[PATCH RFC 1/4] file: add take_fd() cleanup helper

[Christian Brauner <brauner@kernel.org>]

Add a helper that returns the file descriptor and ensures that the old
variable contains a negative value. This makes it easy to rely on
CLASS(get_unused_fd).

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 include/linux/cleanup.h | 13 ++++++++-----
 include/linux/file.h    |  2 ++
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index c2d09bc4f976..80c4181e194a 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -63,17 +63,20 @@
 
 #define __free(_name)	__cleanup(__free_##_name)
 
-#define __get_and_null_ptr(p) \
-	({ __auto_type __ptr = &(p); \
-	   __auto_type __val = *__ptr; \
-	   *__ptr = NULL;  __val; })
+#define __get_and_null(p, nullvalue)   \
+	({                                  \
+		__auto_type __ptr = &(p);   \
+		__auto_type __val = *__ptr; \
+		*__ptr = nullvalue;         \
+		__val;                      \
+	})
 
 static inline __must_check
 const volatile void * __must_check_fn(const volatile void *val)
 { return val; }
 
 #define no_free_ptr(p) \
-	((typeof(p)) __must_check_fn(__get_and_null_ptr(p)))
+	((typeof(p)) __must_check_fn(__get_and_null(p, NULL)))
 
 #define return_ptr(p)	return no_free_ptr(p)
 
diff --git a/include/linux/file.h b/include/linux/file.h
index 45d0f4800abd..3ea7f2452f20 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -97,6 +97,8 @@ extern void put_unused_fd(unsigned int fd);
 DEFINE_CLASS(get_unused_fd, int, if (_T >= 0) put_unused_fd(_T),
 	     get_unused_fd_flags(flags), unsigned flags)
 
+#define take_fd(fd) __get_and_null(fd, -EBADF)
+
 extern void fd_install(unsigned int fd, struct file *file);
 
 int receive_fd(struct file *file, int __user *ufd, unsigned int o_flags);

-- 
2.43.0