Re: [PATCH v2 05/17] workqueue: use WARN_ON_FUNCTION_MISMATCH

[Sami Tolvanen]

On Thu, Mar 18, 2021 at 11:50 AM Nick Desaulniers
<ndesaulniers@xxxxxxxxxx> wrote:
>
> On Thu, Mar 18, 2021 at 10:11 AM Sami Tolvanen <samitolvanen@xxxxxxxxxx> wrote:
> >
> > With CONFIG_CFI_CLANG, a callback function passed to
> > __queue_delayed_work from a module points to a jump table entry
> > defined in the module instead of the one used in the core kernel,
> > which breaks function address equality in this check:
> >
> >   WARN_ON_ONCE(timer->function != delayed_work_timer_fn);
> >
> > Use WARN_ON_FUNCTION_MISMATCH() instead to disable the warning
> > when CFI and modules are both enabled.
>
> Does __cficanonical help with such comparisons? Or would that be a
> very invasive change, if the concern was to try to keep these checks
> in place for CONFIG_CFI_CLANG?

The last time I checked, Clang ignored the __cficanonical attribute in
header files, which means it would still generate a local jump table
entry in each module for such functions, and the comparison here would
fail. We could avoid the issue by using __cficanonical for the
callback function *and* using __va_function() when we take the
function address in modules, but that feels way too invasive for this
particular use case.

Sami