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Date: Tue, 11 Jun 2013 17:26:24 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: [PATCH] Check for existence of the path before processing '..'
Message-ID: <20130611152624.GE9484@calimero.vinschen.de>
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References: <001401ce66a4$bb9029a0$32b07ce0$%fedin AT samsung DOT com> <20130611142002 DOT GA1927 AT ednor DOT casa DOT cgf DOT cx> <20130611150446 DOT GB9484 AT calimero DOT vinschen DOT de> <20130611151708 DOT GB2235 AT ednor DOT casa DOT cgf DOT cx>
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On Jun 11 11:17, Christopher Faylor wrote:
> On Tue, Jun 11, 2013 at 05:04:46PM +0200, Corinna Vinschen wrote:
> >I don't see how do this without calling path_conv, though.  You have to
> >perform the full conversion on the parent path, with symlinks and
> >everything to get the right result.
> 
> Yes, but it is a HUGE stack hit to call path_conv recursively here.

Shouldn't that be much reduced by the fact that the temporary buffers
used by path_conv are tmp_pathbuf buffers?  Originally, when we started
with 1.7, we had all those 64K buffers on the stack and thus a lot of
spurious crashes due to stack overflow.  But since using tmp_pathbuf
buffers we got rid of those.

Maybe we should contemplate the idea to raise the maximum number of
tmp_pathbuf buffers to accommodate situations we're not aware of at
this point in time.

> >However, I'm rather impressed by the low impact of this change.  I moved
> >the check_parent setting so it's only set when a slash occurs, and then
> >I made a couple of runs building coreutils.  As you know, GCC uses ..
> >paths a lot.  The performance hit is almost unnoticable:  72.3 seconds
> >without, 73.4 seconds with the patch.
> 
> If we are considering doing this, then couldn't we somehow just avoid
> eliminating "/.." until after the path is fully parsed and then collapse
> all of them in one final loop?  Also, don't we have the same problem for
> foo/./bar?  We change that to foo/bar but foo may not exist.

The problem with .. is that a path component disappears while normalizing
the path, without checking it's existence.  Therefore foo/./bar is no
problem here, because the final patch still contains foo and thus it's
existence will be checked anyway.

I may be missing something, but if we don't remove .. and . from the
path right at the start of path_conv, then we have exactly the problem
which so far kept us from doing the check:  A . or .. component spoils
the mount_table->conv_to_win32_path path, and it might (I'm *not* sure)
break the sym.check call as well.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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