[newlib-cygwin/main] Cygwin: cygheap: fix fork error after heap has grown

[Corinna Vinschen <corinna@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=a14a0e542d3aa8e53d189c906205c92ad044f750

commit a14a0e542d3aa8e53d189c906205c92ad044f750
Author:     David McFarland <corngood@gmail.com>
AuthorDate: Mon Apr 17 21:41:55 2023 -0300
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Tue Apr 18 10:06:53 2023 +0200

    Cygwin: cygheap: fix fork error after heap has grown
    
    2f9b8ff0 introduced a problem where forks would sometimes fail with:
    
    child_copy: cygheap read copy failed, 0x0..0x80044C750, done 0, windows pid 14032, Win32 error 299
    
    When cygheap_max was > CYGHEAP_STORAGE_INITIAL, commit_size would be set to
    allocsize(cygheap_max), which is an address, not a size.  VirtualAlloc would be
    called to commit commit_size bytes, which would fail, and then child_copy would
    be called with zero as the base address.
    
    Fixes: 2f9b8ff00cce ("Cygwin: decouple cygheap from Cygwin DLL")
    Signed-off-by: David McFarland <corngood@gmail.com>

Diff:
---
 winsup/cygwin/mm/cygheap.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/mm/cygheap.cc b/winsup/cygwin/mm/cygheap.cc
index 6a20c159ab44..a20ee5972aed 100644
--- a/winsup/cygwin/mm/cygheap.cc
+++ b/winsup/cygwin/mm/cygheap.cc
@@ -87,7 +87,8 @@ cygheap_fixup_in_child (bool execed)
   SIZE_T commit_size = CYGHEAP_STORAGE_INITIAL - CYGHEAP_STORAGE_LOW;
 
   if (child_proc_info->cygheap_max > (void *) CYGHEAP_STORAGE_INITIAL)
-    commit_size = allocsize (child_proc_info->cygheap_max);
+    commit_size = allocsize ((char *) child_proc_info->cygheap_max
+                   - CYGHEAP_STORAGE_LOW);
   cygheap = (init_cygheap *) VirtualAlloc ((LPVOID) CYGHEAP_STORAGE_LOW,
 					   CYGHEAP_STORAGE_HIGH
 					   - CYGHEAP_STORAGE_LOW,