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Subject: Re: [PATCH] mm: Move check for SHRINKER_NUMA_AWARE to
 do_shrink_slab()
To:     David Howells <dhowells@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
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Date:   Fri, 3 Aug 2018 13:59:32 +0300
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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.

> ---
> 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;
>  }
>  
>