> On Oct 28, 2019, at 4:51 PM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
>
> On Monday, October 28, 2019 7:31:14 PM CET Kangjie Lu wrote:
>> "obj" is a local variable. Elements are deep-copied from external
>> package to obj and security-checked. The original code is
>> seemingly fine; however, compilers optimize the deep copies into
>> shallow copies, introducing potential race conditions. For
>> example, the checks for type and length may be bypassed.
>
> How exactly?
>
> What compiler(s) do such optimizations in this particular case?
Tested on LLVM. The deep copy is indeed optimized into a shallow copy at optimization level O2.
>
>> The fix tells compilers to not optimize the deep copy by inserting
>> "volatile".
>
> Have you actually analyzed the object code produced by the compiler with and
> without the volatile to determine whether or not it has an effect as expected
> on code generation?
Yes, with “volatile", the deep copy is preserved, and “obj” is created as a local variable.
>
>> Signed-off-by: Kangjie Lu <kjlu@xxxxxxx>
>> ---
>> drivers/acpi/processor_throttling.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/acpi/processor_throttling.c b/drivers/acpi/processor_throttling.c
>> index 532a1ae3595a..6f4d86f8a9ce 100644
>> --- a/drivers/acpi/processor_throttling.c
>> +++ b/drivers/acpi/processor_throttling.c
>> @@ -413,7 +413,7 @@ static int acpi_processor_get_throttling_control(struct acpi_processor *pr)
>> acpi_status status = 0;
>> struct acpi_buffer buffer = { ACPI_ALLOCATE_BUFFER, NULL };
>> union acpi_object *ptc = NULL;
>> - union acpi_object obj = { 0 };
>> + volatile union acpi_object obj = { 0 };
>> struct acpi_processor_throttling *throttling;
>>
>> status = acpi_evaluate_object(pr->handle, "_PTC", NULL, &buffer);
>>
>
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