[PATCH v2] errno.h: Improve ENOSYS's comment

[Andy Lutomirski]

From: Andy Lutomirski <luto@xxxxxxxxxxxxxx>

ENOSYS is the mechanism used by user code to detect whether the
running kernel implements a given system call.  It should not be
returned by anything except an unimplemented system call.

Unfortunately, it is rather frequently used in the kernel to
indicate that various new functions of existing system calls are not
implemented.  This should be discouraged.

Improve the comment in errno.h to help clarify ENOSYS's purpose.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
---

Changes from v1:
 - Fix changelog typo.
 - Further improve the comment.

 include/uapi/asm-generic/errno.h | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/include/uapi/asm-generic/errno.h b/include/uapi/asm-generic/errno.h
index 1e1ea6e6e7a5..88e0914cf2d9 100644
--- a/include/uapi/asm-generic/errno.h
+++ b/include/uapi/asm-generic/errno.h
@@ -6,7 +6,16 @@
 #define	EDEADLK		35	/* Resource deadlock would occur */
 #define	ENAMETOOLONG	36	/* File name too long */
 #define	ENOLCK		37	/* No record locks available */
-#define	ENOSYS		38	/* Function not implemented */
+
+/*
+ * This error code is special: arch syscall entry code will return
+ * -ENOSYS if users try to call a syscall that doesn't exist.  To keep
+ * failures of syscalls that really do exist distinguishable from
+ * failures due to attempts to use a nonexistent syscall, syscall
+ * implementations should refrain from returning -ENOSYS.
+ */
+#define	ENOSYS		38	/* Invalid system call number */
+
 #define	ENOTEMPTY	39	/* Directory not empty */
 #define	ELOOP		40	/* Too many symbolic links encountered */
 #define	EWOULDBLOCK	EAGAIN	/* Operation would block */
-- 
2.3.0

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