This is a note to let you know that I've just added the patch titled
x86/lib: Fix overflow when counting digits
to the 6.7-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-lib-fix-overflow-when-counting-digits.patch
and it can be found in the queue-6.7 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 548ff0f18e11f7cad782e3f35e688178ec5cfb6b
Author: Colin Ian King <colin.i.king@xxxxxxxxx>
Date: Thu Nov 2 17:49:01 2023 +0000
x86/lib: Fix overflow when counting digits
[ Upstream commit a24d61c609813963aacc9f6ec8343f4fcaac7243 ]
tl;dr: The num_digits() function has a theoretical overflow issue.
But it doesn't affect any actual in-tree users. Fix it by using
a larger type for one of the local variables.
Long version:
There is an overflow in variable m in function num_digits when val
is >= 1410065408 which leads to the digit calculation loop to
iterate more times than required. This results in either more
digits being counted or in some cases (for example where val is
1932683193) the value of m eventually overflows to zero and the
while loop spins forever).
Currently the function num_digits is currently only being used for
small values of val in the SMP boot stage for digit counting on the
number of cpus and NUMA nodes, so the overflow is never encountered.
However it is useful to fix the overflow issue in case the function
is used for other purposes in the future. (The issue was discovered
while investigating the digit counting performance in various
kernel helper functions rather than any real-world use-case).
The simplest fix is to make m a long long, the overhead in
multiplication speed for a long long is very minor for small values
of val less than 10000 on modern processors. The alternative
fix is to replace the multiplication with a constant division
by 10 loop (this compiles down to an multiplication and shift)
without needing to make m a long long, but this is slightly slower
than the fix in this commit when measured on a range of x86
processors).
[ dhansen: subject and changelog tweaks ]
Fixes: 646e29a1789a ("x86: Improve the printout of the SMP bootup CPU table")
Signed-off-by: Colin Ian King <colin.i.king@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/all/20231102174901.2590325-1-colin.i.king%40gmail.com
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c
index 92cd8ecc3a2c..40b81c338ae5 100644
--- a/arch/x86/lib/misc.c
+++ b/arch/x86/lib/misc.c
@@ -8,7 +8,7 @@
*/
int num_digits(int val)
{
- int m = 10;
+ long long m = 10;
int d = 1;
if (val < 0) {
