Root directory being a junction fools realpath()

[Corinna Vinschen]

On Feb 15 13:25, Corinna Vinschen via Cygwin-developers wrote:
> On Feb 14 12:56, David Macek via Cygwin-developers wrote:
> > Greetings.
> > 
> > I think I found a way to fool Cygwin into misplacing its root
> > directory. Given recent Cygwin with GCC and a simple test program:
> > 
> > ```
> > #include <stdio.h>
> > #include <limits.h>
> > #include <stdlib.h>
> > int main() {
> > char real[PATH_MAX];
> > realpath("/", real);
> > printf("/ -> %s\n", real);
> > return 0;
> > }
> > ```
> > 
> > ... compiled with:
> > 
> > $ gcc -D_DEFAULT_SOURCE -o new.exe -Wall -Wextra -std=c11 new.c
> > 
> > ... and my set-up with a junction (`mklink /j link target`) realpath()
> > result is dependent on which path I use to invoke the Cygwin root
> > process:
> > 
> > > dir C:\ | findstr cygwin64
> > 10. 02. 2021  12:13    <DIR>          cygwin64
> > 
> > > dir D:\ | findstr cygwin64
> > 14. 02. 2021  12:19    <JUNCTION>     cygwin64 [C:\cygwin64]
> > 
> > > C:\cygwin64\bin\bash -lc /cygdrive/w/new
> > / -> /
> > 
> > > D:\cygwin64\bin\bash -lc /cygdrive/w/new
> > / -> /cygdrive/c/cygwin64
> 
> This isn't realpath's fault.  You're circumventing the mount point
> handling which is automated in terms of /, depending on the path
> returned from GetModuleFileNameW for the Cygwin DLL.  Since you're
> 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.  Thus the result
> you see above.

A potential workaround is this:

diff --git a/winsup/cygwin/cygheap.cc b/winsup/cygwin/cygheap.cc
index 3a32c352d4e9..01b49468e804 100644
--- a/winsup/cygwin/cygheap.cc
+++ b/winsup/cygwin/cygheap.cc
@@ -144,6 +144,19 @@ init_cygheap::init_installation_root ()
     api_fatal ("Can't initialize Cygwin installation root dir.\n"
 	       "GetModuleFileNameW(%p, %p, %u), %E",
 	       cygwin_hmodule, installation_root_buf, PATH_MAX);
+
+  /* We don't care if fetching the final pathname fails, it's non-fatal and
+     the path returned by GetModuleFileNameW is still valid. */
+  HANDLE h;
+  h = CreateFileW (installation_root_buf, GENERIC_READ, FILE_SHARE_VALID_FLAGS,
+		   &sec_none, OPEN_EXISTING, 0, 0);
+  if (h != INVALID_HANDLE_VALUE)
+    {
+      GetFinalPathNameByHandleW (h, installation_root_buf, PATH_MAX,
+				 FILE_NAME_NORMALIZED);
+      CloseHandle (h);
+    }
+
   PWCHAR p = installation_root_buf;
   if (wcsncasecmp (p, L"\\\\", 2))	/* Normal drive letter path */
     {

I just wonder if such a bordercase is worth the extra performance hit
for each Cygwin invocation.


Corinna