Re: [PATCH] perf/x86: Fix integer overflow when left shifting an integer more than 32 bits

[Colin Ian King]

On 20/04/2021 16:31, Peter Zijlstra wrote:
> On Tue, Apr 20, 2021 at 05:03:03PM +0200, Peter Zijlstra wrote:
>> On Tue, Apr 20, 2021 at 03:29:07PM +0100, Colin King wrote:
>>> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
>>>
>>> The 64 bit value read from MSR_ARCH_PERFMON_FIXED_CTR_CTRL is being
>>> bit-wise masked with the value (0x03 << i*4). However, the shifted value
>>> is evaluated using 32 bit arithmetic, so will overflow when i > 8.
>>> Fix this by making 0x03 a ULL so that the shift is performed using
>>> 64 bit arithmetic.
>>>
>>> Addresses-Coverity: ("Unintentional integer overflow")
>>
>> Strange tag that, also inaccurate, wide shifts are UB and don't behave
>> consistently.
>>
>> As is, we've not had hardware with that many fixed counters, but yes,
>> worth fixing I suppose.
> 
> Patch now reads:
> 
> ---
> Subject: perf/x86: Allow for 8<num_fixed_counters<16
> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> Date: Tue, 20 Apr 2021 15:29:07 +0100
> 
> From: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> 
> The 64 bit value read from MSR_ARCH_PERFMON_FIXED_CTR_CTRL is being
> bit-wise masked with the value (0x03 << i*4). However, the shifted value
> is evaluated using 32 bit arithmetic, so will UB when i > 8. Fix this
> by making 0x03 a ULL so that the shift is performed using 64 bit
> arithmetic.
> 
> This makes the arithmetic internally consistent and preparers for the
> day when hardware provides 8<num_fixed_counters<16.

Yep, that's good. Thanks.

> 
> Signed-off-by: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Link: https://lkml.kernel.org/r/20210420142907.382417-1-colin.king@xxxxxxxxxxxxx
> ---
>  arch/x86/events/core.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -261,7 +261,7 @@ static bool check_hw_exists(void)
>  		for (i = 0; i < x86_pmu.num_counters_fixed; i++) {
>  			if (fixed_counter_disabled(i))
>  				continue;
> -			if (val & (0x03 << i*4)) {
> +			if (val & (0x03ULL << i*4)) {
>  				bios_fail = 1;
>  				val_fail = val;
>  				reg_fail = reg;
>