This command allows for comparing the filters pointed to by two seccomp
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.

v2: switch to KCMP_SECCOMP_FD instead of KCMP_FILE_PRIVATE_DATA

Signed-off-by: Tycho Andersen <tycho.ander...@canonical.com>
CC: Kees Cook <keesc...@chromium.org>
CC: Will Drewry <w...@chromium.org>
CC: Oleg Nesterov <o...@redhat.com>
CC: Andy Lutomirski <l...@amacapital.net>
CC: Pavel Emelyanov <xe...@parallels.com>
CC: Serge E. Hallyn <serge.hal...@ubuntu.com>
CC: Alexei Starovoitov <a...@kernel.org>
CC: Daniel Borkmann <dan...@iogearbox.net>
---
 include/uapi/linux/kcmp.h |  1 +
 kernel/kcmp.c             | 27 +++++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/include/uapi/linux/kcmp.h b/include/uapi/linux/kcmp.h
index 84df14b..cd7870b 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_SECCOMP_FD,
 
        KCMP_TYPES,
 };
diff --git a/kernel/kcmp.c b/kernel/kcmp.c
index 0aa69ea..d53db53 100644
--- a/kernel/kcmp.c
+++ b/kernel/kcmp.c
@@ -11,6 +11,7 @@
 #include <linux/bug.h>
 #include <linux/err.h>
 #include <linux/kcmp.h>
+#include <linux/seccomp.h>
 
 #include <asm/unistd.h>
 
@@ -165,6 +166,32 @@ SYSCALL_DEFINE5(kcmp, pid_t, pid1, pid_t, pid2, int, type,
                ret = -EOPNOTSUPP;
 #endif
                break;
+       case KCMP_SECCOMP_FD: {
+               struct file *filp1, *filp2;
+
+               filp1 = get_file_raw_ptr(task1, idx1);
+               filp2 = get_file_raw_ptr(task2, idx2);
+
+               if (filp1 && filp2) {
+                       struct seccomp_filter *filter1, *filter2;
+
+                       filter1 = seccomp_filter_from_file(filp1);
+                       if (IS_ERR(filter1)) {
+                               ret = PTR_ERR(filter1);
+                               break;
+                       }
+
+                       filter2 = seccomp_filter_from_file(filp2);
+                       if (IS_ERR(filter2)) {
+                               ret = PTR_ERR(filter2);
+                               break;
+                       }
+
+                       ret = kcmp_ptr(filter1, filter2, KCMP_SECCOMP_FD);
+               } else
+                       ret = -EBADF;
+               break;
+       }
        default:
                ret = -EINVAL;
                break;
-- 
2.5.0

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