[PATCH 1/2] fcntl: add fcntl(F_CHECK_ORIGINAL_MEMFD)

[Michal Clapinski <mclapinski@google.com>]

Add a way to check if an fd points to the memfd's original open fd
(the one created by memfd_create).
Useful because only the original open fd can be both writable and
executable.

Signed-off-by: Michal Clapinski <mclapinski@google.com>
---
 fs/fcntl.c                 | 3 +++
 include/uapi/linux/fcntl.h | 9 +++++++++
 2 files changed, 12 insertions(+)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index e871009f6c88..301527e07a4d 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -419,6 +419,9 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 	case F_SET_RW_HINT:
 		err = fcntl_rw_hint(filp, cmd, arg);
 		break;
+	case F_CHECK_ORIGINAL_MEMFD:
+		err = !(filp->f_mode & FMODE_WRITER);
+		break;
 	default:
 		break;
 	}
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index 6c80f96049bd..9a10fe3aafa7 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -56,6 +56,15 @@
 #define F_GET_FILE_RW_HINT	(F_LINUX_SPECIFIC_BASE + 13)
 #define F_SET_FILE_RW_HINT	(F_LINUX_SPECIFIC_BASE + 14)
 
+/*
+ * Check if the fd points to the memfd's original open fd (the one created by
+ * memfd_create). Returns 1 if yes, 0 if no.
+ * If the fd doesn't point to a memfd, the value should not be interpreted.
+ * Useful because only the original open fd can be both writable and
+ * executable.
+ */
+#define F_CHECK_ORIGINAL_MEMFD	(F_LINUX_SPECIFIC_BASE + 15)
+
 /*
  * Valid hint values for F_{GET,SET}_RW_HINT. 0 is "not set", or can be
  * used to clear any hints previously set.
-- 
2.42.0.283.g2d96d420d3-goog