Cygwin: lock cygheap during fork

another thread may simultaneously be doing a cmalloc/cfree while the cygheap is being copied to the child. Addresses: https://cygwin.com/pipermail/cygwin/2025-September/258801.html Signed-off-by: Jeremy Drake <cygwin@jdrake.com>

Author: jeremyd2019

winsup/cygwin/fork.cc

@@ -329,6 +329,7 @@ frok::parent (volatile char * volatile stack_here)
  /* NEVER, EVER, call a function which in turn calls malloc&friends while this
  malloc lock is active! */
  __malloc_lock ();
+ cygheap->lock ();
  bool locked = true;
 
  /* Remove impersonation */
@@ -483,6 +484,7 @@ frok::parent (volatile char * volatile stack_here)
  impure, impure_beg, impure_end,
  NULL);
 
+ cygheap->unlock ();
  __malloc_unlock ();
  locked = false;
  if (!rc)
@@ -568,7 +570,10 @@ frok::parent (volatile char * volatile stack_here)
  if (fix_impersonation)
  cygheap->user.reimpersonate ();
  if (locked)
- __malloc_unlock ();
+ {
+ cygheap->unlock ();
+ __malloc_unlock ();
+ }
 
  /* Remember to de-allocate the fd table. */
  if (hchild)

winsup/cygwin/release/3.6.5

@@ -25,3 +25,7 @@ Fixes:
 
 - Fix Ctrl-O (FLUSHO) handling.
  Addresses: https://cygwin.com/pipermail/cygwin/2025-August/258717.html
+
+- Fix multi-thread safety of fork()/exec() by adding the same locking as was
+ done for spawn.
+ Addresses: https://cygwin.com/pipermail/cygwin/2025-September/258801.html