linux-kernel - Re: [PATCH 03/34] brcmfmac: firmware: Support having multiple alt paths

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Date:   Sun, 2 Jan 2022 23:11:32 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Hector Martin <marcan@...can.st>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Len Brown <lenb@...nel.org>,
        Arend van Spriel <aspriel@...il.com>,
        Franky Lin <franky.lin@...adcom.com>,
        Hante Meuleman <hante.meuleman@...adcom.com>,
        Chi-hsien Lin <chi-hsien.lin@...ineon.com>,
        Wright Feng <wright.feng@...ineon.com>,
        Chung-hsien Hsu <chung-hsien.hsu@...ineon.com>
Cc:     Sven Peter <sven@...npeter.dev>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Mark Kettenis <kettenis@...nbsd.org>,
        Rafał Miłecki <zajec5@...il.com>,
        Pieter-Paul Giesberts <pieter-paul.giesberts@...adcom.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Hans de Goede <hdegoede@...hat.com>,
        "John W. Linville" <linville@...driver.com>,
        "Daniel (Deognyoun) Kim" <dekim@...adcom.com>,
        "brian m. carlson" <sandals@...stytoothpaste.net>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, brcm80211-dev-list.pdl@...adcom.com,
        SHA-cyfmac-dev-list@...ineon.com
Subject: Re: [PATCH 03/34] brcmfmac: firmware: Support having multiple alt
 paths

02.01.2022 17:18, Hector Martin пишет:
> On 2022/01/02 15:45, Dmitry Osipenko wrote:
>> 26.12.2021 18:35, Hector Martin пишет:
>>> -static char *brcm_alt_fw_path(const char *path, const char *board_type)
>>> +static const char **brcm_alt_fw_paths(const char *path, const char *board_type)
>>>  {
>>>  	char alt_path[BRCMF_FW_NAME_LEN];
>>> +	char **alt_paths;
>>>  	char suffix[5];
>>>  
>>>  	strscpy(alt_path, path, BRCMF_FW_NAME_LEN);
>>> @@ -609,27 +612,46 @@ static char *brcm_alt_fw_path(const char *path, const char *board_type)
>>>  	strlcat(alt_path, board_type, BRCMF_FW_NAME_LEN);
>>>  	strlcat(alt_path, suffix, BRCMF_FW_NAME_LEN);
>>>  
>>> -	return kstrdup(alt_path, GFP_KERNEL);
>>> +	alt_paths = kzalloc(sizeof(char *) * 2, GFP_KERNEL);
>>
>> array_size()?
> 
> Of what array?

array_size(sizeof(*alt_paths), 2)

>>> +	alt_paths[0] = kstrdup(alt_path, GFP_KERNEL);
>>> +
>>> +	return (const char **)alt_paths;
>>
>> Why this casting is needed?
> 
> Because implicit conversion from char ** to const char ** is not legal
> in C, as that could cause const unsoundness if you do this:
> 
> char *foo[1];
> const char **bar = foo;
> 
> bar[0] = "constant string";
> foo[0][0] = '!'; // clobbers constant string

It's up to a programmer to decide what is right to do. C gives you
flexibility, meanwhile it's easy to shoot yourself in the foot if you
won't be careful.

> But it's fine in this case since the non-const pointer disappears so
> nothing can ever write through it again.
> 

There is indeed no need for the castings in such cases, it's a typical
code pattern in kernel. You would need to do the casting for the other
way around, i.e. if char ** was returned and **alt_paths was a const.