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authorFlorian Weimer <fweimer@redhat.com>2016-05-12 08:54:17 +0200
committerFlorian Weimer <fweimer@redhat.com>2016-05-12 15:26:55 +0200
commit56290d6e762c1194547e73ff0b948cd79d3a1e03 (patch)
treec6025736ee13b24a908e7d12496d47b636592345 /sysdeps/nptl
parentcd065b68439076971640047cc6eaada27dcfd0f7 (diff)
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Increase fork signal safety for single-threaded processes [BZ #19703]
This provides a band-aid and addresses the scenario where fork is called from a signal handler while the process is in the malloc subsystem (or has acquired the libio list lock). It does not address the general issue of async-signal-safety of fork; multi-threaded processes are not covered, and some glibc subsystems have fork handlers which are not async-signal-safe.
Diffstat (limited to 'sysdeps/nptl')
-rw-r--r--sysdeps/nptl/fork.c53
1 files changed, 38 insertions, 15 deletions
diff --git a/sysdeps/nptl/fork.c b/sysdeps/nptl/fork.c
index 1a68cbd647..616d897a36 100644
--- a/sysdeps/nptl/fork.c
+++ b/sysdeps/nptl/fork.c
@@ -54,6 +54,12 @@ __libc_fork (void)
struct used_handler *next;
} *allp = NULL;
+ /* Determine if we are running multiple threads. We skip some fork
+ handlers in the single-thread case, to make fork safer to use in
+ signal handlers. POSIX requires that fork is async-signal-safe,
+ but our current fork implementation is not. */
+ bool multiple_threads = THREAD_GETMEM (THREAD_SELF, header.multiple_threads);
+
/* Run all the registered preparation handlers. In reverse order.
While doing this we build up a list of all the entries. */
struct fork_handler *runp;
@@ -109,12 +115,21 @@ __libc_fork (void)
break;
}
- _IO_list_lock ();
+ /* If we are not running multiple threads, we do not have to
+ preserve lock state. If fork runs from a signal handler, only
+ async-signal-safe functions can be used in the child. These data
+ structures are only used by unsafe functions, so their state does
+ not matter if fork was called from a signal handler. */
+ if (multiple_threads)
+ {
+ _IO_list_lock ();
- /* Acquire malloc locks. This needs to come last because fork
- handlers may use malloc, and the libio list lock has an indirect
- malloc dependency as well (via the getdelim function). */
- __malloc_fork_lock_parent ();
+ /* Acquire malloc locks. This needs to come last because fork
+ handlers may use malloc, and the libio list lock has an
+ indirect malloc dependency as well (via the getdelim
+ function). */
+ __malloc_fork_lock_parent ();
+ }
#ifndef NDEBUG
pid_t ppid = THREAD_GETMEM (THREAD_SELF, tid);
@@ -173,14 +188,18 @@ __libc_fork (void)
# endif
#endif
- /* Release malloc locks. */
- __malloc_fork_unlock_child ();
+ /* Reset the lock state in the multi-threaded case. */
+ if (multiple_threads)
+ {
+ /* Release malloc locks. */
+ __malloc_fork_unlock_child ();
- /* Reset the file list. These are recursive mutexes. */
- fresetlockfiles ();
+ /* Reset the file list. These are recursive mutexes. */
+ fresetlockfiles ();
- /* Reset locks in the I/O code. */
- _IO_list_resetlock ();
+ /* Reset locks in the I/O code. */
+ _IO_list_resetlock ();
+ }
/* Reset the lock the dynamic loader uses to protect its data. */
__rtld_lock_initialize (GL(dl_load_lock));
@@ -217,11 +236,15 @@ __libc_fork (void)
/* Restore the PID value. */
THREAD_SETMEM (THREAD_SELF, pid, parentpid);
- /* Release malloc locks, parent process variant. */
- __malloc_fork_unlock_parent ();
+ /* Release acquired locks in the multi-threaded case. */
+ if (multiple_threads)
+ {
+ /* Release malloc locks, parent process variant. */
+ __malloc_fork_unlock_parent ();
- /* We execute this even if the 'fork' call failed. */
- _IO_list_unlock ();
+ /* We execute this even if the 'fork' call failed. */
+ _IO_list_unlock ();
+ }
/* Run the handlers registered for the parent. */
while (allp != NULL)