netdev - Re: [PATCH v4 1/3] Bluetooth: Fix possible race with userspace of sysfs isoc

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Message-ID: <CABBYNZJSwQJ-KWacoXDGVJ5gni260FTi=XEpNw3ER2CJhpVrKg@mail.gmail.com>
Date: Thu, 13 Feb 2025 10:56:46 -0500
From: Luiz Augusto von Dentz <luiz.dentz@...il.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: Hsin-chen Chuang <chharry@...gle.com>, linux-bluetooth@...r.kernel.org, 
	chromeos-bluetooth-upstreaming@...omium.org, 
	Hsin-chen Chuang <chharry@...omium.org>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Johan Hedberg <johan.hedberg@...il.com>, Marcel Holtmann <marcel@...tmann.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, Ying Hsu <yinghsu@...omium.org>, 
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v4 1/3] Bluetooth: Fix possible race with userspace of
 sysfs isoc_alt

Hi Greg,

On Thu, Feb 13, 2025 at 10:39 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Thu, Feb 13, 2025 at 09:22:28AM -0500, Luiz Augusto von Dentz wrote:
> > Hi Greg,
> >
> > On Thu, Feb 13, 2025 at 8:45 AM Greg KH <gregkh@...uxfoundation.org> wrote:
> > >
> > > On Thu, Feb 13, 2025 at 09:33:34PM +0800, Hsin-chen Chuang wrote:
> > > > On Thu, Feb 13, 2025 at 8:10 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> > > > >
> > > > > A: http://en.wikipedia.org/wiki/Top_post
> > > > > Q: Were do I find info about this thing called top-posting?
> > > > > A: Because it messes up the order in which people normally read text.
> > > > > Q: Why is top-posting such a bad thing?
> > > > > A: Top-posting.
> > > > > Q: What is the most annoying thing in e-mail?
> > > > >
> > > > > A: No.
> > > > > Q: Should I include quotations after my reply?
> > > > >
> > > > > http://daringfireball.net/2007/07/on_top
> > > > >
> > > > > On Thu, Feb 13, 2025 at 07:57:15PM +0800, Hsin-chen Chuang wrote:
> > > > > > The btusb driver data is allocated by devm_kzalloc and is
> > > > > > automatically freed on driver detach, so I guess we don't have
> > > > > > anything to do here.
> > > > >
> > > > > What?  A struct device should NEVER be allocated with devm_kzalloc.
> > > > > That's just not going to work at all.
> > > >
> > > > Noted. Perhaps that needs to be refactored together.
> > > >
> > > > >
> > > > > > Or perhaps we should move btusb_disconnect's content here? Luiz, what
> > > > > > do you think?
> > > > >
> > > > > I think something is really wrong here.  Why are you adding a new struct
> > > > > device to the system?  What requires that?  What is this new device
> > > > > going to be used for?
> > > >
> > > > The new device is only for exposing a new sysfs attribute.
> > >
> > > That feels crazy.
> > >
> > > > So originally we had a device called hci_dev, indicating the
> > > > implementation of the Bluetooth HCI layer. hci_dev is directly the
> > > > child of the usb_interface (the Bluetooth chip connected through USB).
> > > > Now I would like to add an attribute for something that's not defined
> > > > in the HCI layer, but lower layer only in Bluetooth USB.
> > > > Thus we want to rephrase the structure: usb_interface -> btusb (new
> > > > device) -> hci_dev, and then we could place the new attribute in the
> > > > new device.
> > > >
> > > > Basically I kept the memory management in btusb unchanged in this
> > > > patch, as the new device is only used for a new attribute.
> > > > Would you suggest we revise the memory management since we added a
> > > > device in this module?
> > >
> > > If you add a new device in the tree, it HAS to work properly with the
> > > driver core (i.e. life cycles are unique, you can't have empty release
> > > functions, etc.)  Put it on the proper bus it belongs to, bind the
> > > needed drivers to it, and have it work that way, don't make a "fake"
> > > device for no good reason.
> >
> > Well we could just introduce it to USB device, since alternate setting
> > is a concept that is coming from there, but apparently the likes of
> > /sys/bus/usb/devices/usbX/bAlternateSetting is read-only, some
> > Bluetooth profiles (HFP) requires switching the alternate setting and
> > because Google is switching to handle this via userspace thus why
> > there was this request to add a new sysfs to control it.
>
> That's fine, just don't add devices where there shouldn't be devices, or
> if you do, make them actually work properly (i.e. do NOT have empty
> release callbacks...)
>
> If you want to switch alternate settings in a USB device, do it the
> normal way from userspace that has been there for decades!  Don't make
> up some other random sysfs file for this please, that would be crazy,
> and wrong.
>
> So what's wrong with the current api we have today that doesn't work for
> bluetooth devices?

Perhaps it is just lack of knowledge then, how userspace can request
to switch alternate settings? If I recall correctly Hsin-chen tried
with libusb, or something like that, but it didn't work for some
reason.

@Hsin-chen Chuang I hope you can fill in the details since for the
most part this is not a BlueZ feature. I'm just explaining what you
guys are after but ultimately it is up to Google to drive this.

-- 
Luiz Augusto von Dentz