This command allows comparing the underling private data of two fds. This
is useful e.g. to find out if a seccomp filter is inherited, since struct
seccomp_filter are unique across tasks and are the private_data seccomp
fds.

Signed-off-by: Tycho Andersen <redacted>
CC: Kees Cook <redacted>
CC: Will Drewry <wad@chromium.org>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Pavel Emelyanov <redacted>
CC: Serge E. Hallyn <redacted>
CC: Alexei Starovoitov <ast@kernel.org>
CC: Daniel Borkmann <daniel@iogearbox.net>
---
 include/uapi/linux/kcmp.h |  1 +
 kernel/kcmp.c             | 14 ++++++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/uapi/linux/kcmp.h b/include/uapi/linux/kcmp.h
index 84df14b..ed389d2 100644
--- a/include/uapi/linux/kcmp.h
+++ b/include/uapi/linux/kcmp.h

@@ -10,6 +10,7 @@ enum kcmp_type {
 	KCMP_SIGHAND,
 	KCMP_IO,
 	KCMP_SYSVSEM,
+	KCMP_FILE_PRIVATE_DATA,
 
 	KCMP_TYPES,
 };

diff --git a/kernel/kcmp.c b/kernel/kcmp.c
index 0aa69ea..9ae673b 100644
--- a/kernel/kcmp.c
+++ b/kernel/kcmp.c

@@ -165,6 +165,20 @@ SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
 		ret = -EOPNOTSUPP;
 #endif
 		break;
+	case KCMP_FILE_PRIVATE_DATA: {
+		struct file *filp1, *filp2;
+
+		filp1 = get_file_raw_ptr(task1, idx1);
+		filp2 = get_file_raw_ptr(task2, idx2);
+
+		if (filp1 && filp2)
+			ret = kcmp_ptr(filp1->private_data,
+				       filp2->private_data,
+				       KCMP_FILE_PRIVATE_DATA);
+		else
+			ret = -EBADF;
+		break;
+	}
 	default:
 		ret = -EINVAL;
 		break;

-- 
2.5.0