linux-kernel - Re: [PATCH -tip perf/probes 00/10] x86 insn decoder bugfixes

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Message-ID: <20091103072439.GA19928@elte.hu>
Date:	Tue, 3 Nov 2009 08:24:39 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Roland McGrath <roland@...hat.com>
Cc:	Masami Hiramatsu <mhiramat@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jim Keniston <jkenisto@...ibm.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Jason Baron <jbaron@...hat.com>,
	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	systemtap <systemtap@...rces.redhat.com>,
	DLE <dle-develop@...ts.sourceforge.net>
Subject: Re: [PATCH -tip perf/probes 00/10] x86 insn decoder bugfixes

* Roland McGrath <roland@...hat.com> wrote:

> > Thirdly, i think we should expose the build-id of the kernel and the 
> > path to the vmlinux (and modules) via /proc/build-id or so. That way 
> > tooling can find the vmlinux file and can validate that it matches 
> > the kernel's signature. (maybe include a file date as well - that's 
> > a faster check than a full build-id checksum, especially for large 
> > kernels)
> 
> You seem to be confused about what build IDs are.  There is no 
> "checksum validation".  Once the bits are stored there is no 
> calculation ever done again, only exact comparison with an expected 
> build ID bitstring.  The size of an object file is immaterial.
> 
> As Frank mentioned, the kernel's and modules' allocated ELF notes (and 
> thus build IDs) are already exposed in /sys.  Tools like "eu-unstrip 
> -nk" use this information today.

Ah, i didnt realize we link with --build-id already, unconditonally, 
since v2.6.23 (if ld supports it):

 |
 | From 18991197b4b588255ccabf472ebc84db7b66a19c Mon Sep 17 00:00:00 2001
 | From: Roland McGrath <roland@...hat.com>
 | Date: Thu, 19 Jul 2007 01:48:40 -0700
 | Subject: [PATCH] Use --build-id ld option
 |
 | This change passes the --build-id when linking the kernel and when 
 | linking modules, if ld supports it.  This is a new GNU ld option that 
 | synthesizes an ELF note section inside the read-only data.  The note in 
 | this section contains unique identifying bits called the "build ID", 
 | which are generated so as to be different for any two linked ELF files 
 | that aren't identical.
 |

So we have an SHA1 build-id already on the vmlinux and on modules, and 
it's exposed in /sys/*/*/notes. Just have to make use of it in 
tools/perf too.

The other useful addition i mentioned isnt implemented yet: to emit an 
ELF note of the absolute path of the output directory the kernel was 
built in as well. This information is not available right now, and it 
would be a place to look in to search for the vmlinux and the modules.

What do you think? Also, if we do this, is there a standard way to name 
it , or should i just pick a suitably new, Linux-specific name for that?

Thanks,

	Ingo
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