linux-kernel - Re: [PATCH v1 1/1] Firstly, as Andy mentioned, this should be smp_rmb() instead of rmb(). considering that TSYNC is a cross-thread situation, and rmb() is a mandatory barrier which should not be used to control SMP effects, since mandatory barriers impose unnecessary overhead on both SMP and UP systems, as kernel Documentation said.

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Date:   Tue, 2 Feb 2021 22:01:33 +0800
From:   kernel test robot <lkp@...el.com>
To:     wanghongzhe <wanghongzhe@...wei.com>, luto@...capital.net
Cc:     kbuild-all@...ts.01.org, andrii@...nel.org, ast@...nel.org,
        bpf@...r.kernel.org, daniel@...earbox.net,
        john.fastabend@...il.com, kafai@...com, keescook@...omium.org,
        kpsingh@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/1] Firstly, as Andy mentioned, this should be
 smp_rmb() instead of rmb(). considering that TSYNC is a cross-thread
 situation, and rmb() is a mandatory barrier which should not be used to
 control SMP effects, since mandatory barriers impose unnecessary overhead on
 both SMP and UP systems, as kernel Documentation said.

Hi wanghongzhe,

Thank you for the patch! Perhaps something to improve:

[auto build test WARNING on v5.11-rc6]
[also build test WARNING on next-20210125]
[cannot apply to kees/for-next/seccomp]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/0day-ci/linux/commits/wanghongzhe/Firstly-as-Andy-mentioned-this-should-be-smp_rmb-instead-of-rmb-considering-that-TSYNC-is-a-cross-thread-situation-and-r/20210202-173311
base:    1048ba83fb1c00cd24172e23e8263972f6b5d9ac
config: i386-randconfig-s001-20210202 (attached as .config)
compiler: gcc-9 (Debian 9.3.0-15) 9.3.0
reproduce:
        # apt-get install sparse
        # sparse version: v0.6.3-215-g0fb77bb6-dirty
        # https://github.com/0day-ci/linux/commit/f79414957fc8acb6b680bbcd26fa987328a5724a
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review wanghongzhe/Firstly-as-Andy-mentioned-this-should-be-smp_rmb-instead-of-rmb-considering-that-TSYNC-is-a-cross-thread-situation-and-r/20210202-173311
        git checkout f79414957fc8acb6b680bbcd26fa987328a5724a
        # save the attached .config to linux build tree
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__' ARCH=i386 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@...el.com>

All warnings (new ones prefixed by >>):

   kernel/seccomp.c: In function '__secure_computing':
>> kernel/seccomp.c:1313:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
    1313 |   if (unlikely(current->seccomp.mode != SECCOMP_MODE_FILTER))
         |      ^
   kernel/seccomp.c:1315:2: note: here
    1315 |  case SECCOMP_MODE_FILTER:
         |  ^~~~


vim +1313 kernel/seccomp.c

  1283	
  1284	int __secure_computing(const struct seccomp_data *sd)
  1285	{
  1286		int this_syscall;
  1287	
  1288		if (IS_ENABLED(CONFIG_CHECKPOINT_RESTORE) &&
  1289		    unlikely(current->ptrace & PT_SUSPEND_SECCOMP))
  1290			return 0;
  1291	
  1292		this_syscall = sd ? sd->nr :
  1293			syscall_get_nr(current, current_pt_regs());
  1294	
  1295		/*
  1296		 * Make sure that any changes to mode from another thread have
  1297		 * been seen after SYSCALL_WORK_SECCOMP was seen.
  1298		 */
  1299		smp_rmb();
  1300	
  1301		switch (current->seccomp.mode) {
  1302		case SECCOMP_MODE_STRICT:
  1303			__secure_computing_strict(this_syscall);  /* may call do_exit */
  1304			return 0;
  1305		/*
  1306		 * Make sure that change to mode (from SECCOMP_MODE_DISABLED to
  1307		 * SECCOMP_MODE_FILTER) from another thread using TSYNC ability
  1308		 * have been seen after SYSCALL_WORK_SECCOMP was seen. Read mode again behind
  1309		 * smp_rmb(), if it equals SECCOMP_MODE_FILTER, go to the right path.
  1310		 */
  1311		case SECCOMP_MODE_DISABLED:
  1312			smp_rmb();
> 1313			if (unlikely(current->seccomp.mode != SECCOMP_MODE_FILTER))
  1314				BUG();
  1315		case SECCOMP_MODE_FILTER:
  1316			return __seccomp_filter(this_syscall, sd, false);
  1317		default:
  1318			BUG();
  1319		}
  1320	}
  1321	#endif /* CONFIG_HAVE_ARCH_SECCOMP_FILTER */
  1322	

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org

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