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

From: Masami Hiramatsu <mhiramat@kernel.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Evgenii Shatokhin <eshatokhin@virtuozzo.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Kristen Carlson Accardi <kristen@linux.intel.com>,
	live-patching@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>,
	linux-kernel@vger.kernel.org,
	Konstantin Khorenko <khorenko@virtuozzo.com>
Subject: Re: 'perf probe' and symbols from .text.<something>
Date: Tue, 23 Feb 2021 10:23:31 +0900	[thread overview]
Message-ID: <20210223102331.147d62de88886a75013c10e0@kernel.org> (raw)
In-Reply-To: <20210222175150.yxgw3sxxaqjqgq56@treble>

On Mon, 22 Feb 2021 11:51:50 -0600
Josh Poimboeuf <jpoimboe@redhat.com> wrote:

> On Tue, Feb 23, 2021 at 12:05:08AM +0900, Masami Hiramatsu wrote:
> > > 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.
> > 
> > Hmm, if the FGKASLAR really randomizes the symbol address, perf-probe
> > should give up "_text-relative" probe for that kernel, and must fallback
> > to the "symbol-based" probe. (Are there any way to check the FGKASLR is on?)
> > The problem of "symbol-based" probe is that local (static) symbols
> > may share a same name sometimes. In that case, it can not find correct
> > symbol. (Maybe I can find a candidate from its size.)
> > Anyway, sometimes the security and usability are trade-off.
> 
> We had a similar issue with FGKASLR and live patching.  The proposed
> solution is a new linker flag which eliminates duplicates: -z
> unique-symbol.
> 
> https://sourceware.org/bugzilla/show_bug.cgi?id=26391

Interesting, but it might not be enough for perf-probe.
Since the perf-probe has to handle both dwarf and elf, both must be
changed. I think the problem is that the dwarf is generated while
compiling, but this -z seems converting elf symbols in linkage.
As far as I can see, this appends ".COUNT" suffix to the non-unique
symbols in the linkage phase. Is that also applied to dwarf too?

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>