4.19-stable review patch. If anyone has any objections, please let me know.
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From: Colin Ian King colin.i.king@gmail.com
[ 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@gmail.com Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Link: https://lore.kernel.org/all/20231102174901.2590325-1-colin.i.king%40gmail.co... Signed-off-by: Sasha Levin sashal@kernel.org --- arch/x86/lib/misc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/lib/misc.c b/arch/x86/lib/misc.c index a018ec4fba53..c97be9a1430a 100644 --- a/arch/x86/lib/misc.c +++ b/arch/x86/lib/misc.c @@ -6,7 +6,7 @@ */ int num_digits(int val) { - int m = 10; + long long m = 10; int d = 1;
if (val < 0) {