[PATCH] Cygwin: cygwin_conv_path: don't write to `to` before size is validated.

[Jeremy Drake via Cygwin-patches]

In the CCP_POSIX_TO_WIN_W path, when `from` is a device,
cygwin_conv_path would attempt to write to the `to` buffer before the
validation of the `size`.  This resulted in an EFAULT error in the
common use-case of passing `to` as NULL and `size` as 0 to get the
required size of `to` for the conversion (as used in
cygwin_create_path).  Instead, set a boolean and write to `to`
after validation.

Fixes: 43f65cdd7dae ("* Makefile.in (DLL_OFILES): Add fhandler_procsys.o.")
Addresses: https://cygwin.com/pipermail/cygwin/2025-April/258068.html
Signed-off-by: Jeremy Drake <cyg...@jdrake.com>
---
 winsup/cygwin/path.cc       | 5 ++++-
 winsup/cygwin/release/3.6.2 | 3 +++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/path.cc b/winsup/cygwin/path.cc
index 7a08e978ad..d26f99ee7f 100644
--- a/winsup/cygwin/path.cc
+++ b/winsup/cygwin/path.cc
@@ -3911,6 +3911,7 @@ cygwin_conv_path (cygwin_conv_path_t what, const void 
*from, void *to,
   int how = what & CCP_CONVFLAGS_MASK;
   what &= CCP_CONVTYPE_MASK;
   int ret = -1;
+  bool prependglobalroot = false;

   __try
     {
@@ -4019,7 +4020,7 @@ cygwin_conv_path (cygwin_conv_path_t what, const void 
*from, void *to,
            {
              /* Device name points to somewhere else in the NT namespace.
                 Use GLOBALROOT prefix to convert to Win32 path. */
-             to = (void *) wcpcpy ((wchar_t *) to, ro_u_globalroot.Buffer);
+             prependglobalroot = true;
              lsiz += ro_u_globalroot.Length / sizeof (WCHAR);
            }
          /* TODO: Same ".\\" band-aid as in CCP_POSIX_TO_WIN_A case. */
@@ -4075,6 +4076,8 @@ cygwin_conv_path (cygwin_conv_path_t what, const void 
*from, void *to,
          stpcpy ((char *) to, buf);
          break;
        case CCP_POSIX_TO_WIN_W:
+         if (prependglobalroot)
+           to = (void *) wcpcpy ((PWCHAR) to, ro_u_globalroot.Buffer);
          wcpcpy ((PWCHAR) to, path);
          break;
        }
diff --git a/winsup/cygwin/release/3.6.2 b/winsup/cygwin/release/3.6.2
index bceabcab34..de6eae13fc 100644
--- a/winsup/cygwin/release/3.6.2
+++ b/winsup/cygwin/release/3.6.2
@@ -13,3 +13,6 @@ Fixes:

 - Fix setting DOS attributes on devices.
   Addresse: https://cygwin.com/pipermail/cygwin/2025-April/257940.html
+
+- Fix cygwin_conv_path writing to 'to' pointer before size is checked.
+  Addresses: https://cygwin.com/pipermail/cygwin/2025-April/258068.html
-- 
2.49.0.windows.1