Re: [PATCH 07/26] brcmsmac: reduce stack size with KASAN

[Arend Van Spriel <arend.vanspriel@broadcom.com>]

On 6-3-2017 11:38, Arnd Bergmann wrote:
> On Mon, Mar 6, 2017 at 10:16 AM, Arend Van Spriel
> <arend.vanspriel@broadcom.com> wrote:
>> On 2-3-2017 17:38, Arnd Bergmann wrote:
>>> The wlc_phy_table_write_nphy/wlc_phy_table_read_nphy functions always put an object
>>> on the stack, which will each require a redzone with KASAN and lead to possible
>>> stack overflow:
>>>
>>> drivers/net/wireless/broadcom/brcm80211/brcmsmac/phy/phy_n.c: In function 'wlc_phy_workarounds_nphy':
>>> drivers/net/wireless/broadcom/brcm80211/brcmsmac/phy/phy_n.c:17135:1: warning: the frame size of 6312 bytes is larger than 1000 bytes [-Wframe-larger-than=]
>>
>> Looks like this warning text ended up in the wrong commit message. Got
>> me confused for a sec :-p
> 
> What's wrong about the warning?

The warning is about the function 'wlc_phy_workarounds_nphy' (see PATCH
9/26) and not about wlc_phy_table_write_nphy/wlc_phy_table_read_nphy
functions.

>>> This marks the two functions as noinline_for_kasan, avoiding the problem entirely.
>>
>> Frankly I seriously dislike annotating code for the sake of some
>> (dynamic) memory analyzer. To me the whole thing seems rather
>> unnecessary. If the code passes the 2048 stack limit without KASAN it
>> would seem the limit with KASAN should be such that no warning is given.
>> I suspect that it is rather difficult to predict the additional size of
>> the instrumentation code and on some systems there might be a real issue
>> with increased stack usage.
> 
> The frame sizes don't normally change that much. There are a couple of
> drivers like brcmsmac that repeatedly call an inline function which has
> a local variable that it passes by reference to an extern function.
> 
> While normally those variables share a stack location, KASAN forces
> each instance to its own location and adds (in this case) 80 bytes of
> redzone around it to detect out-of-bounds access.
> 
> While most drivers are fine with a 1500 byte warning limit, increasing
> the limit to 7kb would silence brcmsmac (unless more registers
> are accessed from wlc_phy_workarounds_nphy) but also risk a
> stack overflow to go unnoticed.

Given the amount of local variables maybe just tag the functions with
noinline instead.

Regards,
Arend