On Monday, October 28, 2019 10:32:26 PM CET Kangjie Lu wrote:
>
> > 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?
Not answered.
> > 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.
OK, that should have been mentioned in the changelog.
> >
> >> 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.
OK, but does it actually make a practical difference?
> >
> >> 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 };
Why don't you change obj to a pointer instead?
> >> struct acpi_processor_throttling *throttling;
> >>
> >> status = acpi_evaluate_object(pr->handle, "_PTC", NULL, &buffer);
> >>
