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.
