linux-kernel - Re: [peterz-queue:sched/core 13/19] kernel/sched/debug.c:453 print_cfs_group_stats() warn: variable dereferenced before check 'se' (see line 444)

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Date:   Mon, 13 Sep 2021 19:04:14 +0800
From:   Yafang Shao <laoar.shao@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     kbuild@...ts.01.org, kbuild test robot <lkp@...el.com>,
        kbuild-all@...ts.01.org, LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [peterz-queue:sched/core 13/19] kernel/sched/debug.c:453
 print_cfs_group_stats() warn: variable dereferenced before check 'se' (see
 line 444)

On Mon, Sep 13, 2021 at 4:48 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
>
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git sched/core
> head:   2dfdb3d20ad50e2ae2cb84cbceb0f0fc75e79e5d
> commit: 445d9e8ba05d5e9e4b26956b7fe529223e29d8d1 [13/19] sched: make struct sched_statistics independent of fair sched class
> config: x86_64-randconfig-m001-20210910 (attached as .config)
> compiler: gcc-9 (Debian 9.3.0-22) 9.3.0
>
> If you fix the issue, kindly add following tag as appropriate
> Reported-by: kernel test robot <lkp@...el.com>
> Reported-by: Dan Carpenter <dan.carpenter@...cle.com>
>
> smatch warnings:
> kernel/sched/debug.c:453 print_cfs_group_stats() warn: variable dereferenced before check 'se' (see line 444)
>

Sorry, my bad.

Hi Peter,

Could you pls. help amend below change to the original commit ?

diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
index 52b4426414c0..d59f33ac06fe 100644
--- a/kernel/sched/debug.c
+++ b/kernel/sched/debug.c
@@ -440,11 +440,8 @@ void dirty_sched_domain_sysctl(int cpu)
 #ifdef CONFIG_FAIR_GROUP_SCHED
 static void print_cfs_group_stats(struct seq_file *m, int cpu, struct
task_group *tg)
 {
-       struct sched_statistics __maybe_unused *stats;
        struct sched_entity *se = tg->se[cpu];

-       stats = __schedstats_from_se(se);
-
 #define P(F)           SEQ_printf(m, "  .%-30s: %lld\n",       #F,
(long long)F)
 #define P_SCHEDSTAT(F) SEQ_printf(m, "  .%-30s: %lld\n",       \
                #F, (long long)schedstat_val(stats->F))
@@ -460,6 +457,8 @@ static void print_cfs_group_stats(struct seq_file
*m, int cpu, struct task_group
        PN(se->sum_exec_runtime);

        if (schedstat_enabled()) {
+               struct sched_statistics *stats =  __schedstats_from_se(se);
+
                PN_SCHEDSTAT(wait_start);
                PN_SCHEDSTAT(sleep_start);
                PN_SCHEDSTAT(block_start);



> vim +/se +453 kernel/sched/debug.c
>
> 3866e845ed5222 kernel/sched/debug.c Steven Rostedt (Red Hat  2016-02-22  439)
> ff9b48c3598732 kernel/sched_debug.c Bharata B Rao            2008-11-10  440  #ifdef CONFIG_FAIR_GROUP_SCHED
> 5091faa449ee0b kernel/sched_debug.c Mike Galbraith           2010-11-30  441  static void print_cfs_group_stats(struct seq_file *m, int cpu, struct task_group *tg)
> ff9b48c3598732 kernel/sched_debug.c Bharata B Rao            2008-11-10  442  {
> ff9b48c3598732 kernel/sched_debug.c Bharata B Rao            2008-11-10  443    struct sched_entity *se = tg->se[cpu];
> 445d9e8ba05d5e kernel/sched/debug.c Yafang Shao              2021-09-05 @444    struct sched_statistics *stats = __schedstats_from_se(se);
>                                                                                                                                       ^^
> New unchecked dereference.
>
> ff9b48c3598732 kernel/sched_debug.c Bharata B Rao            2008-11-10  445
> 97fb7a0a8944bd kernel/sched/debug.c Ingo Molnar              2018-03-03  446  #define P(F)              SEQ_printf(m, "  .%-30s: %lld\n",       #F, (long long)F)
> 445d9e8ba05d5e kernel/sched/debug.c Yafang Shao              2021-09-05  447  #define P_SCHEDSTAT(F)    SEQ_printf(m, "  .%-30s: %lld\n",       \
> 445d9e8ba05d5e kernel/sched/debug.c Yafang Shao              2021-09-05  448            #F, (long long)schedstat_val(stats->F))
> 97fb7a0a8944bd kernel/sched/debug.c Ingo Molnar              2018-03-03  449  #define PN(F)             SEQ_printf(m, "  .%-30s: %lld.%06ld\n", #F, SPLIT_NS((long long)F))
> 445d9e8ba05d5e kernel/sched/debug.c Yafang Shao              2021-09-05  450  #define PN_SCHEDSTAT(F)   SEQ_printf(m, "  .%-30s: %lld.%06ld\n", \
> 445d9e8ba05d5e kernel/sched/debug.c Yafang Shao              2021-09-05  451            #F, SPLIT_NS((long long)schedstat_val(stats->F)))
> ff9b48c3598732 kernel/sched_debug.c Bharata B Rao            2008-11-10  452
> cd126afe838d7e kernel/sched/debug.c Yuyang Du                2015-07-15 @453    if (!se)
>                                                                                     ^^^
> The old code assumed "se" can be NULL.
>
> 18bf2805d9b30c kernel/sched/debug.c Ben Segall               2012-10-04  454            return;
> 18bf2805d9b30c kernel/sched/debug.c Ben Segall               2012-10-04  455
> ff9b48c3598732 kernel/sched_debug.c Bharata B Rao            2008-11-10  456    PN(se->exec_start);
> ff9b48c3598732 kernel/sched_debug.c Bharata B Rao            2008-11-10  457    PN(se->vruntime);
>
> ---
> 0-DAY CI Kernel Test Service, Intel Corporation
> https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org
>


--
Thanks
Yafang