errno.h: Improve ENOSYS's comment

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@amacapital.net>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/uapi/asm-generic/errno.h b/include/uapi/asm-generic/errno.h
index 1e1ea6e..88e0914 100644 (file)
--- 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 */