From: Andrew Morton <akpm@linux-foundation.org> To: Rik van Riel <riel@redhat.com> Cc: Michal Hocko <mhocko@suse.cz>, Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, fengguang.wu@intel.com, mpatlasov@parallels.com, Motohiro.Kosaki@us.fujitsu.com Subject: Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom Date: Wed, 30 Apr 2014 12:35:26 -0700 [thread overview] Message-ID: <20140430123526.bc6a229c1ea4addad1fb483d@linux-foundation.org> (raw) In-Reply-To: <53614F3C.8020009@redhat.com> On Wed, 30 Apr 2014 15:30:04 -0400 Rik van Riel <riel@redhat.com> wrote: > On 04/30/2014 03:00 PM, Andrew Morton wrote: > > On Wed, 30 Apr 2014 10:41:14 -0400 Rik van Riel <riel@redhat.com> wrote: > > > >> It is possible for "limit - setpoint + 1" to equal zero, leading to a > >> divide by zero error. Blindly adding 1 to "limit - setpoint" is not > >> working, so we need to actually test the divisor before calling div64. > >> > >> ... > >> > >> --- a/mm/page-writeback.c > >> +++ b/mm/page-writeback.c > >> @@ -598,10 +598,15 @@ static inline long long pos_ratio_polynom(unsigned long setpoint, > >> unsigned long limit) > >> { > >> long long pos_ratio; > >> + long divisor; > >> long x; > >> > >> + divisor = limit - setpoint; > >> + if (!(s32)divisor) > >> + divisor = 1; /* Avoid div-by-zero */ > >> + > >> x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, > >> - limit - setpoint + 1); > >> + (s32)divisor); > > > > Doesn't this just paper over the bug one time in four billion? The > > other 3999999999 times, pos_ratio_polynom() returns an incorect result? > > > > If it is indeed the case that pos_ratio_polynom() callers are > > legitimately passing a setpoint which is more than 2^32 less than limit > > then it would be better to handle that input correctly. > > The easy way would be by calling div64_s64 and div64_u64, > which are 64 bit all the way through. > > Any objections? Sounds good to me. > The inlined bits seem to be stubs calling the _rem variants > of the functions, and discarding the remainder. I was referring to pos_ratio_polynom(). The compiler will probably be uninlining it anyway, but still...
WARNING: multiple messages have this Message-ID (diff)
From: Andrew Morton <akpm@linux-foundation.org> To: Rik van Riel <riel@redhat.com> Cc: Michal Hocko <mhocko@suse.cz>, Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>, linux-kernel@vger.kernel.org, linux-mm@kvack.org, sandeen@redhat.com, jweiner@redhat.com, kosaki.motohiro@jp.fujitsu.com, fengguang.wu@intel.com, mpatlasov@parallels.com, Motohiro.Kosaki@us.fujitsu.com Subject: Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom Date: Wed, 30 Apr 2014 12:35:26 -0700 [thread overview] Message-ID: <20140430123526.bc6a229c1ea4addad1fb483d@linux-foundation.org> (raw) In-Reply-To: <53614F3C.8020009@redhat.com> On Wed, 30 Apr 2014 15:30:04 -0400 Rik van Riel <riel@redhat.com> wrote: > On 04/30/2014 03:00 PM, Andrew Morton wrote: > > On Wed, 30 Apr 2014 10:41:14 -0400 Rik van Riel <riel@redhat.com> wrote: > > > >> It is possible for "limit - setpoint + 1" to equal zero, leading to a > >> divide by zero error. Blindly adding 1 to "limit - setpoint" is not > >> working, so we need to actually test the divisor before calling div64. > >> > >> ... > >> > >> --- a/mm/page-writeback.c > >> +++ b/mm/page-writeback.c > >> @@ -598,10 +598,15 @@ static inline long long pos_ratio_polynom(unsigned long setpoint, > >> unsigned long limit) > >> { > >> long long pos_ratio; > >> + long divisor; > >> long x; > >> > >> + divisor = limit - setpoint; > >> + if (!(s32)divisor) > >> + divisor = 1; /* Avoid div-by-zero */ > >> + > >> x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT, > >> - limit - setpoint + 1); > >> + (s32)divisor); > > > > Doesn't this just paper over the bug one time in four billion? The > > other 3999999999 times, pos_ratio_polynom() returns an incorect result? > > > > If it is indeed the case that pos_ratio_polynom() callers are > > legitimately passing a setpoint which is more than 2^32 less than limit > > then it would be better to handle that input correctly. > > The easy way would be by calling div64_s64 and div64_u64, > which are 64 bit all the way through. > > Any objections? Sounds good to me. > The inlined bits seem to be stubs calling the _rem variants > of the functions, and discarding the remainder. I was referring to pos_ratio_polynom(). The compiler will probably be uninlining it anyway, but still... -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>
next prev parent reply other threads:[~2014-04-30 19:35 UTC|newest] Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top 2014-04-29 19:19 [PATCH] mm,writeback: fix divide by zero in pos_ratio_polynom Rik van Riel 2014-04-29 19:19 ` Rik van Riel 2014-04-29 19:43 ` Motohiro Kosaki 2014-04-29 19:43 ` Motohiro Kosaki 2014-04-29 22:39 ` Andrew Morton 2014-04-29 22:39 ` Andrew Morton 2014-04-29 22:48 ` Rik van Riel 2014-04-29 22:48 ` Rik van Riel 2014-04-29 22:53 ` Andrew Morton 2014-04-29 22:53 ` Andrew Morton 2014-04-30 8:04 ` Maxim Patlasov 2014-04-30 8:04 ` Maxim Patlasov 2014-04-30 8:12 ` Michal Hocko 2014-04-30 8:12 ` Michal Hocko 2014-04-30 8:34 ` Maxim Patlasov 2014-04-30 8:34 ` Maxim Patlasov 2014-04-30 10:01 ` Masayoshi Mizuma 2014-04-30 10:01 ` Masayoshi Mizuma 2014-04-30 13:30 ` [PATCH v2] " Rik van Riel 2014-04-30 13:30 ` Rik van Riel 2014-04-30 13:48 ` Michal Hocko 2014-04-30 13:48 ` Michal Hocko 2014-04-30 14:26 ` Rik van Riel 2014-04-30 14:26 ` Rik van Riel 2014-04-30 14:31 ` Rik van Riel 2014-04-30 14:31 ` Rik van Riel 2014-04-30 14:49 ` Michal Hocko 2014-04-30 14:49 ` Michal Hocko 2014-04-30 14:52 ` Rik van Riel 2014-04-30 14:52 ` Rik van Riel 2014-04-30 14:41 ` [PATCH v3] " Rik van Riel 2014-04-30 14:41 ` Rik van Riel 2014-04-30 19:00 ` Andrew Morton 2014-04-30 19:00 ` Andrew Morton 2014-04-30 19:30 ` Rik van Riel 2014-04-30 19:30 ` Rik van Riel 2014-04-30 19:35 ` Andrew Morton [this message] 2014-04-30 19:35 ` Andrew Morton 2014-04-30 20:02 ` [PATCH v4] " Rik van Riel 2014-04-30 20:02 ` Rik van Riel 2014-04-30 20:13 ` Andrew Morton 2014-04-30 20:13 ` Andrew Morton 2014-04-30 20:32 ` Rik van Riel 2014-04-30 20:32 ` Rik van Riel 2014-04-30 20:42 ` [PATCH v5] " Rik van Riel 2014-04-30 20:42 ` Rik van Riel 2014-04-30 21:00 ` Andrew Morton 2014-04-30 21:00 ` Andrew Morton 2014-04-30 21:21 ` Rik van Riel 2014-04-30 21:21 ` Rik van Riel 2014-04-30 21:32 ` Andrew Morton 2014-04-30 21:32 ` Andrew Morton 2014-05-02 9:16 ` Michal Hocko 2014-05-02 9:16 ` Michal Hocko 2014-05-08 10:17 ` Masayoshi Mizuma 2014-05-08 10:17 ` Masayoshi Mizuma
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