linux-kernel - Re: [PATCH perf/core ] perf-probe: Check kprobes blacklist when adding new events

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Date:	Wed, 25 Feb 2015 18:50:05 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, namhyung@...nel.org,
	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH perf/core ] perf-probe: Check kprobes blacklist when
 adding new events

Em Thu, Feb 19, 2015 at 11:31:13PM +0900, Masami Hiramatsu escreveu:
> Recent linux kernel provides a blacklist of the functions which
> can not be probed. perf probe can now check this blacklist before
> setting new events and indicate better error message for users.
> 
> Without this patch,
>   ----
>   # perf probe --add vmalloc_fault
>   Added new event:
>   Failed to write event: Invalid argument
>     Error: Failed to add events.
>   ----
> With this patch
>   ----
>   # perf probe --add vmalloc_fault
>   Added new event:
>   Warning: Skipped probing on blacklisted function: vmalloc_fault
>   ----
> 
> Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
> Reported-by: Arnaldo Carvalho de Melo <acme@...nel.org>

Ok, I really need to use a newer kernel on this RHEL7 box, a 4.0-rc one,
as when trying this I saw no difference, but probably that is because it
didn't find the kprobes blacklist, tried inserting the probe and then
failed with the exact previous message...

A follow up patch could try to improve the situation by saying something
like:

"Failed to insert the probe, maybe this is a blacklisted function and
you're running an old kernel?

Please check the function definition, it should not be marked as, for
instance:

  NOKPROBE_SYMBOL(vmalloc_fault);"

Anyway, applying your patch, clearly an improvement when the blacklist
is available.

Thanks,

- Arnaldo
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