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Subject: Re: [PATCH] mm: Move check for SHRINKER_NUMA_AWARE to
 do_shrink_slab()
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
To:     David Howells <dhowells@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, vdavydov.dev@gmail.com,
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Date:   Fri, 3 Aug 2018 14:04:42 +0300
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On 03.08.2018 13:59, Kirill Tkhai wrote:
> On 03.08.2018 13:31, David Howells wrote:
>> The reproducer can be reduced to:
>>
>> 	#define _GNU_SOURCE
>> 	#include <endian.h>
>> 	#include <stdint.h>
>> 	#include <string.h>
>> 	#include <stdio.h>
>> 	#include <sys/syscall.h>
>> 	#include <sys/stat.h>
>> 	#include <sys/mount.h>
>> 	#include <unistd.h>
>> 	#include <fcntl.h>
>>
>> 	const char path[] = "./file0";
>>
>> 	int main()
>> 	{
>> 		mkdir(path, 0);
>> 		mount(path, path, "cgroup2", 0, 0);
>> 		chroot(path);
>> 		umount2(path, 0);
>> 		return 0;
>> 	}
>>
>> and I've found two bugs (see attached patch).  The issue is that
>> do_remount_sb() is called with fc == NULL from umount(), but both
>> cgroup_reconfigure() and do_remount_sb() dereference fc unconditionally.
>>
>> But!  I'm not sure why the reproducer works at all because the umount2() call
>> is *after* the chroot, so should fail on ENOENT before it even gets that far.
>> In fact, umount2() can be called multiple times, apparently successfully, and
>> doesn't actually unmount anything.
> 
> Before I also try to check why it works; just reporting you that the patch
> works the problem in my environment. Thanks, David.

patch *fixes* the problem

> 
>> ---
>> diff --git a/fs/super.c b/fs/super.c
>> index 3fe5d12b7697..321fbc244570 100644
>> --- a/fs/super.c
>> +++ b/fs/super.c
>> @@ -978,7 +978,10 @@ int do_remount_sb(struct super_block *sb, int sb_flags, void *data,
>>  	    sb->s_op->remount_fs) {
>>  		if (sb->s_op->reconfigure) {
>>  			retval = sb->s_op->reconfigure(sb, fc);
>> -			sb_flags = fc->sb_flags;
>> +			if (fc)
>> +				sb_flags = fc->sb_flags;
>> +			else
>> +				sb_flags = sb->s_flags;
>>  			if (retval == 0)
>>  				security_sb_reconfigure(fc);
>>  		} else {
>> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
>> index f3238f38d152..48275fdce053 100644
>> --- a/kernel/cgroup/cgroup.c
>> +++ b/kernel/cgroup/cgroup.c
>> @@ -1796,9 +1796,11 @@ static void apply_cgroup_root_flags(unsigned int root_flags)
>>  
>>  static int cgroup_reconfigure(struct kernfs_root *kf_root, struct fs_context *fc)
>>  {
>> -	struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
>> +	if (fc) {
>> +		struct cgroup_fs_context *ctx = cgroup_fc2context(fc);
>>  
>> -	apply_cgroup_root_flags(ctx->flags);
>> +		apply_cgroup_root_flags(ctx->flags);
>> +	}
>>  	return 0;
>>  }
>>  
>>