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From: ebiederm@xmission.com (Eric W. Biederman)
To: Dan Carpenter <dan.carpenter@oracle.com>
Cc: kbuild-all@lists.01.org, lkp@intel.com,
	Christian Brauner <christian.brauner@ubuntu.com>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	Linux Containers <containers@lists.linux-foundation.org>,
	Jann Horn <jannh@google.com>, LKML <linux-kernel@vger.kernel.org>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	legion@kernel.org, kbuild@lists.01.org
Subject: [PATCH] ucounts: Silence warning in dec_rlimit_ucounts
Date: Fri, 30 Apr 2021 13:09:55 -0500
Message-ID: <m1tunny77w.fsf@fess.ebiederm.org> (raw)
In-Reply-To: <202104272256.9Y5ZQxrO-lkp@intel.com> (Dan Carpenter's message of "Thu, 29 Apr 2021 17:04:23 +0300")


Dan Carpenter <dan.carpenter@oracle.com> wrote:
>
> url:    https://github.com/0day-ci/linux/commits/legion-kernel-org/Count-rlimits-in-each-user-namespace/20210427-162857
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git next
> config: arc-randconfig-m031-20210426 (attached as .config)
> compiler: arceb-elf-gcc (GCC) 9.3.0
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@intel.com>
> Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
>
> smatch warnings:
> kernel/ucount.c:270 dec_rlimit_ucounts() error: uninitialized symbol 'new'.
>
> vim +/new +270 kernel/ucount.c
>
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  260  bool dec_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  261  {
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  262   struct ucounts *iter;
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  263   long new;
>                                                 ^^^^^^^^
>
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  264   for (iter = ucounts; iter; iter = iter->ns->ucounts) {
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  265    long dec = atomic_long_add_return(-v, &iter->ucount[type]);
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  266    WARN_ON_ONCE(dec < 0);
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  267    if (iter == ucounts)
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  268     new = dec;
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  269   }
> 176ec2b092cc22 Alexey Gladkov 2021-04-22 @270   return (new == 0);
>                                                         ^^^^^^^^
> I don't know if this is a bug or not, but I can definitely tell why the
> static checker complains about it.
>
> 176ec2b092cc22 Alexey Gladkov 2021-04-22  271  }

In the only two cases that care about the return value of
dec_rlimit_ucounts the code first tests to see that ucounts is not
NULL.  In those cases it is guaranteed at least one iteration of the
loop will execute guaranteeing the variable new will be initialized.

Initialize new to -1 so that the return value is well defined even
when the loop does not execute and the static checker is silenced.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
 kernel/ucount.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/ucount.c b/kernel/ucount.c
index d316bac3e520..12a48457bb86 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -263,7 +263,7 @@ long inc_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
 bool dec_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
 {
 	struct ucounts *iter;
-	long new;
+	long new = -1; /* Silence compiler warnings */
 	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
 		long dec = atomic_long_add_return(-v, &iter->ucount[type]);
 		WARN_ON_ONCE(dec < 0);
-- 
2.30.1

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Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-29 14:04 [PATCH v11 4/9] Reimplement RLIMIT_NPROC on top of ucounts Dan Carpenter
2021-04-30 18:09 ` Eric W. Biederman [this message]
  -- strict thread matches above, loose matches on Subject: below --
2021-04-22 12:27 [PATCH v11 0/9] Count rlimits in each user namespace legion
2021-04-22 12:27 ` [PATCH v11 1/9] Increase size of ucounts to atomic_long_t legion
2021-04-22 12:27 ` [PATCH v11 2/9] Add a reference to ucounts for each cred legion
2021-04-22 12:27 ` [PATCH v11 3/9] Use atomic_t for ucounts reference counting legion
2021-04-22 12:27 ` [PATCH v11 5/9] Reimplement RLIMIT_MSGQUEUE on top of ucounts legion
2021-04-22 12:27 ` [PATCH v11 6/9] Reimplement RLIMIT_SIGPENDING " legion
2021-04-22 12:27 ` [PATCH v11 7/9] Reimplement RLIMIT_MEMLOCK " legion
2021-04-22 12:27 ` [PATCH v11 8/9] kselftests: Add test to check for rlimit changes in different user namespaces legion
2021-04-22 12:27 ` [PATCH v11 9/9] ucounts: Set ucount_max to the largest positive value the type can hold legion

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