linux-kernel - Re: 'perf probe' and symbols from .text.<something>

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Message-ID: <20210218193508.mw6ugmh6bjlplbwe@treble>
Date:   Thu, 18 Feb 2021 13:35:08 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Evgenii Shatokhin <eshatokhin@...tuozzo.com>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        live-patching@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Konstantin Khorenko <khorenko@...tuozzo.com>
Subject: Re: 'perf probe' and symbols from .text.<something>

On Thu, Feb 18, 2021 at 08:09:17PM +0300, Evgenii Shatokhin wrote:
> Is there a way to allow probing of functions in .text.<something> ?
> 
> Of course, one could place probes using absolute addresses of the functions
> but that would be less convenient.
> 
> This also affects many livepatch modules where the kernel code can be
> compiled with -ffunction-sections and each function may end up in a separate
> section .text.<function_name>. 'perf probe' cannot be used there, except
> with the absolute addresses.
> 
> Moreover, if FGKASLR patches are merged (https://lwn.net/Articles/832434/)
> and the kernel is built with FGKASLR enabled, -ffunction-sections will be
> used too. 'perf probe' will be unable to see the kernel functions then.

A hack fix like the below would probably work, but as you pointed out,
FGKASLR is going to be a problem.  I suspect the proper fix is for perf
to learn how to deal with multiple executable ELF sections.

diff --git a/scripts/module.lds.S b/scripts/module.lds.S
index 69b9b71a6a47..0c522a87f6ce 100644
--- a/scripts/module.lds.S
+++ b/scripts/module.lds.S
@@ -4,6 +4,10 @@
  * combine them automatically.
  */
 SECTIONS {
+	.text : {
+		*(.text)
+		*(.text.*)
+	}
 	/DISCARD/ : {
 		*(.discard)
 		*(.discard.*)