[PATCH] Cygwin: cygheap: fix fork error after heap has grown

[David McFarland]

David McFarland <corngood@gmail.com> writes:

> 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.
> ---
>  winsup/cygwin/mm/cygheap.cc | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/winsup/cygwin/mm/cygheap.cc b/winsup/cygwin/mm/cygheap.cc
> index 6a20c159a..d614fc7a9 100644
> --- a/winsup/cygwin/mm/cygheap.cc
> +++ b/winsup/cygwin/mm/cygheap.cc
> @@ -87,7 +87,7 @@ 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 (child_proc_info->cygheap_max) - CYGHEAP_STORAGE_LOW;
>    cygheap = (init_cygheap *) VirtualAlloc ((LPVOID) CYGHEAP_STORAGE_LOW,
>  					   CYGHEAP_STORAGE_HIGH
>  					   - CYGHEAP_STORAGE_LOW,

It might be better to do the subtraction before the call to allocsize,
but this should still work in practice. I'll wait for more feedback
before I post another patch.