linux-kernel - Re: [PATCH] userns: suppress kmemleak message

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Date:   Thu, 03 Nov 2016 09:54:25 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc:     Jakub Kicinski <kubakici@...pl>, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Johannes Berg <johannes@...solutions.net>,
        Maciej Żenczykowski <zenczykowski@...il.com>
Subject: Re: [PATCH] userns: suppress kmemleak message

Dmitry Torokhov <dmitry.torokhov@...il.com> writes:

> We do not ever intend to unregister "user" sysctl table, unfortunately
> it leads kmemleak to believe that we are leaking memory:

Sounds like an issue with kmemleak because we do retain references.

So no we don't intend to unregister the table.

As for the patch.

Nacked-by: "Eric W. Biederman" <ebiederm@...ssion.com>

I can't see the using kmemleak_not_leak is possibly good form.  I
would much rather have suggestions about constructs that won't confuse
kmemleak and won't need ugly annotations that serve no purpose but to
appease a tool.  Perhaps the user_header variable needs to be moved out
of user_namespace_sysctl_init.

Eric

> unreferenced object 0xffff8807383bfd48 (size 96):
>   comm "swapper/0", pid 1, jiffies 4294894636 (age 278.320s)
>   hex dump (first 32 bytes):
>     a0 b4 b0 ba ff ff ff ff 00 00 00 00 01 00 00 00  ................
>     01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
>   backtrace:
>     [<ffffffffb7de59e8>] kmemleak_alloc+0x28/0x50
>     [<ffffffffb676e2f6>] __kmalloc+0x206/0x5a0
>     [<ffffffffb69be2d3>] __register_sysctl_table+0xb3/0x1130
>     [<ffffffffb69bf36b>] register_sysctl+0x1b/0x20
>     [<ffffffffba840de1>] user_namespace_sysctl_init+0x17/0x4c
>     [<ffffffffb60022b7>] do_one_initcall+0xb7/0x2a0
>     [<ffffffffba7eb102>] kernel_init_freeable+0x597/0x636
>     [<ffffffffb7de0433>] kernel_init+0x13/0x140
>     [<ffffffffb7dfb36a>] ret_from_fork+0x2a/0x40t show
>     [<ffffffffffffffff>] 0xffffffffffffffff
>
> Let's annotate the pointer as kmemleak_not_leak() to suppress the
> kmemleak false positive.
>
> Reported-by: Jakub Kicinski <kubakici@...pl>
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> ---
>
> This was only compiled; Jakub, could you give it a spin?
>
>  kernel/ucount.c | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/kernel/ucount.c b/kernel/ucount.c
> index 9d20d5d..07d69b2 100644
> --- a/kernel/ucount.c
> +++ b/kernel/ucount.c
> @@ -5,6 +5,7 @@
>   *  License.
>   */
>  
> +#include <linux/kmemleak.h>
>  #include <linux/stat.h>
>  #include <linux/sysctl.h>
>  #include <linux/slab.h>
> @@ -226,6 +227,7 @@ static __init int user_namespace_sysctl_init(void)
>  	 */
>  	user_header = register_sysctl("user", empty);
>  	BUG_ON(!user_header);
> +	kmemleak_not_leak(user_header);
>  	BUG_ON(!setup_userns_sysctls(&init_user_ns));
>  #endif
>  	return 0;
> -- 
> 2.8.0.rc3.226.g39d4020