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Date: Tue, 15 Jul 2008 11:40:38 -0700
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To: Amir Hardon <amirh@metacafe.com>
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Subject: Re: [PHP-DEV] lstat call on each directory level
From: rasmus@lerdorf.com (Rasmus Lerdorf)

Amir Hardon wrote:
> I've noticed a weird behavior when doing file access from PHP:
> PHP seems to make an lstat call on each of the parent directories of the
> accessed file, for example see this script:
> 
> <?php
> $fp=fopen("/var/www/metacafe/test","r");
> fclose($fp);
> ?>
> 
> When running with strace -e lstat I see this:
> lstat("/var", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
> lstat("/var/www", {st_mode=S_IFDIR|0755, st_size=12288, ...}) = 0
> lstat("/var/www/metacafe", {st_mode=S_IFDIR|0755, st_size=4096, ...}) =
> 0
> lstat("/var/www/metacafe/test", 0x7fbfff9b10) = -1 ENOENT (No such file
> or directory)
> 
> Measuring total syscalls time for an apache process on a production
> server, I found out
> that ~33% of the time it spends in syscalls is spent on lstat.
> 
> I did a pretty deep web search on the issue and came out with nothing.
> I'll also note that I did a small experiment - moving our root portal
> folder to /,
> this gave an amazing performance improvement!
> 
> So my questions are:
> What is the reason for doing these lstat calls?
> How can it be disabled? if not by configuration, maybe by patching php
> (can you direct me to where is this being done in php's source?)

That's a realpath() call and it should be getting cached by the realpath 
cache, so if you are seeing these on every request, try increasing your 
realpath_cache size in your .ini.  Without checking the realpath, you 
would be able to circumvent open_basedir checking really easily with a 
symlink.

-Rasmus