linux-kernel - Re: [PATCH] perf, tool, record: Fix the header generation for pipe

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Date:	Mon, 22 Aug 2011 17:51:06 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jiri Olsa <jolsa@...hat.com>
Cc:	acme@...hat.com, a.p.zijlstra@...llo.nl, mingo@...e.hu,
	paulus@...ba.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf, tool, record: Fix the header generation for pipe

Le lundi 22 août 2011 à 16:52 +0200, Jiri Olsa a écrit :

> - fd is always file.. the descriptor, which might be pipe, is in output_fd variable
>   but, maybe the die call is not necessary.. this should not fail
> 
> - the record_file function is called only on debugfs or procfs files:
> 	events/header_page
> 	events/header_event
> 	events/**/format
> 	printk_formats
> 	/proc/kallsyms
> 
> so I think I need to read the whole file as in current code.
> 

You read the file twice. Is it the right fix ?

Once to compute the length to be able to write the output header, once
to process the content.

Are you sure length cannot change between the two phases ?

/proc/kallsyms can definitely change when a module is loaded.

Usually, when input "file" is not seekable or procfs based (fstat()
returns st_size=0), we must load/cache it in memory (or using a
temporary file) to get a consistent view of it.

This is what is done by following commands :

cat /proc/kallsyms | less
less /proc/kallsyms

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