From: Jason Gunthorpe <jgg@ziepe.ca>
To: "Mika Penttilä" <mpenttil@redhat.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
apopple@nvidia.com, jhubbard@nvidia.com, rcampbell@nvidia.com,
vbabka@suse.cz
Subject: Re: [PATCH v2] mm/hmm/test: simplify hmm test code: use miscdevice instead of char dev
Date: Tue, 15 Mar 2022 15:39:22 -0300 [thread overview]
Message-ID: <20220315183922.GC64706@ziepe.ca> (raw)
In-Reply-To: <d9b9c82e-4012-cf0a-d966-d9669a684a27@redhat.com>
On Tue, Mar 15, 2022 at 05:22:15AM +0200, Mika Penttilä wrote:
> Hi Jason and thanks for your comments..
>
> On 14.3.2022 20.24, Jason Gunthorpe wrote:
> > On Fri, Mar 11, 2022 at 05:30:50AM +0200, mpenttil@redhat.com wrote:
> > > From: Mika Penttilä <mpenttil@redhat.com>
> > >
> > > HMM selftests use an in-kernel pseudo device to emulate device private
> > > memory. The pseudo device registers a major device range for two pseudo
> > > device instances. User space has a script that reads /proc/devices in
> > > order to find the assigned major number, and sends that to mknod(1),
> > > once for each node.
> > >
> > > This duplicates a fair amount of boilerplate that misc device can do
> > > instead.
> > >
> > > Change this to use misc device, which makes the device node names appear
> > > for us. This also enables udev-like processing if desired.
> >
> > This is borderline the wrong way to use misc devices, they should
> > never be embedded into other structs like this. It works out here
> > because they are eventually only placed in a static array, but still
> > it is a generally bad pattern to see.
>
> Could you elaborate on this one? We have many in-tree usages of the same
> pattern, like:
The kernel is full of bugs
> drivers/video/fbdev/pxa3xx-gcu.c
ie this is broken because it allocates like this:
priv = devm_kzalloc(dev, sizeof(struct pxa3xx_gcu_priv), GFP_KERNEL);
if (!priv)
return -ENOMEM;
And free's via devm:
static int pxa3xx_gcu_remove(struct platform_device *pdev)
{
struct pxa3xx_gcu_priv *priv = platform_get_drvdata(pdev);
misc_deregister(&priv->misc_dev);
return 0;
}
But this will UAF if it races fops open with misc_desregister.
Proper use of cdevs with proper struct devices prevent this bug.
> You mention "placed in a static array", are you seeing a potential lifetime
> issue or what? Many of the examples above embed miscdevice in a dynamically
> allocated object also.
>
> The file object's private_data holds a pointer to the miscdevice, and
> fops_get() pins the module. So freeing the objects miscdevice is embedded in
> at module_exit time should be fine. But, as you said, in this case the
> miscdevices are statically allocated, so that shouldn't be an issue
> either.
Correct, it is OK here because the module refcounts prevent the
miscdevice memory from being freed, the above cases with dynamic
allocations do not have that protection and are wrong.
This is why I don't care for the pattern of putting misc devices
inside other structs, it suggests this is perhaps generally safe but
it is not.
> I think using cdev_add ends up in the same results in device_* api
> sense.
Nope, everything works right once you use cdev_device_add on a
properly registered struct device.
> miscdevice acting like a mux at a higher abstraction level simplifies the
> code.
It does avoid the extra struct device, but at the cost of broken
memory lifetime
Jason
next prev parent reply other threads:[~2022-03-15 18:39 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-11 3:30 [PATCH v2] mm/hmm/test: simplify hmm test code: use miscdevice instead of char dev mpenttil
2022-03-11 6:15 ` John Hubbard
2022-03-11 6:18 ` Mika Penttilä
2022-03-14 18:24 ` Jason Gunthorpe
2022-03-15 3:22 ` Mika Penttilä
2022-03-15 18:39 ` Jason Gunthorpe [this message]
2022-03-17 5:47 ` Mika Penttilä
2022-03-17 6:58 ` Mika Penttilä
2022-03-17 14:15 ` Jason Gunthorpe
2022-03-18 2:34 ` Mika Penttilä
2022-03-18 2:58 ` John Hubbard
2022-03-18 12:40 ` Jason Gunthorpe
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20220315183922.GC64706@ziepe.ca \
--to=jgg@ziepe.ca \
--cc=apopple@nvidia.com \
--cc=jhubbard@nvidia.com \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=mpenttil@redhat.com \
--cc=rcampbell@nvidia.com \
--cc=vbabka@suse.cz \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).