[newlib-cygwin] Cygwin: exec: don't access cygheap before it's initialized

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=30add3e6b3e3211b3e2d4a093f45bee5c6e24b8b

commit 30add3e6b3e3211b3e2d4a093f45bee5c6e24b8b
Author:     Corinna Vinschen <corinna@vinschen.de>
AuthorDate: Thu Dec 1 21:33:14 2022 +0100
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Thu Dec 1 22:34:53 2022 +0100

    Cygwin: exec: don't access cygheap before it's initialized
    
    This is a long-standing thinko.
    
    When you exec a process, dll_crt0_0 in the child process calls
    child_info_spawn::handle_spawn().  handle_spawn() initialises the
    cygheap.
    
    Now consider calling strace.  Strace is a non-Cygwin process dynamically
    loading cygwin1.dll via LoadLibrary.  This in turn initializes the DLL:
    
    - dll_crt0_0 finds that the process it attaches to has been exec'd, so
      child_info_spawn::handle_spawn() is called.
    
    - If the DLL is being dynamically loaded, handle_spawn() calls
      child_info_spawn::get_parent_handle().  This in turn tries to set
      the moreinfo->myself_pinfo value inside the cygheap to NULL.
    
    - However, at this time, the cygheap has not yet been initialized.  This
      only occurs in the cygheap_fixup_in_child() call after get_parent_handle()
      returns.
    
    --> SEGV
    
    This thinko never had a negative side effect, because the cygheap was
    pre-allocated at DLL load time until commit 2f9b8ff00cce ("Cygwin:
    decouple cygheap from Cygwin DLL").  With 2f9b8ff00cce, the cygheap
    actually doesn't exist until after the call to cygheap_fixup_in_child().
    
    Fix this problem by moving the assignment after the call to
    cygheap_fixup_in_child().
    
    Fixes: 3de7be4c1deb ("* DevNotes: Add entry cgf-000007. [...]")
    Fixes: 2f9b8ff00cce ("Cygwin: decouple cygheap from Cygwin DLL")
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/dcrt0.cc | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/dcrt0.cc b/winsup/cygwin/dcrt0.cc
index e5f19f1f930d..5c5a280cce9d 100644
--- a/winsup/cygwin/dcrt0.cc
+++ b/winsup/cygwin/dcrt0.cc
@@ -620,7 +620,6 @@ bool
 child_info_spawn::get_parent_handle ()
 {
   parent = OpenProcess (PROCESS_VM_READ, FALSE, parent_winpid);
-  moreinfo->myself_pinfo = NULL;
   return !!parent;
 }
 
@@ -632,6 +631,8 @@ child_info_spawn::handle_spawn ()
   if (!dynamically_loaded || get_parent_handle ())
       {
 	cygheap_fixup_in_child (true);
+	if (dynamically_loaded)
+	  moreinfo->myself_pinfo = NULL;
 	memory_init ();
       }