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Subject: Re: [PATCH v3 3/9] brcmfmac: firmware: Do not crash on a NULL
 board_type
To:     Andy Shevchenko <andy.shevchenko@gmail.com>
Cc:     Kalle Valo <kvalo@codeaurora.org>,
        "David S. Miller" <davem@davemloft.net>,
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        Arend van Spriel <aspriel@gmail.com>,
        Franky Lin <franky.lin@broadcom.com>,
        Hante Meuleman <hante.meuleman@broadcom.com>,
        Chi-hsien Lin <chi-hsien.lin@infineon.com>,
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        Dmitry Osipenko <digetx@gmail.com>,
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        Alyssa Rosenzweig <alyssa@rosenzweig.io>,
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References: <20220117142919.207370-1-marcan@marcan.st>
 <20220117142919.207370-4-marcan@marcan.st>
 <CAHp75VfZ+thU+AWeOQSC9Dqq3MO+GMb_8oPxqMEbaxYTH0PH5A@mail.gmail.com>
From:   Hector Martin <marcan@marcan.st>
Message-ID: <d3d46724-6e36-95d1-b614-90ac1811f672@marcan.st>
Date:   Thu, 20 Jan 2022 15:13:11 +0900
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On 20/01/2022 06.45, Andy Shevchenko wrote:
> On Mon, Jan 17, 2022 at 4:30 PM Hector Martin <marcan@marcan.st> wrote:
>>
>> This unbreaks support for USB devices, which do not have a board_type
>> to create an alt_path out of and thus were running into a NULL
>> dereference.
> 
> In v5.16 we have two call sites:
> 
> 1.
>   if (cur->type == BRCMF_FW_TYPE_NVRAM && fwctx->req->board_type) {
>     ...
>     alt_path = brcm_alt_fw_path(cur->path, fwctx->req->board_type);
> 
> 2.
>   alt_path = brcm_alt_fw_path(first->path, fwctx->req->board_type);
>   if (alt_path) {
>     ...
> 
> Looking at them I would rather expect to see (as a quick fix, the
> better solution is to unify those call sites by splitting out a common
> helper):
> 
>   if (fwctx->req->board_type) {
>     alt_path = brcm_alt_fw_path(first->path, fwctx->req->board_type);
>   else
>     alt_path = NULL;
>    ...
> 

Since brcm_alt_fw_path can fail anyway, and its return value is already
NULL-checked, it makes sense to propagate the NULL board_path there
rather than doing it at all the callsites. That's a common pattern, e.g.
the entire DT API is designed to accept NULL nodes. That does mean that
the first callsite has a redundant NULL check, yes, but that doesn't hurt.

This is all going to change with subsequent patches anyway; the point of
this patch is just to fix the regression.

-- 
Hector Martin (marcan@marcan.st)
Public Key: https://mrcn.st/pub