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# Re: [PATCH] Check for existence of the path before processing '..'

• From: Corinna Vinschen <corinna-cygwin at cygwin dot com>
• To: cygwin at cygwin dot com
• Date: Tue, 11 Jun 2013 17:04:46 +0200
• Subject: Re: [PATCH] Check for existence of the path before processing '..'
• References: <001401ce66a4\$bb9029a0\$32b07ce0\$%fedin at samsung dot com> <20130611142002 dot GA1927 at ednor dot casa dot cgf dot cx>
• Reply-to: cygwin at cygwin dot com

On Jun 11 10:20, Christopher Faylor wrote:
> On Tue, Jun 11, 2013 at 05:08:13PM +0400, Fedin Pavel wrote:
> > Hello!
> >
> > Some time ago i reported ability to access things like
> >"/usr/nonexistent/..bin". I still had this problem and i tried my hands on
> >fixing it.
> > The patch works by checking the actual existence of the path before
> >removing the last component from it. For performance reasons, only one check
> >is done for things like "../..". Because, obviously, if "/foo/bar/baz"
> >exists, then "/foo/bar" exists too. Also, the check is done only after some
> >components have been added to the path. So, for example, current directory
> >(obtained when processing relative paths), will not be checked.
> > I tried to add a similar test also to normalize_win32_path() function,
> >however this broke things like "cd /usr/src/..". For some reason, a POSIX
> >version of the path (but with reversed slashes) is passed to this routine
> >when expanding mount points, so, consequently, test for "\usr\src" using
> >GetFileType() fails.
> > I think it's ok, at least POSIX paths now behave in POSIX way. I have
> >tested against performance, there is some loss (~0.2 seconds), but only for
> >referencing '..'.
> > With this patch i am able to compile the latest version of glibc with no
> >problems.
>
> You introduce a check_parent flag which is set every time a non-slash
> character is found.  That doesn't seem right.  It seems like it should
> be set whenever you see a slash.

Indeed.  I moved setting check_parent before the while expression in
the else branch instead and it still works.

> Also you are calling path_conv recursively.  I assume that is where you
> are seeing a performance hit.

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.

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.

Corinna

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

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