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From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: /proc/pid/fd/ shows strange mode when executed via sudo.
Date: Fri, 18 May 2012 22:23:02 +0100
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Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>,
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To: Linus Torvalds <torvalds@linux-foundation.org>
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On Fri, May 18, 2012 at 01:49:18PM -0700, Linus Torvalds wrote:
> I think you're optimizing the wrong case, and adding code to do so.
> 
> The ENOENT case never happens in practice - there's no sane situation
> where you'd look up a non-existend /proc/xyz/fd/X file.

I would agree if we only called that on lookup.  We also do that on
readdir(), for every descriptor in range 0..files->max_fds-1.  So
if you have sufficiently sparse set of descriptors, it will be
called a _lot_.  Moreover, that's the usual path for calling it,
exactly because we do getdents before trying to open/lstat/anything
else.