Re: [selinuxproject-selinux:stable-5.15 1/1] include/linux/rcupdate.h:395:2: warning: passing argument 1 of 'security_locked_down' discards 'const' qualifier from pointer target type

From: Ondrej Mosnacek
Date: Thu Sep 16 2021 - 08:22:20 EST

On Thu, Sep 16, 2021 at 2:08 PM kernel test robot <lkp@xxxxxxxxx> wrote:
> tree: https://github.com/SELinuxProject/selinux-kernel stable-5.15
> head: c491f0a471580712a4254adece400c3ebb3d8e44
> commit: c491f0a471580712a4254adece400c3ebb3d8e44 [1/1] lockdown,selinux: fix wrong subject in some SELinux lockdown checks
> config: um-x86_64_defconfig (attached as .config)
> compiler: gcc-9 (Debian 9.3.0-22) 9.3.0
> reproduce (this is a W=1 build):
> # https://github.com/SELinuxProject/selinux-kernel/commit/c491f0a471580712a4254adece400c3ebb3d8e44
> git remote add selinuxproject-selinux https://github.com/SELinuxProject/selinux-kernel
> git fetch --no-tags selinuxproject-selinux stable-5.15
> git checkout c491f0a471580712a4254adece400c3ebb3d8e44
> # save the attached .config to linux build tree
> make W=1 ARCH=um SUBARCH=x86_64
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@xxxxxxxxx>
>
> All warnings (new ones prefixed by >>):
>
> In file included from include/linux/rbtree.h:24,
> from include/linux/mm_types.h:10,
> from include/linux/mmzone.h:21,
> from include/linux/gfp.h:6,
> from include/linux/mm.h:10,
> from drivers/char/mem.c:12:
> drivers/char/mem.c: In function 'open_port':
> >> include/linux/rcupdate.h:395:2: warning: passing argument 1 of 'security_locked_down' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
> 395 | ({ \
> | ~^~~
> 396 | RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
> | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 397 | rcu_check_sparse(p, space); \
> | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 398 | ((typeof(*p) __force __kernel *)(p)); \
> | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 399 | })
> | ~~
> include/linux/rcupdate.h:587:2: note: in expansion of macro '__rcu_dereference_protected'
> 587 | __rcu_dereference_protected((p), (c), __rcu)
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~
> include/linux/cred.h:299:2: note: in expansion of macro 'rcu_dereference_protected'
> 299 | rcu_dereference_protected(current->cred, 1)
> | ^~~~~~~~~~~~~~~~~~~~~~~~~
> drivers/char/mem.c:620:28: note: in expansion of macro 'current_cred'
> 620 | rc = security_locked_down(current_cred(), LOCKDOWN_DEV_MEM);
> | ^~~~~~~~~~~~
> In file included from include/linux/fs_context.h:14,
> from include/linux/fs_parser.h:11,
> from include/linux/shmem_fs.h:11,
> from drivers/char/mem.c:25:
> include/linux/security.h:1347:53: note: expected 'struct cred *' but argument is of type 'const struct cred *'
> 1347 | static inline int security_locked_down(struct cred *cred, enum lockdown_reason what)
> | ~~~~~~~~~~~~~^~~~

Ah, I forgot to add the const qualifier to the function definition in
the CONFIG_SECURITY=n branch... Paul, will you amend the commit or
should I send an updated patch?

>
>
> vim +395 include/linux/rcupdate.h
>
> 76c8eaafe4f061 Paul E. McKenney 2021-04-21 379
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 380 #define __rcu_access_pointer(p, space) \
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 381 ({ \
> 7d0ae8086b8283 Paul E. McKenney 2015-03-03 382 typeof(*p) *_________p1 = (typeof(*p) *__force)READ_ONCE(p); \
> 423a86a610cad1 Joel Fernandes (Google 2018-12-12 383) rcu_check_sparse(p, space); \
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 384 ((typeof(*p) __force __kernel *)(_________p1)); \
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 385 })
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 386 #define __rcu_dereference_check(p, c, space) \
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 387 ({ \
> ac59853c06993a Pranith Kumar 2014-11-13 388 /* Dependency order vs. p above. */ \
> 506458efaf153c Will Deacon 2017-10-24 389 typeof(*p) *________p1 = (typeof(*p) *__force)READ_ONCE(p); \
> f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 390 RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_check() usage"); \
> 423a86a610cad1 Joel Fernandes (Google 2018-12-12 391) rcu_check_sparse(p, space); \
> ac59853c06993a Pranith Kumar 2014-11-13 392 ((typeof(*p) __force __kernel *)(________p1)); \
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 393 })
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 394 #define __rcu_dereference_protected(p, c, space) \
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 @395 ({ \
> f78f5b90c4ffa5 Paul E. McKenney 2015-06-18 396 RCU_LOCKDEP_WARN(!(c), "suspicious rcu_dereference_protected() usage"); \
> 423a86a610cad1 Joel Fernandes (Google 2018-12-12 397) rcu_check_sparse(p, space); \
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 398 ((typeof(*p) __force __kernel *)(p)); \
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 399 })
> 995f1405610bd8 Paul E. McKenney 2016-07-01 400 #define rcu_dereference_raw(p) \
> 995f1405610bd8 Paul E. McKenney 2016-07-01 401 ({ \
> 995f1405610bd8 Paul E. McKenney 2016-07-01 402 /* Dependency order vs. p above. */ \
> 506458efaf153c Will Deacon 2017-10-24 403 typeof(p) ________p1 = READ_ONCE(p); \
> 995f1405610bd8 Paul E. McKenney 2016-07-01 404 ((typeof(*p) __force __kernel *)(________p1)); \
> 995f1405610bd8 Paul E. McKenney 2016-07-01 405 })
> ca5ecddfa8fcbd Paul E. McKenney 2010-04-28 406
>
> :::::: The code at line 395 was first introduced by commit
> :::::: ca5ecddfa8fcbd948c95530e7e817cee9fb43a3d rcu: define __rcu address space modifier for sparse
>
> :::::: TO: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> :::::: CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>
> ---
> 0-DAY CI Kernel Test Service, Intel Corporation
> https://lists.01.org/hyperkitty/list/kbuild-all@xxxxxxxxxxxx

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.