Re: [PATCH v3 03/17] mm: add generic __va_function and __pa_function macros

On Wed, Mar 24, 2021 at 08:54:18AM -0700, Sami Tolvanen wrote:

On Wed, Mar 24, 2021 at 12:14 AM Christoph Hellwig [off-list ref] wrote:

quoted

On Tue, Mar 23, 2021 at 01:39:32PM -0700, Sami Tolvanen wrote:

quoted

With CONFIG_CFI_CLANG, the compiler replaces function addresses
in instrumented C code with jump table addresses. This means that
__pa_symbol(function) returns the physical address of the jump table
entry instead of the actual function, which may not work as the jump
table code will immediately jump to a virtual address that may not be
mapped.

To avoid this address space confusion, this change adds generic
definitions for __va_function and __pa_function, which architectures
that support CFI can override. The typical implementation of the
__va_function macro would use inline assembly to take the function
address, which avoids compiler instrumentation.

I think these helper are sensible, but shouldn't they have somewhat
less arcane names and proper documentation?

Good point, I'll add comments in the next version. I thought
__pa_function would be a fairly straightforward replacement for
__pa_symbol, but I'm fine with renaming these. Any suggestions for
less arcane names?

I think dropping 'nocfi' into the name would be clear enough. I think
that given the usual fun with {symbol,module,virt}->phys conversions
it's not worth having the __pa_* form, and we can leave the phys
conversion to the caller that knows where the function lives.

How about we just add `function_nocfi()` ?

Callers can then do `__pa_symbol(function_nocfi(foo))` and similar.

Thanks,
Mark.