linux-kernel - Re: [tip:perf/urgent] perf symbols: Handle /proc/sys/kernel/kptr

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Date:	Thu, 26 May 2011 18:20:40 -0300
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org, paulus@...ba.org, hpa@...or.com,
	mingo@...hat.com, eranian@...gle.com, tzanussi@...il.com,
	penberg@...helsinki.fi, torvalds@...ux-foundation.org,
	efault@....de, peterz@...radead.org, davej@...hat.com,
	davem@...emloft.net, fweisbec@...il.com, kees.cook@...onical.com,
	tglx@...utronix.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/urgent] perf symbols: Handle
	/proc/sys/kernel/kptr_restrict

Em Thu, May 26, 2011 at 09:39:46PM +0200, Ingo Molnar escreveu:
> 
> * tip-bot for Arnaldo Carvalho de Melo <acme@...hat.com> wrote:
> 
> > Commit-ID:  ec80fde746e3ccf93895d25ae1a7071c9af52585
> > Gitweb:     http://git.kernel.org/tip/ec80fde746e3ccf93895d25ae1a7071c9af52585
> > Author:     Arnaldo Carvalho de Melo <acme@...hat.com>
> > AuthorDate: Thu, 26 May 2011 09:53:51 -0300
> > Committer:  Arnaldo Carvalho de Melo <acme@...hat.com>
> > CommitDate: Thu, 26 May 2011 11:15:25 -0300
> > 
> > perf symbols: Handle /proc/sys/kernel/kptr_restrict
> 
> Ok, things are much better now with kptr_restrict turned on - but i 
> still see a few rough edges in practice.
> 
> For example 'perf top' says:
> 
>   aldebaran:~/linux> perf top
>   [kernel.kallsyms] with build id 122214021a666675f6e5ff97d70a85ce7139c0e7 not found, continuing without symbols
>   The (null) file can't be used

please wait a bit more, I completely forgot testing 'perf top' :-\
 
> Now we've confused the user, havent we? :-)
> 
> Also, if i run 'perf top' with the proper vmlinux in the cwd, i do 
> not get any warning messages - despite both module symbols not being 
> available and potetial relocations not being considered.

Right, it has to provide similar warning.
 
> Secondly, even though i have the proper 'vmlinux' in cwd, i get half 
> a page long warnings on perf record warning me about the vmlinux:
> 
>   WARNING: Kernel address maps (/proc/{kallsyms,modules}) are restricted, check /proc/sys/kernel/kptr_restrict.
> 
>   Samples in kernel functions may not be resolved if a suitable 
>   vmlinux file is not found in the buildid cache or in the vmlinux 
>   path.
> 
>   ...
> 
> But a vmlinux file *is* in the cwd.
> 
> One detail i noticed in the patch:
> 
> >  	symbol__init();
> >  
> > +	if (symbol_conf.kptr_restrict)
> > +		pr_warning("WARNING: Kernel address maps "
> > +			   "(/proc/{kallsyms,modules}) are restricted, "
> > +			   "check /proc/sys/kernel/kptr_restrict.\n\n"
> > +			   "Samples in kernel functions may not be resolved "
> > +			   "if a suitable vmlinux file is not found in the "
> > +			   "buildid cache or in the vmlinux path.\n\n"
> > +			   "Samples in kernel modules won't be resolved "
> > +			   "at all.\n\n"
> > +			   "If some relocation was applied (e.g. kexec) "
> > +			   "symbols may be misresolved even with a suitable "
> > +			   "vmlinux or kallsyms file.\n\n");
> 
> 
> messages broken mid-line just to make it look better is really bad 
> and can break git grep searches for the string as seen on the screen, 
> such as:
> 
>    git grep="are restricted, check"
> 
> The solution i use in such cases is to deviate in a common-sense way 
> from the standard coding style:
> 
> 	if (symbol_conf.kptr_restrict) {
> 		pr_warning(
> "WARNING: Kernel address maps (/proc/{kallsyms,modules}) are restricted, check /proc/sys/kernel/kptr_restrict.\n\n"
> "Samples in kernel functions may not be resolved if a suitable vmlinux file is not found in the buildid cache or in the vmlinux path.\n\n"
> "Samples in kernel modules won't be resolved at all.\n\n"
> "If some relocation was applied (e.g. kexec) symbols may be misresolved even with a suitable vmlinux or kallsyms file.\n\n");
> 	}
> 
> This has the big advantage that you can check how the *user* will see 
> this message.
> 
> Had you done this you'd have immediately noticed that the individual 
> lines are *way* too large to fit on even large terminals. Please 
> break such messages in a much shorter fashion - we want the error 
> messages we output to be readable even on relatively small (80col) 
> terminals.

But it would reflow, no?
 
> Thanks,
> 
> 	Ingo
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