linux-kernel - [PATCH 3.10 081/268] cred/userns: define current_user

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Date:   Mon, 19 Jun 2017 20:29:40 +0200
From:   Willy Tarreau <w@....eu>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        linux@...ck-us.net
Cc:     Arnd Bergmann <arnd@...db.de>, David Howells <dhowells@...hat.com>,
        Yaowei Bai <baiyaowei@...s.chinamobile.com>,
        James Morris <james.l.morris@...cle.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Willy Tarreau <w@....eu>
Subject: [PATCH 3.10 081/268] cred/userns: define current_user_ns() as a function

From: Arnd Bergmann <arnd@...db.de>

commit 0335695dfa4df01edff5bb102b9a82a0668ee51e upstream.

The current_user_ns() macro currently returns &init_user_ns when user
namespaces are disabled, and that causes several warnings when building
with gcc-6.0 in code that compares the result of the macro to
&init_user_ns itself:

  fs/xfs/xfs_ioctl.c: In function 'xfs_ioctl_setattr_check_projid':
  fs/xfs/xfs_ioctl.c:1249:22: error: self-comparison always evaluates to true [-Werror=tautological-compare]
    if (current_user_ns() == &init_user_ns)

This is a legitimate warning in principle, but here it isn't really
helpful, so I'm reprasing the definition in a way that shuts up the
warning.  Apparently gcc only warns when comparing identical literals,
but it can figure out that the result of an inline function can be
identical to a constant expression in order to optimize a condition yet
not warn about the fact that the condition is known at compile time.
This is exactly what we want here, and it looks reasonable because we
generally prefer inline functions over macros anyway.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
Acked-by: Serge Hallyn <serge.hallyn@...onical.com>
Cc: David Howells <dhowells@...hat.com>
Cc: Yaowei Bai <baiyaowei@...s.chinamobile.com>
Cc: James Morris <james.l.morris@...cle.com>
Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Willy Tarreau <w@....eu>
---
 include/linux/capability.h | 2 --
 include/linux/cred.h       | 5 ++++-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/capability.h b/include/linux/capability.h
index 9b4378a..eeb43c4 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -40,8 +40,6 @@ struct inode;
 struct dentry;
 struct user_namespace;
 
-struct user_namespace *current_user_ns(void);
-
 extern const kernel_cap_t __cap_empty_set;
 extern const kernel_cap_t __cap_init_eff_set;
 
diff --git a/include/linux/cred.h b/include/linux/cred.h
index 6c58dd7..cd3fb73 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -345,7 +345,10 @@ extern struct user_namespace init_user_ns;
 #ifdef CONFIG_USER_NS
 #define current_user_ns()	(current_cred_xxx(user_ns))
 #else
-#define current_user_ns()	(&init_user_ns)
+static inline struct user_namespace *current_user_ns(void)
+{
+	return &init_user_ns;
+}
 #endif
 
 
-- 
2.8.0.rc2.1.gbe9624a