Re: [PATCH RFC 0/4] static key support for error injection functions

[Mark Rutland]

Hi,

On Fri, May 31, 2024 at 11:33:31AM +0200, Vlastimil Babka wrote:
> Incomplete, help needed from ftrace/kprobe and bpf folks.

> - the generic error injection using kretprobes with
>   override_function_with_return is handled in patch 2. The
>   ALLOW_ERROR_INJECTION() annotation is extended so that static key
>   address can be passed, and the framework controls it when error
>   injection is enabled or disabled in debugfs for the function.
> 
> There are two more users I know of but am not familiar enough to fix up
> myself. I hope people that are more familiar can help me here.
> 
> - ftrace seems to be using override_function_with_return from
>   #define ftrace_override_function_with_return but I found no place
>   where the latter is used. I assume it might be hidden behind more
>   macro magic? But the point is if ftrace can be instructed to act like
>   an error injection, it would also have to use some form of metadata
>   (from patch 2 presumably?) to get to the static key and control it.

I don't think you've missed anything; nothing currently uses
ftrace_override_function_with_return(). I added that in commit:

  94d095ffa0e16bb7 ("ftrace: abstract DYNAMIC_FTRACE_WITH_ARGS accesses")

.. so that it was possible to do anything that was possible with
FTRACE_WITH_REGS and/or kprobes, under the expectation that we might
want to move fault injection and BPF probes over to fprobes in future,
as ftrace/fprobes is generally faster than kprobes (e.g. for
architectures that can't do KPROBES_ON_FTRACE or OPTPROBES).

That's just the mechanism for the handler to use; I'd expect whatever
registered the handler to be responsible for flipping the static key,
and I don't think anything needs to change within ftrace itself.

>   If ftrace can only observe the function being called, maybe it
>   wouldn't be wrong to just observe nothing if the static key isn't
>   enabled because nobody is doing the fault injection?

Yep, that sounds right to me.

Mark.