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Subject: Re: [PATCH AUTOSEL 5.4 39/52] brcmfmac: properly check for bus
 register errors
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     linux-wireless <linux-wireless@vger.kernel.org>,
        Kalle Valo <kvalo@codeaurora.org>,
        Sasha Levin <sashal@kernel.org>
References: <20210524144903.2498518-1-sashal@kernel.org>
 <20210524144903.2498518-39-sashal@kernel.org>
 <437445ee-a360-597f-f058-3b913984e064@broadcom.com>
 <YKycgrI+zh4seh7U@kroah.com>
From:   Arend Van Spriel <arend.vanspriel@broadcom.com>
Message-ID: <81b5dc11-4dfe-76d6-f822-0adcfb3a9e30@broadcom.com>
Date:   Tue, 25 May 2021 09:04:59 +0200
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On 5/25/2021 8:43 AM, Greg Kroah-Hartman wrote:
> On Tue, May 25, 2021 at 08:38:34AM +0200, Arend van Spriel wrote:
>> On 5/24/2021 4:48 PM, Sasha Levin wrote:
>>> From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>>>
>>> [ Upstream commit 419b4a142a7ece36cebcd434f8ce2af59ef94b85 ]
>>>
>>> The brcmfmac driver ignores any errors on initialization with the
>>> different busses by deferring the initialization to a workqueue and
>>> ignoring all possible errors that might happen.  Fix up all of this by
>>> only allowing the module to load if all bus registering worked properly.
>>
>> Hi Greg,
>>
>> Saw this one flying by for stable kernel. Actually the first time I saw this
>> patch, because I don't follow LKML as much as linux-wireless. The patch is
>> fine, but wanted to give some context on the workqueue approach. It was
>> there for historic reasons. Back then we had the UMH to provide firmware
>> loading and because we request firmware during driver probe we could cause
>> kernel boot to show significant delay when driver was built-in. Hence the
>> workqueue which allowed kernel boot to proceed and driver probe was running
>> in another thread context. These days we have direct firmware loading from
>> the kernel and brcmfmac uses the asynchronous firmware loading API so there
>> is indeed no longer a need for the workqueue.
>>
>> Just for my understanding could you explain the motivation behind this
>> change. In the preceding revert patch I saw this remark:
>>
>> """
>> The original commit here did nothing to actually help if usb_register()
>> failed, so it gives a "false sense of security" when there is none.  The
>> correct solution is to correctly unwind from this error.
>> """
>>
>> Does this mean the patch is addressing some security issue. Before your
>> patch the module would remain loaded despite a bus register failure. I guess
>> there is a story behind this that I am curious about.
> 
> The module would remain loaded, yes, but nothing would work, and so no
> one would have any idea that something went wrong.  The original commit
> was wrong, it did not actually solve anything.

Agree.

> This commit properly propagates any error that happens back to the user,
> like any other module being loaded.

I understand, but this might cause a regression for the user. For 
instance if the usb_register() fails, but the other driver registrations 
succeed and the user has a wireless PCIe device. Before this change the 
user would have a functioning wifi device, but with this change it does not?

Regards,
Arend

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