netdev - Re: [PATCH v2 net-next 1/2] net: dsa: lan9303: Add port_fast_age and port_fdb

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Date:   Thu, 19 Oct 2017 18:52:38 +0200
From:   Egil Hjelmeland <privat@...l-hjelmeland.no>
To:     David Laight <David.Laight@...LAB.COM>,
        'Andrew Lunn' <andrew@...n.ch>,
        Vivien Didelot <vivien.didelot@...oirfairelinux.com>
Cc:     "f.fainelli@...il.com" <f.fainelli@...il.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 net-next 1/2] net: dsa: lan9303: Add port_fast_age and
 port_fdb_dump methods



Den 19. okt. 2017 17:42, skrev Egil Hjelmeland:
> On 19. okt. 2017 17:15, David Laight wrote:
>> From: Andrew Lunn
>>> Sent: 19 October 2017 15:15
>>>>> +/* Clear learned (non-static) entry on given port */
>>>>> +static void alr_loop_cb_del_port_learned(struct lan9303 *chip, u32 
>>>>> dat0,
>>>>> +                     u32 dat1, int portmap, void *ctx)
>>>>> +{
>>>>> +    int *port = ctx;
>>>>
>>>> You can get the value directly to make the line below more readable:
>>>>
>>>>      int port = *(int *)ctx;
>>>
>>> You have to be a bit careful with this. You often see people
>>> submitting patches taking away casts for void * pointers.
>>> If they do that here, it should at least not compile...
>>>
>>> So maybe do it in two steps?
>>>
>>>     int * pport = ctx;
>>>     int port = *pport;
>>
>> IMHO it is best to define a struct for the 'ctx and then do:
>>     ..., void *v_ctx)
>> {
>>     foo_ctx *ctx = v_ctx;
>>     int port = ctx->port;
>>
>> That stops anyone having to double-check that the *(int *)
>> is operating on a pointer to an integer of the correct size.
>>
> 
> Does casting to a struct pointer require less manual double-check than
> to a int-pointer? In neither cases the compiler can protect us, IFAIK.
> But on the other hand, a the text "foo_ctx" can searched in the editor.
> So in that respect it can somewhat aid to the double-checking.
> 
> So I can do that.
> 
> 

I understand now that the caller side (lan9303_port_fast_age) is
vulnerable. Say somebody has the idea to change the "port" param
of .port_fast_age from int to u8, then my code is a trap.

Thanks for the education.

Egil