[PATCH v3] checkpatch.pl: New instances of ENOSYS are errors

[Andy Lutomirski <luto@amacapital.net>]

ENOSYS means that a nonexistent system call was called.  We have a
bad habit of using it for things like invalid operations on
otherwise valid syscalls.  We should avoid this in new code.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---

Pervasive incorrect usage of ENOSYS came up at the kernel summit ABI
review discussion.  Let's see if checkpatch can help.

I'll submit a separate patch for include/uapi/asm-generic/errno.h.

Changes from v2:
 - Reduce severity to WARN.
 - Remove unnecessary clarification.
 - Rebase onto Linus' tree instead of v3.16.

Changes from v1:
 - Moved later so that it won't warn on context lines.
 - Use $herecur.
 - Improve regex pattern.

 scripts/checkpatch.pl | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
index 31a731e..448d075 100755
--- a/scripts/checkpatch.pl
+++ b/scripts/checkpatch.pl
@@ -3214,6 +3214,14 @@ sub process {
 			     "Prefer dev_$level(... to dev_printk(KERN_$orig, ...\n" . $herecurr);
 		}
 
+# ENOSYS means "bad syscall nr" and nothing else.  This will have a small
+# number of false positives, but assembly files are not checked, so at
+# least the arch entry code will not trigger this warning.
+		if ($line =~ /\bENOSYS\b/) {
+			WARN("ENOSYS",
+			     "ENOSYS means 'invalid syscall nr' and nothing else\n" . $herecurr);
+		}
+
 # function brace can't be on same line, except for #defines of do while,
 # or if closed on same line
 		if (($line=~/$Type\s*$Ident\(.*\).*\s*{/) and
-- 
1.9.3