linux-kernel - Re: [PATCH][next] ftrace: Fix -Wcast-function-type warnings on powerpc64

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Date:   Tue, 5 Oct 2021 14:35:57 -0500
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...nel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH][next] ftrace: Fix -Wcast-function-type warnings on
 powerpc64

On Tue, Oct 05, 2021 at 03:08:07PM -0400, Steven Rostedt wrote:
[..]
> Or did you not remove your patch first?

Yep; that was the problem. 

I now applied it to a clean tree and the warnings went away.

However, I'm a bit concerned about the following Jann's comments:

"the real issue here is that ftrace_func_t is defined as a fixed
type, but actually has different types depending on the architecture?
If so, it might be cleaner to define ftrace_func_t differently
depending on architecture, or something like that?"[1]

"Would it not be possible to have two function types (#define'd as the
same if ARCH_SUPPORTS_FTRACE_OPS), and then ensure that ftrace_func_t
is only used as ftrace_asm_func_t if ARCH_SUPPORTS_FTRACE_OPS?"[2]

"Essentially my idea here is to take the high-level rule "you can only
directly call ftrace_func_t-typed functions from assembly if
ARCH_SUPPORTS_FTRACE_OPS", and encode it in the type system. And then
the compiler won't complain as long as we make sure that we never cast
between the two types under ARCH_SUPPORTS_FTRACE_OPS==0."[3]

So, is this linker approach really a good solution to this problem? :)

What's the main problem with what Jann suggests?

Thanks!
--
Gustavo

[1] https://lore.kernel.org/all/CAG48ez2pOns4vF9M_4ubMJ+p9YFY29udMaH0wm8UuCwGQ4ZZAQ@mail.gmail.com/
[2] https://lore.kernel.org/all/CAG48ez04Fj=1p61KAxAQWZ3f_z073fVUr8LsQgtKA9c-kcHmDQ@mail.gmail.com/#t
[3] https://lore.kernel.org/all/CAG48ez1LoTLmHnAKFZCQFSvcb13Em6kc8y1xO8sNwyvzB=D2Lg@mail.gmail.com/