Re: [PATCH] netfilter: xt_hashlimit: avoid 64-bit division

On Wed, Sep 6, 2017 at 10:22 PM, Vishwanath Pai [off-list ref] wrote:

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

quoted

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