linux-kernel - Re: debugfs_create_u32

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Date:   Mon, 22 Jun 2020 21:31:09 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Ronald Tschalär <ronald@...ovation.ch>,
        Nicolai Stange <nicstange@...il.com>,
        David Rientjes <rientjes@...gle.com>,
        Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org
Subject: Re: debugfs_create_u32_array() memory leaks

On Mon, Jun 22, 2020 at 12:23:32PM -0700, Jakub Kicinski wrote:
> On Sat, 20 Jun 2020 09:45:42 +0200 Greg Kroah-Hartman wrote:
> > On Fri, Jun 19, 2020 at 04:17:34PM -0700, Jakub Kicinski wrote:
> > > Hi!
> > > 
> > > I'm trying to use debugfs_create_u32_array() in drivers/net/netdevsim
> > > and it causes memory leaks:
> > > 
> > > unreferenced object 0xffff8880546642a0 (size 16):
> > >   comm "test_udp_tuns.s", pid 2146, jiffies 4294928368 (age 3772.435s)
> > >   hex dump (first 16 bytes):
> > >     84 52 6a 4d 80 88 ff ff 04 00 00 00 f3 78 7e 89  .RjM.........x~.
> > >   backtrace:
> > >     [<000000006962a447>] debugfs_create_u32_array+0x3f/0x90
> > > 
> > > I can see that debugfs_create_u32_array() allocates a structure at
> > > create time that ends up assigned to inode->i_private, but I don't 
> > > see it freed anywhere.
> > > 
> > > Am I missing something? I'm pretty sure files get removed, cause the
> > > driver calls debugfs_remove_recursive() and no other file types leaks.  
> > 
> > Yeah, that's a bug, nice catch.  The debugfs_create*() functions should
> > not allocate local memory as we can't know to free that memory when the
> > file is removed.
> > 
> > Can you fix this up, or do you want me to?  I only see one in-kernel
> > user of this, so it shouldn't be that tough to do so.  The one user
> > never removes that file so that's why no one noticed this before.
> 
> Ah, I wasn't sure how to fix but since you say that create functions
> shouldn't allocate memory seems like the fix will be to make callers
> pass an equivalent of struct debugfs_blob_wrapper for u32.

Sounds good.

> I'm happy to send a patch to that effect - I have a process question
> tho - I need this change in net-next, should I sent the patch to you?
> Can it still make it into 5.8 (debugfs -> Linus -> net -> net-next) or
> perhaps can it go via net-next since there is no de facto bug in 5.8?

I can take a fix now, and get it into 5.8 if that makes things easier
for you.

thanks,

greg k-h