netfilter: xt_hashlimit: avoid 64-bit division

Message ID 20170906195825.3715290-1-arnd@arndb.de
State New
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  • netfilter: xt_hashlimit: avoid 64-bit division
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Commit Message

Arnd Bergmann Sept. 6, 2017, 7:57 p.m.
64-bit division is expensive on 32-bit architectures, and
requires a special function call to avoid a link error like:

net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common':
xt_hashlimit.c:(.text+0x1328): undefined reference to `__aeabi_uldivmod'

In the case of hashlimit_mt_common, we don't actually need a
64-bit operation, we can simply rewrite the function slightly
to make that clear to the compiler.

Fixes: bea74641e378 ("netfilter: xt_hashlimit: add rate match mode")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

---
 net/netfilter/xt_hashlimit.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

-- 
2.9.0

Comments

Vishwanath Pai Sept. 6, 2017, 8:22 p.m. | #1
On 09/06/2017 03:57 PM, Arnd Bergmann wrote:
> 64-bit division is expensive on 32-bit architectures, and

> requires a special function call to avoid a link error like:

> 

> net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common':

> xt_hashlimit.c:(.text+0x1328): undefined reference to `__aeabi_uldivmod'

> 

> In the case of hashlimit_mt_common, we don't actually need a

> 64-bit operation, we can simply rewrite the function slightly

> to make that clear to the compiler.

> 

> Fixes: bea74641e378 ("netfilter: xt_hashlimit: add rate match mode")

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

> ---

>  net/netfilter/xt_hashlimit.c | 5 ++++-

>  1 file changed, 4 insertions(+), 1 deletion(-)

> 

> diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c

> index 10d48234f5f4..50b53d86eef5 100644

> --- a/net/netfilter/xt_hashlimit.c

> +++ b/net/netfilter/xt_hashlimit.c

> @@ -531,7 +531,10 @@ static u64 user2rate_bytes(u64 user)

>  {

>  	u64 r;

>  

> -	r = user ? 0xFFFFFFFFULL / user : 0xFFFFFFFFULL;

> +	if (user > 0xFFFFFFFFULL)

> +		return 0;

> +

> +	r = user ? 0xFFFFFFFFULL / (u32)user : 0xFFFFFFFFULL;

>  	r = (r - 1) << 4;

>  	return r;

>  }

> 


I have submitted another patch to fix this:
https://patchwork.ozlabs.org/patch/809881/

We have seen this problem before, I was careful not to introduce this
again in the new patch but clearly I overlooked this particular line :(

In the other cases we fixed it by replacing division with div64_u64().

-Vishwanath
Arnd Bergmann Sept. 6, 2017, 8:48 p.m. | #2
On Wed, Sep 6, 2017 at 10:22 PM, Vishwanath Pai <vpai@akamai.com> wrote:
> On 09/06/2017 03:57 PM, Arnd Bergmann wrote:

>> 64-bit division is expensive on 32-bit architectures, and

>> requires a special function call to avoid a link error like:

>>

>> net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common':

>> xt_hashlimit.c:(.text+0x1328): undefined reference to `__aeabi_uldivmod'

>>

>> In the case of hashlimit_mt_common, we don't actually need a

>> 64-bit operation, we can simply rewrite the function slightly

>> to make that clear to the compiler.

>>

>> Fixes: bea74641e378 ("netfilter: xt_hashlimit: add rate match mode")

>> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

>> ---

>>  net/netfilter/xt_hashlimit.c | 5 ++++-

>>  1 file changed, 4 insertions(+), 1 deletion(-)

>>

>> diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c

>> index 10d48234f5f4..50b53d86eef5 100644

>> --- a/net/netfilter/xt_hashlimit.c

>> +++ b/net/netfilter/xt_hashlimit.c

>> @@ -531,7 +531,10 @@ static u64 user2rate_bytes(u64 user)

>>  {

>>       u64 r;

>>

>> -     r = user ? 0xFFFFFFFFULL / user : 0xFFFFFFFFULL;

>> +     if (user > 0xFFFFFFFFULL)

>> +             return 0;

>> +

>> +     r = user ? 0xFFFFFFFFULL / (u32)user : 0xFFFFFFFFULL;

>>       r = (r - 1) << 4;

>>       return r;

>>  }

>>

>

> I have submitted another patch to fix this:

> https://patchwork.ozlabs.org/patch/809881/

>

> We have seen this problem before, I was careful not to introduce this

> again in the new patch but clearly I overlooked this particular line :(

>

> In the other cases we fixed it by replacing division with div64_u64().


div64_u64() seems needlessly expensive here since the dividend
is known to be a 32-bit number. I guess the function is not called
frequently though, so it doesn't matter much.

      Arnd
Pablo Neira Ayuso Sept. 7, 2017, 10:19 a.m. | #3
On Wed, Sep 06, 2017 at 10:48:22PM +0200, Arnd Bergmann wrote:
> On Wed, Sep 6, 2017 at 10:22 PM, Vishwanath Pai <vpai@akamai.com> wrote:

> > On 09/06/2017 03:57 PM, Arnd Bergmann wrote:

> >> 64-bit division is expensive on 32-bit architectures, and

> >> requires a special function call to avoid a link error like:

> >>

> >> net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common':

> >> xt_hashlimit.c:(.text+0x1328): undefined reference to `__aeabi_uldivmod'

> >>

> >> In the case of hashlimit_mt_common, we don't actually need a

> >> 64-bit operation, we can simply rewrite the function slightly

> >> to make that clear to the compiler.

> >>

> >> Fixes: bea74641e378 ("netfilter: xt_hashlimit: add rate match mode")

> >> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

> >> ---

> >>  net/netfilter/xt_hashlimit.c | 5 ++++-

> >>  1 file changed, 4 insertions(+), 1 deletion(-)

> >>

> >> diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c

> >> index 10d48234f5f4..50b53d86eef5 100644

> >> --- a/net/netfilter/xt_hashlimit.c

> >> +++ b/net/netfilter/xt_hashlimit.c

> >> @@ -531,7 +531,10 @@ static u64 user2rate_bytes(u64 user)

> >>  {

> >>       u64 r;

> >>

> >> -     r = user ? 0xFFFFFFFFULL / user : 0xFFFFFFFFULL;

> >> +     if (user > 0xFFFFFFFFULL)

> >> +             return 0;

> >> +

> >> +     r = user ? 0xFFFFFFFFULL / (u32)user : 0xFFFFFFFFULL;

> >>       r = (r - 1) << 4;

> >>       return r;

> >>  }

> >>

> >

> > I have submitted another patch to fix this:

> > https://patchwork.ozlabs.org/patch/809881/

> >

> > We have seen this problem before, I was careful not to introduce this

> > again in the new patch but clearly I overlooked this particular line :(

> >

> > In the other cases we fixed it by replacing division with div64_u64().

> 

> div64_u64() seems needlessly expensive here since the dividend

> is known to be a 32-bit number. I guess the function is not called

> frequently though, so it doesn't matter much.


This is called from the packet path, only for the first packet for
each new destination IP entry in the hashtable, still from the
datapath. So if we can take something faster (for 32 bit arches) that
is correct, I think it's sensible to take.

Let me know in any case.
Arnd Bergmann Sept. 7, 2017, 11:16 a.m. | #4
On Thu, Sep 7, 2017 at 12:19 PM, Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> On Wed, Sep 06, 2017 at 10:48:22PM +0200, Arnd Bergmann wrote:

>> On Wed, Sep 6, 2017 at 10:22 PM, Vishwanath Pai <vpai@akamai.com> wrote:

>> > On 09/06/2017 03:57 PM, Arnd Bergmann wrote:

>> >> 64-bit division is expensive on 32-bit architectures, and

>> >> requires a special function call to avoid a link error like:

>> >>

>> >> net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common':

>> >> xt_hashlimit.c:(.text+0x1328): undefined reference to `__aeabi_uldivmod'

>> >>

>> >> In the case of hashlimit_mt_common, we don't actually need a

>> >> 64-bit operation, we can simply rewrite the function slightly

>> >> to make that clear to the compiler.

>> >>

>> >> Fixes: bea74641e378 ("netfilter: xt_hashlimit: add rate match mode")

>> >> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

>> >> ---

>> >>  net/netfilter/xt_hashlimit.c | 5 ++++-

>> >>  1 file changed, 4 insertions(+), 1 deletion(-)

>> >>

>> >> diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c

>> >> index 10d48234f5f4..50b53d86eef5 100644

>> >> --- a/net/netfilter/xt_hashlimit.c

>> >> +++ b/net/netfilter/xt_hashlimit.c

>> >> @@ -531,7 +531,10 @@ static u64 user2rate_bytes(u64 user)

>> >>  {

>> >>       u64 r;

>> >>

>> >> -     r = user ? 0xFFFFFFFFULL / user : 0xFFFFFFFFULL;

>> >> +     if (user > 0xFFFFFFFFULL)

>> >> +             return 0;

>> >> +

>> >> +     r = user ? 0xFFFFFFFFULL / (u32)user : 0xFFFFFFFFULL;

>> >>       r = (r - 1) << 4;

>> >>       return r;

>> >>  }

>> >>

>> >

>> > I have submitted another patch to fix this:

>> > https://patchwork.ozlabs.org/patch/809881/

>> >

>> > We have seen this problem before, I was careful not to introduce this

>> > again in the new patch but clearly I overlooked this particular line :(

>> >

>> > In the other cases we fixed it by replacing division with div64_u64().

>>

>> div64_u64() seems needlessly expensive here since the dividend

>> is known to be a 32-bit number. I guess the function is not called

>> frequently though, so it doesn't matter much.

>

> This is called from the packet path, only for the first packet for

> each new destination IP entry in the hashtable, still from the

> datapath. So if we can take something faster (for 32 bit arches) that

> is correct, I think it's sensible to take.

>

> Let me know in any case.


I think my version should be slightly better then, unless someone
finds something wrong with it.

       Arnd
Geert Uytterhoeven Sept. 7, 2017, 2:16 p.m. | #5
Hi Arnd,

On Wed, Sep 6, 2017 at 9:57 PM, Arnd Bergmann <arnd@arndb.de> wrote:
> 64-bit division is expensive on 32-bit architectures, and

> requires a special function call to avoid a link error like:

>

> net/netfilter/xt_hashlimit.o: In function `hashlimit_mt_common':

> xt_hashlimit.c:(.text+0x1328): undefined reference to `__aeabi_uldivmod'

>

> In the case of hashlimit_mt_common, we don't actually need a

> 64-bit operation, we can simply rewrite the function slightly

> to make that clear to the compiler.

>

> Fixes: bea74641e378 ("netfilter: xt_hashlimit: add rate match mode")

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>


Thanks, this fixes a similar issue (__udivdi3 undefined) on m68k.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>


Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds

Patch

diff --git a/net/netfilter/xt_hashlimit.c b/net/netfilter/xt_hashlimit.c
index 10d48234f5f4..50b53d86eef5 100644
--- a/net/netfilter/xt_hashlimit.c
+++ b/net/netfilter/xt_hashlimit.c
@@ -531,7 +531,10 @@  static u64 user2rate_bytes(u64 user)
 {
 	u64 r;
 
-	r = user ? 0xFFFFFFFFULL / user : 0xFFFFFFFFULL;
+	if (user > 0xFFFFFFFFULL)
+		return 0;
+
+	r = user ? 0xFFFFFFFFULL / (u32)user : 0xFFFFFFFFULL;
 	r = (r - 1) << 4;
 	return r;
 }