Subject: Re: [PATCH net-next 08/11] ath9k: work around false-positive gcc warning

From: Kalle Valo <[email-masked]>
Date: Sat, 07 Nov 2020 13:18:10 +0200

Johannes Berg <[email-masked]> writes:

> On Mon, 2020-11-02 at 18:26 +0200, Kalle Valo wrote:
>> Arnd Bergmann <[email-masked]> writes:
>> 
>> > From: Arnd Bergmann <[email-masked]>
>> > 
>> > gcc-10 shows a false-positive warning with CONFIG_KASAN:
>> > 
>> > drivers/net/wireless/ath/ath9k/dynack.c: In function 'ath_dynack_sample_tx_ts':
>> > include/linux/etherdevice.h:290:14: warning: writing 4 bytes into a region of size 0 [-Wstringop-overflow=]
>> >   290 |  *(u32 *)dst = *(const u32 *)src;
>> >       |  ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~
>> > 
>> > Until gcc is fixed, work around this by using memcpy() in place
>> > of ether_addr_copy(). Hopefully gcc-11 will not have this problem.
>> > 
>> > Link: https://godbolt.org/z/sab1MK
>> > Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97490
>> > Signed-off-by: Arnd Bergmann <[email-masked]>
>> > ---
>> >  drivers/net/wireless/ath/ath9k/dynack.c | 6 ++++++
>> >  1 file changed, 6 insertions(+)
>> > 
>> > diff --git a/drivers/net/wireless/ath/ath9k/dynack.c b/drivers/net/wireless/ath/ath9k/dynack.c
>> > index fbeb4a739d32..e4eb96b26ca4 100644
>> > --- a/drivers/net/wireless/ath/ath9k/dynack.c
>> > +++ b/drivers/net/wireless/ath/ath9k/dynack.c
>> > @@ -247,8 +247,14 @@ void ath_dynack_sample_tx_ts(struct ath_hw *ah, struct sk_buff *skb,
>> >  	ridx = ts->ts_rateindex;
>> >  
>> >  	da->st_rbf.ts[da->st_rbf.t_rb].tstamp = ts->ts_tstamp;
>> > +#if defined(CONFIG_KASAN) && (CONFIG_GCC_VERSION >= 100000) && (CONFIG_GCC_VERSION < 110000)
>> > +	/* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97490 */
>> > +	memcpy(da->st_rbf.addr[da->st_rbf.t_rb].h_dest, hdr->addr1, ETH_ALEN);
>> > +	memcpy(da->st_rbf.addr[da->st_rbf.t_rb].h_src, hdr->addr2, ETH_ALEN);
>> > +#else
>> >  	ether_addr_copy(da->st_rbf.addr[da->st_rbf.t_rb].h_dest, hdr->addr1);
>> >  	ether_addr_copy(da->st_rbf.addr[da->st_rbf.t_rb].h_src, hdr->addr2);
>> > +#endif
>> 
>> Isn't there a better way to handle this? I really would not want
>> checking for GCC versions become a common approach in drivers.
>> 
>> I even think that using memcpy() always is better than the ugly ifdef.
>
> If you put memcpy() always somebody will surely go and clean it up to
> use ether_addr_copy() soon ...

I can always add a comment and hope that the cleanup people read
comments :) I did that now in the pending branch:

https://git.kernel.org/pub/scm/linux/kernel/git/kvalo/ath.git/commit/?h=pending&id=25cfc077bd7a798d1aa527ad2aa9932bb3284376

Does that look ok? I prefer that over the ifdef.

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