From 717c36c0a4e387a73146c1181b7b89f091176e85 Mon Sep 17 00:00:00 2001 From: Corinna Vinschen Date: Mon, 29 Aug 2022 12:25:24 +0200 Subject: [PATCH] Cygwin: fork: fix a potential hang in fork while debugging a problem introduced in commit 63b503916d42 ("Cygwin: tls_pathbuf: Use Windows heap") a hang in fork was encountered using the original implementation of tls_pathbuf: Using tmp_pathbuf inside the code block guarded by __malloc_trylock may call malloc from tmp_pathbuf::w_get and thus trying to lock an exclusive SRW lock recursively, which results in a deadlock. Allocate a small SECURITY_ATTRIBUTES block on the stack rather than allocating a 64K tmp_pathbuf. This avoids the potential malloc call. Drop the __malloc_trylock call entirely. There must not be a malloc call inside the frok::parent block guarded by __malloc_lock, and just trying to lock is too dangerous inside fork while other threads might actually chage the content of the heap. Additionally, add a comment frowning on malloc usage inside tis code block. Fixes: 44a79a6eca3d ("Cygwin: convert malloc lock to SRWLOCK") Signed-off-by: Corinna Vinschen --- winsup/cygwin/fork.cc | 8 +++++--- winsup/cygwin/local_includes/cygmalloc.h | 1 - 2 files changed, 5 insertions(+), 4 deletions(-) diff --git a/winsup/cygwin/fork.cc b/winsup/cygwin/fork.cc index e4931a286..e553c015a 100644 --- a/winsup/cygwin/fork.cc +++ b/winsup/cygwin/fork.cc @@ -296,7 +296,10 @@ frok::parent (volatile char * volatile stack_here) si.lpReserved2 = (LPBYTE) &ch; si.cbReserved2 = sizeof (ch); - bool locked = __malloc_trylock (); + /* NEVER, EVER, call a function which in turn calls malloc&friends while this + malloc lock is active! */ + __malloc_lock (); + bool locked = true; /* Remove impersonation */ cygheap->user.deimpersonate (); @@ -308,8 +311,7 @@ frok::parent (volatile char * volatile stack_here) ch.silentfail (!*with_forkables); /* fail silently without forkables */ - tmp_pathbuf tp; - PSECURITY_ATTRIBUTES sa = (PSECURITY_ATTRIBUTES) tp.w_get (); + PSECURITY_ATTRIBUTES sa = (PSECURITY_ATTRIBUTES) alloca (1024); if (!sec_user_nih (sa, cygheap->user.saved_sid (), well_known_authenticated_users_sid, PROCESS_QUERY_LIMITED_INFORMATION)) diff --git a/winsup/cygwin/local_includes/cygmalloc.h b/winsup/cygwin/local_includes/cygmalloc.h index 73a2f060b..5e1fe8154 100644 --- a/winsup/cygwin/local_includes/cygmalloc.h +++ b/winsup/cygwin/local_includes/cygmalloc.h @@ -32,7 +32,6 @@ extern "C" void __set_ENOMEM (); #elif defined (__INSIDE_CYGWIN__) # define __malloc_lock() AcquireSRWLockExclusive (&mallock) -# define __malloc_trylock() TryAcquireSRWLockExclusive (&mallock) # define __malloc_unlock() ReleaseSRWLockExclusive (&mallock) extern SRWLOCK NO_COPY mallock; void malloc_init ();