DMARC-Filter:	OpenDMARC Filter v1.4.2 delorie.com 55NJlYL83788151
Authentication-Results:	delorie.com; dmarc=pass (p=none dis=none) header.from=cygwin.com
Authentication-Results:	delorie.com; spf=pass smtp.mailfrom=cygwin.com
DKIM-Filter:	OpenDKIM Filter v2.11.0 delorie.com 55NJlYL83788151
Authentication-Results:	delorie.com;
	dkim=pass (1024-bit key, unprotected) header.d=cygwin.com header.i=@cygwin.com header.a=rsa-sha256 header.s=default header.b=cG36AMrr
X-Recipient:	archive-cygwin AT delorie DOT com
DKIM-Filter:	OpenDKIM Filter v2.11.0 sourceware.org 10BC0384671B
DKIM-Signature:	v=1; a=rsa-sha256; c=relaxed/relaxed; d=cygwin.com;
	s=default; t=1750708053;
	bh=RH7hR7h6pAsm4TYx6+8mLObCmF3dHYZn3gwSsdUMIyQ=;
	h=Date:To:Subject:References:In-Reply-To:List-Id:List-Unsubscribe:
	List-Archive:List-Post:List-Help:List-Subscribe:From:Reply-To:Cc:
	From;
	b=cG36AMrr20+uIspNbjrustEkmK+Pb29wDpDNr/AySMIsTDnPXupGIfdqrvgwODRzT
	is9FKOwP/8Q/3r8r0qWvuZsI31uWBo3F4JbdcZIY2TIUqDY0qQhEdkQFu/D6mjZMQf
	Bia5WuQ9NBoWal6b4AjayNCuBq50euwbqG+94q4s=
X-Original-To:	cygwin AT cygwin DOT com
Delivered-To:	cygwin AT cygwin DOT com
DKIM-Filter:	OpenDKIM Filter v2.11.0 sourceware.org 2CAE43865466
Date:	Mon, 23 Jun 2025 21:46:31 +0200
To:	cygwin-patches AT cygwin DOT com, cygwin AT cygwin DOT com
Subject:	Re: symlink_native() bug with case-sensitive file-systems Re:
	[PATCH] symlink_native: allow linking to `..`
Message-ID:	<aFmvF9cWsr8UCqQ6@calimero.vinschen.de>
Mail-Followup-To:	cygwin-patches AT cygwin DOT com, cygwin AT cygwin DOT com
References:	<6058889e2ae8c9c827a8d6678f09b3b1741e2fcf DOT 1750413578 DOT git DOT johannes DOT schindelin AT gmx DOT de>
	<CAHnbEGLjsy4MZD+oqjGbd=JrX+q8an3mhT38xndEgjmTpWyOnw AT mail DOT gmail DOT com>
	<aFkPUI22HlYnYhZh AT calimero DOT vinschen DOT de>
	<CAHnbEG+7T8K50WkDN4=xBA_ir8N3M32=ZGJnYvCFSpH7UquZ=Q AT mail DOT gmail DOT com>
MIME-Version:	1.0
In-Reply-To:	<CAHnbEG+7T8K50WkDN4=xBA_ir8N3M32=ZGJnYvCFSpH7UquZ=Q@mail.gmail.com>
X-BeenThere:	cygwin AT cygwin DOT com
X-Mailman-Version:	2.1.30
List-Id:	General Cygwin discussions and problem reports <cygwin.cygwin.com>
List-Unsubscribe:	<https://cygwin.com/mailman/options/cygwin>,
	<mailto:cygwin-request AT cygwin DOT com?subject=unsubscribe>
List-Archive:	<https://cygwin.com/pipermail/cygwin/>
List-Post:	<mailto:cygwin AT cygwin DOT com>
List-Help:	<mailto:cygwin-request AT cygwin DOT com?subject=help>
List-Subscribe:	<https://cygwin.com/mailman/listinfo/cygwin>,
	<mailto:cygwin-request AT cygwin DOT com?subject=subscribe>
From:	Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	cygwin-patches AT cygwin DOT com
Cc:	Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
Errors-To:	cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com
Sender:	"Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>
X-MIME-Autoconverted:	from base64 to 8bit by delorie.com id 55NJlYL83788151

On Jun 23 20:22, Sebastian Feld wrote:
> On Mon, Jun 23, 2025 at 10:52 AM Corinna Vinschen
> <corinna-cygwin AT cygwin DOT com> wrote:
> >
> > On Jun 20 13:33, Sebastian Feld wrote:
> > > On Fri, Jun 20, 2025 at 12:03 PM Johannes Schindelin
> > > <johannes DOT schindelin AT gmx DOT de> wrote:
> > > >  winsup/cygwin/path.cc | 21 ++++++++++++++++-----
> > > >  1 file changed, 16 insertions(+), 5 deletions(-)
> > > >
> > > > diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
> > > > index 42919a7cf5..ed08398930 100644
> > > > --- a/winsup/cygwin/path.cc
> > > > +++ b/winsup/cygwin/path.cc
> > > > @@ -1855,9 +1855,18 @@ symlink_native (const char *oldpath, path_conv &win32_newpath)
> > > >        while (towupper (*++c_old) == towupper (*++c_new))
> > >
> > > 1 unrelated issue:
> > > I think this towupper() code is WRONG if the filesystem (e.g. WSL) is
> > > case-sensitive!
> >
> > The preceding comment tries to explain why we always compare case
> > insensitive.  There's a high probability that the symlink will be used
> > by native (non-Cygwin) processes which are insensitive.
> 
> OK, but this is at least bad for performance.
> 
> Some stats from a profiling tool I am working on:
> German language, multibyte locale, codepage 65001:
> Each towupper() traverses 11 functions, covering between 8002 and
> 11722 instructions, and between 260 and 469 branches, on 64bit.
> If the code could just use the per-volume case sensitive flag, then
> this could be reduced to 20-30 instructions just to do the indirect
> load (2 times) and compare.

Yes, but real life is working against you.  I would bet that most
users never touch case sensitivity settings and most scenarios
are touching case insensitive paths.

Worse, consider a case-sensitive dir:

  C:\foo\bar\baz\i_am_a_case_sensitive_dir

All path components preceeding the i_am_a_case_sensitive_dir have
to be compared case insensitive!  *Only* the i_am_a_case_sensitive_dir
and path components below that have to be compared case sensitive.

No fun.  No fun at all.

> > > How can code in cygwin.dll test whether the current path is on a
> > > case-sensitive volume, or not?
> >
> > There's a twist here.  NTFS or ReFS or other filesystems (but not FAT)
> > are usually case sensitive.  It's the OS which makes them case insensitve
> > by using a specific flag at open time, combined with a kernel registry
> > key.  So apart from FAT, the creator of a file decides if it's created
> > sensitive or insensitive, and the one searching for and opening a file
> > is deciding if the search/open is sensitive or insensitive.
> >
> > Also, we're creating the symlink via CreateSymbolicLinkW, which is
> > probably acting case insensitive anyway...
> >
> > What if the perr-dir case-sensitive
> > > feature is ON, should that be probed and handled too?
> >
> > ...unless the symlink is created in a case sensitive dir, I assume.
> >
> > Right now we don't handle case sensitive dirs in the path_conv code.  We
> > only check for the kernel registry key and the FILE_CASE_SENSITIVE_SEARCH
> > filesystem flag.
> >
> > To add the sensitive dirs to the picture, path_conv() would have to
> > check every directory on NTFS for
> > NtQueryInformationFile(FileCaseSensitiveInformation). It would then
> > set the path_conv::caseinsensitive flag accordingly.
> 
> Yikes. Does Windows cache this per-dir info somewhere?

I honestly don't know.


Corinna

-- 
Problem reports:      https://cygwin.com/problems.html
FAQ:                  https://cygwin.com/faq/
Documentation:        https://cygwin.com/docs.html
Unsubscribe info:     https://cygwin.com/ml/#unsubscribe-simple

- Raw text -