Root directory being a junction fools realpath()
Corinna Vinschen
corinna-cygwin@cygwin.com
Mon Feb 15 13:20:53 GMT 2021
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
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