Fixing O_TRUNC behavior to match Linux.

PiperOrigin-RevId: 224351139
Change-Id: I9453bd75e5a8d38db406bb47fdc01038ac60922e

pkg/sentry/syscalls/linux/flags.go

@@ -22,6 +22,9 @@ import (
 // flagsToPermissions returns a Permissions object from Linux flags.
 // This includes truncate permission if O_TRUNC is set in the mask.
 func flagsToPermissions(mask uint) (p fs.PermMask) {
+	if mask&linux.O_TRUNC != 0 {
+		p.Write = true
+	}
 	switch mask & linux.O_ACCMODE {
 	case linux.O_WRONLY:
 		p.Write = true

pkg/sentry/syscalls/linux/sys_file.go

@@ -170,7 +170,7 @@ func openAt(t *kernel.Task, dirFD kdefs.FD, addr usermem.Addr, flags uint) (fd u
 			if dirPath {
 				return syserror.ENOTDIR
 			}
-			if fileFlags.Write && flags&linux.O_TRUNC != 0 {
+			if flags&linux.O_TRUNC != 0 {
 				if err := d.Inode.Truncate(t, d, 0); err != nil {
 					return err
 				}