[PATCH] cred/userns: define current_user_ns() as a function

[PATCH] cred/userns: define current_user_ns() as a function

[Arnd Bergmann]

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@arndb.de>
---
 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 f314275d4e3f..00690ff92edf 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 8d70e1361ecd..257db64562e5 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -377,7 +377,10 @@ 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
 
 
-- 
2.7.0

Re: [PATCH] cred/userns: define current_user_ns() as a function

[Serge E. Hallyn]

On Mon, Mar 14, 2016 at 10:47:33PM +0100, Arnd Bergmann wrote:
> 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@arndb.de>

Was a bit worried about the first hunk since capability.h doesn't
include cred.h explicitly, but I guess if that was a problem it
would've not compiled long ago due to current_cred_xxx().

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>

> ---
>  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 f314275d4e3f..00690ff92edf 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 8d70e1361ecd..257db64562e5 100644
> --- a/include/linux/cred.h
> +++ b/include/linux/cred.h
> @@ -377,7 +377,10 @@ 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
>  
>  
> -- 
> 2.7.0