On Thu, Mar 18, 2021 at 11:50 AM Nick Desaulniers
[off-list ref] wrote:

On Thu, Mar 18, 2021 at 10:11 AM Sami Tolvanen [off-list ref] wrote:

quoted

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

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