == f_bavail == 0 from
+	// statfs(2). However, many applications treat this as having a size limit
+	// of 0. To work around this, claim to have a very large but non-zero size,
+	// chosen to ensure that BlockSize * Blocks does not overflow int64 (which
+	// applications may also handle incorrectly).
 	// TODO(b/29637826): allow configuring a tmpfs size and enforce it.
-	TotalBlocks: 0,
-	FreeBlocks:  0,
+	TotalBlocks: math.MaxInt64 / usermem.PageSize,
+	FreeBlocks:  math.MaxInt64 / usermem.PageSize,
 }
 
 // rename implements fs.InodeOperations.Rename for tmpfs nodes.

pkg/sentry/fsimpl/tmpfs/filesystem.go

@@ -25,7 +25,6 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/socket/unix/transport"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/syserror"
-	"gvisor.dev/gvisor/pkg/usermem"
 )
 
 // Sync implements vfs.FilesystemImpl.Sync.
@@ -706,16 +705,7 @@ func (fs *filesystem) StatFSAt(ctx context.Context, rp *vfs.ResolvingPath) (linu
 	if _, err := resolveLocked(ctx, rp); err != nil {
 		return linux.Statfs{}, err
 	}
-	statfs := linux.Statfs{
-		Type:         linux.TMPFS_MAGIC,
-		BlockSize:    usermem.PageSize,
-		FragmentSize: usermem.PageSize,
-		NameLength:   linux.NAME_MAX,
-		// TODO(b/29637826): Allow configuring a tmpfs size and enforce it.
-		Blocks:     0,
-		BlocksFree: 0,
-	}
-	return statfs, nil
+	return globalStatfs, nil
 }
 
 // SymlinkAt implements vfs.FilesystemImpl.SymlinkAt.

pkg/sentry/fsimpl/tmpfs/tmpfs.go

@@ -201,6 +201,25 @@ func (fs *filesystem) Release(ctx context.Context) {
 	fs.vfsfs.VirtualFilesystem().PutAnonBlockDevMinor(fs.devMinor)
 }
 
+// immutable
+var globalStatfs = linux.Statfs{
+	Type:         linux.TMPFS_MAGIC,
+	BlockSize:    usermem.PageSize,
+	FragmentSize: usermem.PageSize,
+	NameLength:   linux.NAME_MAX,
+
+	// tmpfs currently does not support configurable size limits. In Linux,
+	// such a tmpfs mount will return f_blocks == f_bfree == f_bavail == 0 from
+	// statfs(2). However, many applications treat this as having a size limit
+	// of 0. To work around this, claim to have a very large but non-zero size,
+	// chosen to ensure that BlockSize * Blocks does not overflow int64 (which
+	// applications may also handle incorrectly).
+	// TODO(b/29637826): allow configuring a tmpfs size and enforce it.
+	Blocks:          math.MaxInt64 / usermem.PageSize,
+	BlocksFree:      math.MaxInt64 / usermem.PageSize,
+	BlocksAvailable: math.MaxInt64 / usermem.PageSize,
+}
+
 // dentry implements vfs.DentryImpl.
 type dentry struct {
 	vfsd vfs.Dentry
@@ -698,6 +717,11 @@ func (fd *fileDescription) SetStat(ctx context.Context, opts vfs.SetStatOptions)
 	return nil
 }
 
+// StatFS implements vfs.FileDescriptionImpl.StatFS.
+func (fd *fileDescription) StatFS(ctx context.Context) (linux.Statfs, error) {
+	return globalStatfs, nil
+}
+
 // Listxattr implements vfs.FileDescriptionImpl.Listxattr.
 func (fd *fileDescription) Listxattr(ctx context.Context, size uint64) ([]string, error) {
 	return fd.inode().listxattr(size)