linux-kernel - [PATCH v2] posix_types.h: make __NFDBITS compatible with glibc definition

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Date:	Tue, 24 Jul 2012 14:32:31 -0400
From:	Josh Boyer <jwboyer@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, law@...hat.com,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2] posix_types.h: make __NFDBITS compatible with glibc
 definition

Recent glibc made a change to suppress sign-conversion warnings from FD_SET
(glibc commit ceb9e56b3d1).  That patch solved the particular error it was
aiming to, however applications that #include <linux/types.h> after
including <sys/select.h> can now hit a build failure if -Werror=sign-compare
and -D_FORTIFY_SOURCE=2 is passed to gcc.  This can be seen when building
this trivial application against a recent enough glibc:

| #include <sys/select.h>
| #include <linux/types.h>
|
| int main(int argc, char **argv)
| {
|   fd_set fds;
|   FD_ZERO(&fds);
|   FD_SET(0, &fds);
|   return FD_ISSET(0, &fds);
| }

It was suggested the kernel should either match the glibc definition of
__NFDBITS in linux/posix_types.h or remove it entirely.  Given that we
don't know what applications may be relying on the header having a
definition, we make our definition compatible with glibc.

This resolves https://bugzilla.redhat.com/show_bug.cgi?id=837641

Reported-by: Jeff Law <law@...hat.com>
CC: <stable@...r.kernel.org>
Signed-off-by: Josh Boyer <jwboyer@...hat.com>
---

v2: Avoid the type change to long int

 include/linux/posix_types.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/posix_types.h b/include/linux/posix_types.h
index f04c98c..0bfc9cc 100644
--- a/include/linux/posix_types.h
+++ b/include/linux/posix_types.h
@@ -19,7 +19,7 @@
  * use the ones here. 
  */
 #undef __NFDBITS
-#define __NFDBITS	(8 * sizeof(unsigned long))
+#define __NFDBITS	(8 * (int) sizeof(unsigned long))

 #undef __FD_SETSIZE
 #define __FD_SETSIZE	1024
-- 
1.7.10.4

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