linux-kernel - Re: "Dentry still in use" splats in v5.2-rc3

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Message-ID: <20190610183031.GE63833@gmail.com>
Date:   Mon, 10 Jun 2019 11:30:32 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Mark Rutland <mark.rutland@....com>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: "Dentry still in use" splats in v5.2-rc3

On Wed, Jun 05, 2019 at 02:54:01PM +0100, Mark Rutland wrote:
> Hi All,
> 
> While fuzzing arm64 v5.2-rc3, Syzkaller started hitting splats of the
> form:
> 
>     BUG: Dentry (____ptrval____){i=1,n=/}  still in use (2) [unmount of bpf bpf]
> 
> ... which I can reliably reproduce with the following C program
> (partially minimized from what Syzkaller auto-generated).
> 
> It looks like any filesystem will do. I've seen splats with "bpf",
> "hugetlbfs", "rpc_pipefs", and "tmpfs", and can reproduce the problem
> with any of these.
> 
> Any ideas?
> 
> I'm using the config from my fuzzing/5.2-rc3 branch on kernel.org [1].
> 
> Thanks,
> Mark.
> 
> ----
> #include <unistd.h>
> #include <sys/syscall.h>
> 
> /*
>  * NOTE: these are the arm64 numbers
>  */
> #ifndef __NR_fsconfig
> #define __NR_fsconfig 431
> #endif
> #ifndef __NR_fsmount
> #define __NR_fsmount 432
> #endif
> #ifndef __NR_fsopen
> #define __NR_fsopen 430
> #endif
> 
> int main(void)
> {
>         int fs, mnt;
> 
>         fs = syscall(__NR_fsopen, "bpf", 0);
>         syscall(__NR_fsconfig, fs, 6, 0, 0, 0);
>         mnt = syscall(__NR_fsmount, fs, 0, 0);
>         fchdir(mnt);
> 
>         close(fs);
>         close(mnt);
> }
> 

David and Al, is sys_fsmount() missing a call to mntget()?

- Eric