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Cygwin: fork: fix a potential hang in fork

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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 <corinna@vinschen.de>
This commit is contained in:
Corinna Vinschen 2022-08-29 12:25:24 +02:00
parent f59ff93046
commit 717c36c0a4
2 changed files with 5 additions and 4 deletions
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8
winsup/cygwin/fork.cc
View File

@ -296,7 +296,10 @@ frok::parent (volatile char * volatile stack_here)
si.lpReserved2 = (LPBYTE) &ch; si.lpReserved2 = (LPBYTE) &ch;
si.cbReserved2 = sizeof (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 */ /* Remove impersonation */
cygheap->user.deimpersonate (); cygheap->user.deimpersonate ();
@ -308,8 +311,7 @@ frok::parent (volatile char * volatile stack_here)
ch.silentfail (!*with_forkables); /* fail silently without forkables */ ch.silentfail (!*with_forkables); /* fail silently without forkables */
tmp_pathbuf tp; PSECURITY_ATTRIBUTES sa = (PSECURITY_ATTRIBUTES) alloca (1024);
PSECURITY_ATTRIBUTES sa = (PSECURITY_ATTRIBUTES) tp.w_get ();
if (!sec_user_nih (sa, cygheap->user.saved_sid (), if (!sec_user_nih (sa, cygheap->user.saved_sid (),
well_known_authenticated_users_sid, well_known_authenticated_users_sid,
PROCESS_QUERY_LIMITED_INFORMATION)) PROCESS_QUERY_LIMITED_INFORMATION))

1
winsup/cygwin/local_includes/cygmalloc.h
View File

@ -32,7 +32,6 @@ extern "C" void __set_ENOMEM ();
#elif defined (__INSIDE_CYGWIN__) #elif defined (__INSIDE_CYGWIN__)
# define __malloc_lock() AcquireSRWLockExclusive (&mallock) # define __malloc_lock() AcquireSRWLockExclusive (&mallock)
# define __malloc_trylock() TryAcquireSRWLockExclusive (&mallock)
# define __malloc_unlock() ReleaseSRWLockExclusive (&mallock) # define __malloc_unlock() ReleaseSRWLockExclusive (&mallock)
extern SRWLOCK NO_COPY mallock; extern SRWLOCK NO_COPY mallock;
void malloc_init (); void malloc_init ();