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

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Message-ID: <20210222175150.yxgw3sxxaqjqgq56@treble>
Date:   Mon, 22 Feb 2021 11:51:50 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Evgenii Shatokhin <eshatokhin@...tuozzo.com>,
        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 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

-- 
Josh