avoid overflows in kernel/time.c

When the conversion factor between jiffies and milli- or microseconds is
not a single multiply or divide, as for the case of HZ == 300, we currently
do a multiply followed by a divide.  The intervening result, however, is
subject to overflows, especially since the fraction is not simplified (for
HZ == 300, we multiply by 300 and divide by 1000).

This is exposed to the user when passing a large timeout to poll(), for
example.

This patch replaces the multiply-divide with a reciprocal multiplication on
32-bit platforms.  When the input is an unsigned long, there is no portable
way to do this on 64-bit platforms there is no portable way to do this
since it requires a 128-bit intermediate result (which gcc does support on
64-bit platforms but may generate libgcc calls, e.g.  on 64-bit s390), but
since the output is a 32-bit integer in the cases affected, just simplify
the multiply-divide (*3/10 instead of *300/1000).

The reciprocal multiply used can have off-by-one errors in the upper half
of the valid output range.  This could be avoided at the expense of having
to deal with a potential 65-bit intermediate result.  Since the intent is
to avoid overflow problems and most of the other time conversions are only
semiexact, the off-by-one errors were considered an acceptable tradeoff.

At Ralf Baechle's suggestion, this version uses a Perl script to compute
the necessary constants.  We already have dependencies on Perl for kernel
compiles.  This does, however, require the Perl module Math::BigInt, which
is included in the standard Perl distribution starting with version 5.8.0.
In order to support older versions of Perl, include a table of canned
constants in the script itself, and structure the script so that
Math::BigInt isn't required if pulling values from said table.

Running the script requires that the HZ value is available from the
Makefile.  Thus, this patch also adds the Kconfig variable CONFIG_HZ to the
architectures which didn't already have it (alpha, cris, frv, h8300, m32r,
m68k, m68knommu, sparc, v850, and xtensa.) It does *not* touch the sh or
sh64 architectures, since Paul Mundt has dealt with those separately in the
sh tree.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Ralf Baechle <ralf@linux-mips.org>,
Cc: Sam Ravnborg <sam@ravnborg.org>,
Cc: Paul Mundt <lethal@linux-sh.org>,
Cc: Richard Henderson <rth@twiddle.net>,
Cc: Michael Starvik <starvik@axis.com>,
Cc: David Howells <dhowells@redhat.com>,
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>,
Cc: Hirokazu Takata <takata@linux-m32r.org>,
Cc: Geert Uytterhoeven <geert@linux-m68k.org>,
Cc: Roman Zippel <zippel@linux-m68k.org>,
Cc: William L. Irwin <sparclinux@vger.kernel.org>,
Cc: Chris Zankel <chris@zankel.net>,
Cc: H. Peter Anvin <hpa@zytor.com>,
Cc: Jan Engelhardt <jengelh@computergmbh.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

10 files changed, 45 insertions, 0 deletions

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index d9df913a464d..5b7dcd5a0e75 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -615,6 +615,11 @@ config VERBOSE_MCHECK_ON
 
           Take the default (1) unless you want more control or more info.
 
+config HZ
+        int
+        default 1200 if ALPHA_RAWHIDE
+        default 1024
+
 source "drivers/pci/Kconfig"
 source "drivers/eisa/Kconfig"
 
diff --git a/arch/cris/Kconfig b/arch/cris/Kconfig
index 27b082ac7f11..ff078e60e76d 100644
--- a/arch/cris/Kconfig
+++ b/arch/cris/Kconfig
@@ -55,6 +55,10 @@ config CRIS
         bool
         default y
 
+config HZ
+        int
+        default 100
+
 source "init/Kconfig"
 
 menu "General setup"
diff --git a/arch/frv/Kconfig b/arch/frv/Kconfig
index 96f7d70f4473..9e561ede0925 100644
--- a/arch/frv/Kconfig
+++ b/arch/frv/Kconfig
@@ -57,6 +57,10 @@ config ARCH_HAS_ILOG2_U64
         bool
         default y
 
+config HZ
+        int
+        default 1000
+
 mainmenu "Fujitsu FR-V Kernel Configuration"
 
 source "init/Kconfig"
diff --git a/arch/h8300/Kconfig b/arch/h8300/Kconfig
index b4ba4f8b505c..f69e5ea38558 100644
--- a/arch/h8300/Kconfig
+++ b/arch/h8300/Kconfig
@@ -82,6 +82,10 @@ config PCI
         bool
         default n
 
+config HZ
+        int
+        default 100
+
 source "init/Kconfig"
 
 source "arch/h8300/Kconfig.cpu"
diff --git a/arch/m32r/Kconfig b/arch/m32r/Kconfig
index fe61e00a604f..d4679ab55b96 100644
--- a/arch/m32r/Kconfig
+++ b/arch/m32r/Kconfig
@@ -38,6 +38,10 @@ config NO_DMA
 config ARCH_SUPPORTS_AOUT
         def_bool y
 
+config HZ
+        int
+        default 100
+
 source "init/Kconfig"
 
 
diff --git a/arch/m68k/Kconfig b/arch/m68k/Kconfig
index bcbf3e4ee9d4..2b0ed89cd173 100644
--- a/arch/m68k/Kconfig
+++ b/arch/m68k/Kconfig
@@ -55,6 +55,10 @@ config NO_DMA
 config ARCH_SUPPORTS_AOUT
         def_bool y
 
+config HZ
+        int
+        default 100
+
 mainmenu "Linux/68k Kernel Configuration"
 
 source "init/Kconfig"
diff --git a/arch/m68knommu/Kconfig b/arch/m68knommu/Kconfig
index 24f732342d3e..548a7b321633 100644
--- a/arch/m68knommu/Kconfig
+++ b/arch/m68knommu/Kconfig
@@ -525,6 +525,11 @@ config 4KSTACKS
           running more threads on a system and also reduces the pressure
           on the VM subsystem for higher order allocations.
 
+config HZ
+        int
+        default 1000 if CLEOPATRA
+        default 100
+
 comment "RAM configuration"
 
 config RAMBASE
diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index 08821b078ecc..7c674a3503b6 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -30,6 +30,10 @@ config OF
 config ARCH_SUPPORTS_AOUT
         def_bool y
 
+config HZ
+        int
+        default 100
+
 source "init/Kconfig"
 
 menu "General machine setup"
diff --git a/arch/v850/Kconfig b/arch/v850/Kconfig
index 225f30d7cb32..7b6d3716efca 100644
--- a/arch/v850/Kconfig
+++ b/arch/v850/Kconfig
@@ -215,6 +215,13 @@ menu "Processor type and features"
           bool
           default !V850E_CACHE && !V850E2_CACHE
 
+   # HZ depends on the platform
+   config HZ
+          int
+          default 24  if V850E_SIM || V850E2_SIM85E2
+          default 122 if V850E2_FPGA85E2C
+          default 100
+
    #### Misc config
 
    config ROM_KERNEL
diff --git a/arch/xtensa/Kconfig b/arch/xtensa/Kconfig
index 5d5546ce88fe..fd36764d7fb7 100644
--- a/arch/xtensa/Kconfig
+++ b/arch/xtensa/Kconfig
@@ -49,6 +49,10 @@ config ARCH_HAS_ILOG2_U64
 config NO_IOPORT
         def_bool y
 
+config HZ
+        int
+        default 100
+
 source "init/Kconfig"
 
 menu "Processor type and features"