Unable to execute Cygwin application within UDF format

[Takashi Yano]

On Fri, 27 May 2022 18:18:49 +0900
Takashi Yano wrote:
> On Fri, 27 May 2022 16:47:09 +0800
> Sam Lin wrote:
> > I have also successfully executed that is to move cygwin1.dll to the root
> > directory to execute.
> > 
> > Will cygwin1.dll be supported to execute in UDF format in the future?
> 
> As I wrote earlier, it doesn't matter that the file system
> is UDF. cygwin1.dll 3.2.0 and later seems to cause problems
> if it is placed in the root directory.
> 
> Even with UDF format, the problem does not occur if hello.exe
> and cygwin1.dll are placed in any directory other than root
> directory.

I looked into this problem and found the problem occurs after
the commit:

commit 6d898f43fc87a7bf3ab100d75538e381bd86f657
Author: Corinna Vinschen <corinna@vinschen.de>
Date:   Fri Feb 19 18:15:58 2021 +0100

    Cygwin: realpath: fix cygwin installation dir being access via junction
    
    Consider this case:
    
    - Cygwin installed in C:\cygwin64
    
    - mklink /j D:\cygwin64 C:\cygwin64
    
    - create testcase calling
    
        realpath("/", result);
        printf ("%s\n", result);
    
    - start cmd
    
        >C:\cygwin64\bin\bash -lc <path-to-testcase>
        /
        >D\cygwin64\bin\bash -lc <path-to-testcase>
        /cygdrive/c/cygwin64
    
    This scenario circumventing the mount point handling which is automated
    in terms of /, depending on the path returned from GetModuleFileNameW
    for the Cygwin DLL.  When calling D:\cygwin64\bin\bash the dir returned
    from GetModuleFileNameW is D:\cygwin64\bin, thus root is D:\cygwin64.
    
    However, junctions are treated as symlinks in Cygwin which explains why
    the path gets converted to a cygdrive path.
    
    Fix this by calling GetFinalPathNameByHandleW on the result from
    GetModuleFileNameW to get the correct root path, even if accessed via
    a junction point.
    
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

And also found the following patch fixes the issue.

diff --git a/winsup/cygwin/cygheap.cc b/winsup/cygwin/cygheap.cc
index 01b49468e..c4031b919 100644
--- a/winsup/cygwin/cygheap.cc
+++ b/winsup/cygwin/cygheap.cc
@@ -183,6 +183,8 @@ init_cygheap::init_installation_root ()
 	  if (p)
 	    p = wcschr (p + 1, L'\\');  /* Skip share name */
 	}
+      else
+	p = installation_root_buf + 4; /* 4 is the length of "\\\\?\\" */
     }
   installation_root_buf[1] = L'?';
   RtlInitEmptyUnicodeString (&installation_key, installation_key_buf,

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>