[PATCH] ipvs: use div_s64 for signed division

[PATCH] ipvs: use div_s64 for signed division

[Arnd Bergmann]

From: Arnd Bergmann <arnd@arndb.de>

do_div() is only well-behaved for positive numbers, and now warns
when the first argument is a an s64:

net/netfilter/ipvs/ip_vs_est.c: In function 'ip_vs_est_calc_limits':
include/asm-generic/div64.h:222:35: error: comparison of distinct pointer types lacks a cast [-Werror]
  222 |         (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
      |                                   ^~
net/netfilter/ipvs/ip_vs_est.c:694:17: note: in expansion of macro 'do_div'
  694 |                 do_div(val, loops);

Convert to using the more appropriate div_s64(), which also
simplifies the code a bit.

Fixes: 705dd3444081 ("ipvs: use kthreads for stats estimation")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 net/netfilter/ipvs/ip_vs_est.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/net/netfilter/ipvs/ip_vs_est.c b/net/netfilter/ipvs/ip_vs_est.c
index ce2a1549b304..dbc32f8cf1f9 100644
--- a/net/netfilter/ipvs/ip_vs_est.c
+++ b/net/netfilter/ipvs/ip_vs_est.c
@@ -691,15 +691,13 @@ static int ip_vs_est_calc_limits(struct netns_ipvs *ipvs, int *chain_max)
 		}
 		if (diff >= NSEC_PER_SEC)
 			continue;
-		val = diff;
-		do_div(val, loops);
+		val = div_s64(diff, loops);
 		if (!min_est || val < min_est) {
 			min_est = val;
 			/* goal: 95usec per chain */
 			val = 95 * NSEC_PER_USEC;
 			if (val >= min_est) {
-				do_div(val, min_est);
-				max = (int)val;
+				max = div_s64(val, min_est);
 			} else {
 				max = 1;
 			}
-- 
2.35.1

Re: [PATCH] ipvs: use div_s64 for signed division

[Julian Anastasov]

	Hello,

On Thu, 15 Dec 2022, Arnd Bergmann wrote:

> From: Arnd Bergmann <arnd@arndb.de>
> 
> do_div() is only well-behaved for positive numbers, and now warns
> when the first argument is a an s64:
> 
> net/netfilter/ipvs/ip_vs_est.c: In function 'ip_vs_est_calc_limits':
> include/asm-generic/div64.h:222:35: error: comparison of distinct pointer types lacks a cast [-Werror]
>   222 |         (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
>       |                                   ^~
> net/netfilter/ipvs/ip_vs_est.c:694:17: note: in expansion of macro 'do_div'
>   694 |                 do_div(val, loops);

	net-next already contains fix for this warning
and changes val to u64.

> Convert to using the more appropriate div_s64(), which also
> simplifies the code a bit.
> 
> Fixes: 705dd3444081 ("ipvs: use kthreads for stats estimation")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
>  net/netfilter/ipvs/ip_vs_est.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_est.c b/net/netfilter/ipvs/ip_vs_est.c
> index ce2a1549b304..dbc32f8cf1f9 100644
> --- a/net/netfilter/ipvs/ip_vs_est.c
> +++ b/net/netfilter/ipvs/ip_vs_est.c
> @@ -691,15 +691,13 @@ static int ip_vs_est_calc_limits(struct netns_ipvs *ipvs, int *chain_max)
>  		}
>  		if (diff >= NSEC_PER_SEC)
>  			continue;
> -		val = diff;
> -		do_div(val, loops);
> +		val = div_s64(diff, loops);

	On CONFIG_X86_32 both versions execute single divl
for our case but div_s64 is not inlined. I'm not expert in
this area but if you think div_u64 is more appropriate then
post another patch. Note that now val is u64 and
min_est is still s32 (can be u32).

>  		if (!min_est || val < min_est) {
>  			min_est = val;
>  			/* goal: 95usec per chain */
>  			val = 95 * NSEC_PER_USEC;
>  			if (val >= min_est) {
> -				do_div(val, min_est);
> -				max = (int)val;
> +				max = div_s64(val, min_est);
>  			} else {
>  				max = 1;
>  			}
> -- 
> 2.35.1

Regards

--
Julian Anastasov <ja@ssi.bg>

Re: [PATCH] ipvs: use div_s64 for signed division

[Pablo Neira Ayuso]

Hi Julian,

On Thu, Dec 15, 2022 at 09:01:59PM +0200, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Thu, 15 Dec 2022, Arnd Bergmann wrote:
> 
> > From: Arnd Bergmann <arnd@arndb.de>
> > 
> > do_div() is only well-behaved for positive numbers, and now warns
> > when the first argument is a an s64:
> > 
> > net/netfilter/ipvs/ip_vs_est.c: In function 'ip_vs_est_calc_limits':
> > include/asm-generic/div64.h:222:35: error: comparison of distinct pointer types lacks a cast [-Werror]
> >   222 |         (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
> >       |                                   ^~
> > net/netfilter/ipvs/ip_vs_est.c:694:17: note: in expansion of macro 'do_div'
> >   694 |                 do_div(val, loops);
> 
> 	net-next already contains fix for this warning
> and changes val to u64.

Arnd's patch applies fine on top of net-next, maybe he is addressing
something else?

> > Convert to using the more appropriate div_s64(), which also
> > simplifies the code a bit.
> > 
> > Fixes: 705dd3444081 ("ipvs: use kthreads for stats estimation")
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> > ---
> >  net/netfilter/ipvs/ip_vs_est.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/netfilter/ipvs/ip_vs_est.c b/net/netfilter/ipvs/ip_vs_est.c
> > index ce2a1549b304..dbc32f8cf1f9 100644
> > --- a/net/netfilter/ipvs/ip_vs_est.c
> > +++ b/net/netfilter/ipvs/ip_vs_est.c
> > @@ -691,15 +691,13 @@ static int ip_vs_est_calc_limits(struct netns_ipvs *ipvs, int *chain_max)
> >  		}
> >  		if (diff >= NSEC_PER_SEC)
> >  			continue;
> > -		val = diff;
> > -		do_div(val, loops);
> > +		val = div_s64(diff, loops);
> 
> 	On CONFIG_X86_32 both versions execute single divl
> for our case but div_s64 is not inlined. I'm not expert in
> this area but if you think div_u64 is more appropriate then
> post another patch. Note that now val is u64 and
> min_est is still s32 (can be u32).
> 
> >  		if (!min_est || val < min_est) {
> >  			min_est = val;
> >  			/* goal: 95usec per chain */
> >  			val = 95 * NSEC_PER_USEC;
> >  			if (val >= min_est) {
> > -				do_div(val, min_est);
> > -				max = (int)val;
> > +				max = div_s64(val, min_est);
> >  			} else {
> >  				max = 1;
> >  			}
> > -- 
> > 2.35.1
> 
> Regards
> 
> --
> Julian Anastasov <ja@ssi.bg>
> 

Re: [PATCH] ipvs: use div_s64 for signed division

[Arnd Bergmann]

On Fri, Dec 16, 2022, at 11:10, Pablo Neira Ayuso wrote:
> Hi Julian,
>
> On Thu, Dec 15, 2022 at 09:01:59PM +0200, Julian Anastasov wrote:
>> 
>> 	Hello,
>> 
>> On Thu, 15 Dec 2022, Arnd Bergmann wrote:
>> 
>> > From: Arnd Bergmann <arnd@arndb.de>
>> > 
>> > do_div() is only well-behaved for positive numbers, and now warns
>> > when the first argument is a an s64:
>> > 
>> > net/netfilter/ipvs/ip_vs_est.c: In function 'ip_vs_est_calc_limits':
>> > include/asm-generic/div64.h:222:35: error: comparison of distinct pointer types lacks a cast [-Werror]
>> >   222 |         (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
>> >       |                                   ^~
>> > net/netfilter/ipvs/ip_vs_est.c:694:17: note: in expansion of macro 'do_div'
>> >   694 |                 do_div(val, loops);
>> 
>> 	net-next already contains fix for this warning
>> and changes val to u64.
>
> Arnd's patch applies fine on top of net-next, maybe he is addressing
> something else?

No, it's the same bug. I had prepared my patch before the other fix
went in, and only one of the two is needed.

FWIW, I find my version slightly more readable, but Jakub's fix
is probably more efficient, because the unsigned 64-bit division
is better optimized on 32-bit, while div_s64() goes through an
extern function.

     Arnd