Change the notation from pointer-to-array to pointer-to-pointer.
With this, we avoid the compiler complaining about trying
to access a region of size zero as an argument during function
calls.

This is a workaround to prevent the compiler complaining about
accessing an array of size zero when evaluating the arguments
of a couple of function calls. See below:

kernel/cgroup/cgroup.c: In function 'find_css_set':
kernel/cgroup/cgroup.c:1206:16: warning: 'find_existing_css_set' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
 1206 |         cset = find_existing_css_set(old_cset, cgrp, template);
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
kernel/cgroup/cgroup.c:1206:16: note: referencing argument 3 of type 'struct cgroup_subsys_state *[0]'
kernel/cgroup/cgroup.c:1071:24: note: in a call to function 'find_existing_css_set'
 1071 | static struct css_set *find_existing_css_set(struct css_set *old_cset,
      |                        ^~~~~~~~~~~~~~~~~~~~~

With the change to pointer-to-pointer, the functions are not prevented
from being executed, and they will do what they have to do when
CGROUP_SUBSYS_COUNT == 0.

Address the following -Wstringop-overflow warnings seen when
built with ARM architecture and aspeed_g4_defconfig configuration
(notice that under this configuration CGROUP_SUBSYS_COUNT == 0):

kernel/cgroup/cgroup.c:1208:16: warning: 'find_existing_css_set' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
kernel/cgroup/cgroup.c:1258:15: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
kernel/cgroup/cgroup.c:6089:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
kernel/cgroup/cgroup.c:6153:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]

This results in no differences in binary output.

Link: https://github.com/KSPP/linux/issues/316
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v3:
 - Update changelog text to point out that this is a workaround.

Changes in v2:
 - Use pointer-to-pointer instead of pointer-to-array.
 - Update changelog text.
 - Link: https://lore.kernel.org/linux-hardening/ZN02iLcZYgxHFrEN@work/

v1:
 - Link: https://lore.kernel.org/linux-hardening/ZIpm3pcs3iCP9UaR@work/

 kernel/cgroup/cgroup.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index ccbbba06da5b..68e2d9812e3f 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -929,7 +929,7 @@ static void css_set_move_task(struct task_struct *task,
 #define CSS_SET_HASH_BITS	7
 static DEFINE_HASHTABLE(css_set_table, CSS_SET_HASH_BITS);
 
-static unsigned long css_set_hash(struct cgroup_subsys_state *css[])
+static unsigned long css_set_hash(struct cgroup_subsys_state **css)
 {
 	unsigned long key = 0UL;
 	struct cgroup_subsys *ss;
@@ -1070,7 +1070,7 @@ static bool compare_css_sets(struct css_set *cset,
  */
 static struct css_set *find_existing_css_set(struct css_set *old_cset,
 					struct cgroup *cgrp,
-					struct cgroup_subsys_state *template[])
+					struct cgroup_subsys_state **template)
 {
 	struct cgroup_root *root = cgrp->root;
 	struct cgroup_subsys *ss;
-- 
2.34.1

On Thu, Aug 17, 2023 at 11:19:13AM -0600, Gustavo A. R. Silva wrote:
> Change the notation from pointer-to-array to pointer-to-pointer.
> With this, we avoid the compiler complaining about trying
> to access a region of size zero as an argument during function
> calls.
> 
> This is a workaround to prevent the compiler complaining about
> accessing an array of size zero when evaluating the arguments
> of a couple of function calls. See below:
> 
> kernel/cgroup/cgroup.c: In function 'find_css_set':
> kernel/cgroup/cgroup.c:1206:16: warning: 'find_existing_css_set' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
>  1206 |         cset = find_existing_css_set(old_cset, cgrp, template);
>       |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> kernel/cgroup/cgroup.c:1206:16: note: referencing argument 3 of type 'struct cgroup_subsys_state *[0]'
> kernel/cgroup/cgroup.c:1071:24: note: in a call to function 'find_existing_css_set'
>  1071 | static struct css_set *find_existing_css_set(struct css_set *old_cset,
>       |                        ^~~~~~~~~~~~~~~~~~~~~
> 
> With the change to pointer-to-pointer, the functions are not prevented
> from being executed, and they will do what they have to do when
> CGROUP_SUBSYS_COUNT == 0.
> 
> Address the following -Wstringop-overflow warnings seen when
> built with ARM architecture and aspeed_g4_defconfig configuration
> (notice that under this configuration CGROUP_SUBSYS_COUNT == 0):
> 
> kernel/cgroup/cgroup.c:1208:16: warning: 'find_existing_css_set' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> kernel/cgroup/cgroup.c:1258:15: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> kernel/cgroup/cgroup.c:6089:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> kernel/cgroup/cgroup.c:6153:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> 
> This results in no differences in binary output.
> 
> Link: https://github.com/KSPP/linux/issues/316
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Applied to cgroup/for-6.6.

Thanks.

-- 
tejun

On Thu, Aug 17, 2023 at 11:19:13AM -0600, Gustavo A. R. Silva wrote:
> Change the notation from pointer-to-array to pointer-to-pointer.
> With this, we avoid the compiler complaining about trying
> to access a region of size zero as an argument during function
> calls.
> 
> This is a workaround to prevent the compiler complaining about
> accessing an array of size zero when evaluating the arguments
> of a couple of function calls. See below:
> 
> kernel/cgroup/cgroup.c: In function 'find_css_set':
> kernel/cgroup/cgroup.c:1206:16: warning: 'find_existing_css_set' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
>  1206 |         cset = find_existing_css_set(old_cset, cgrp, template);
>       |                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> kernel/cgroup/cgroup.c:1206:16: note: referencing argument 3 of type 'struct cgroup_subsys_state *[0]'
> kernel/cgroup/cgroup.c:1071:24: note: in a call to function 'find_existing_css_set'
>  1071 | static struct css_set *find_existing_css_set(struct css_set *old_cset,
>       |                        ^~~~~~~~~~~~~~~~~~~~~
> 
> With the change to pointer-to-pointer, the functions are not prevented
> from being executed, and they will do what they have to do when
> CGROUP_SUBSYS_COUNT == 0.
> 
> Address the following -Wstringop-overflow warnings seen when
> built with ARM architecture and aspeed_g4_defconfig configuration
> (notice that under this configuration CGROUP_SUBSYS_COUNT == 0):
> 
> kernel/cgroup/cgroup.c:1208:16: warning: 'find_existing_css_set' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> kernel/cgroup/cgroup.c:1258:15: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> kernel/cgroup/cgroup.c:6089:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> kernel/cgroup/cgroup.c:6153:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
> 
> This results in no differences in binary output.
> 
> Link: https://github.com/KSPP/linux/issues/316
> Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook