linux-kernel - Re: [PATCH v2 1/4] misc: fastrpc: Add NULL check to fastrpc_buf

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Message-ID: <a7fc9f94-4a64-4e5f-8c9d-858e7e9df7f8@oss.qualcomm.com>
Date: Tue, 3 Feb 2026 19:00:17 +0800
From: Jianping <jianping.li@....qualcomm.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: srini@...nel.org, amahesh@....qualcomm.com, arnd@...db.de,
        linux-arm-msm@...r.kernel.org,
        Ekansh Gupta <ekansh.gupta@....qualcomm.com>,
        thierry.escande@...aro.org, abelvesa@...nel.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        quic_chennak@...cinc.com, stable@...nel.org
Subject: Re: [PATCH v2 1/4] misc: fastrpc: Add NULL check to fastrpc_buf_free
 to prevent crash



On 2/2/2026 4:41 PM, Greg KH wrote:
> On Mon, Feb 02, 2026 at 03:13:10PM +0800, Jianping wrote:
>>
>>
>> On 1/16/2026 10:49 PM, Greg KH wrote:
>>> On Thu, Jan 15, 2026 at 04:28:48PM +0800, Jianping Li wrote:
>>>> From: Ekansh Gupta <ekansh.gupta@....qualcomm.com>
>>>>
>>>> The fastrpc_buf_free function currently does not handle the case where
>>>> the input buffer pointer (buf) is NULL. This can lead to a null pointer
>>>> dereference, causing a crash or undefined behavior when the function
>>>> attempts to access members of the buf structure. Add a NULL check to
>>>> ensure safe handling of NULL pointers and prevent potential crashes.
>>>
>>> What caller passes in NULL here?  I did a quick look, and see where the
>>> callers check this properly if it could be NULL, otherwise it all looks
>>> sane to me.  What in-kernel user is causing a crash here?  Why not fix
>>> the caller up instead?
>>>
>>> thanks,
>>>
>>> greg k-h
>>
>> It's a saftety coding: to eliminate NULL checks on the caller side, as we do
>> in a lot of other kernel API.
> 
> But you do not do that for all functions in the kernel, otherwise the
> kernel would be full of checks that are never hit at all.
To clarify the intention: this change was not triggered by any real 
crash in current callers. The motivation came from the v1 review 
discussion [1], where it was suggested that a NULL check in 
fastrpc_buf_free() would allow simplifying some of the caller paths.

[1]https://lore.kernel.org/all/c80c48a1-f1b6-4520-9d7c-3a83915c7717@oss.qualcomm.com/
> 
>> And it was pointed out in the v1 patch discussion that this change was
>> needed:
>> https://lore.kernel.org/all/c80c48a1-f1b6-4520-9d7c-3a83915c7717@oss.qualcomm.com/
> 
> Were the checks removed from the caller side like was asked for?
Currently, I have placed the check inside the API and removed all the 
checks outside the API.
> 
> Also, your changelog makes it sound like this is a real bugfix for
> something, when it is not at all, which is what I object to the most.
> Don't make scary changelogs for things that are not actually happening.
You are correct, I will modify the commit text that caused the 
misunderstanding.
> 
> thanks,
> 
> greg k-h