system/musl: integrate MT->single thread transition fix from upstream

https://www.openwall.com/lists/musl/2020/05/22/10

2 files changed, 270 insertions, 1 deletions

diff --git a/system/musl/APKBUILD b/system/musl/APKBUILD
index f7596ebb5..8517b148b 100644
--- a/system/musl/APKBUILD
+++ b/system/musl/APKBUILD
@@ -1,7 +1,7 @@
 # Maintainer: A. Wilcox <awilfox@adelielinux.org>
 pkgname=musl
 pkgver=1.2.0
-pkgrel=0
+pkgrel=1
 pkgdesc="System library (libc) implementation"
 url="https://www.musl-libc.org/"
 arch="all"
@@ -26,6 +26,7 @@ source="https://musl.libc.org/releases/$pkgname-$pkgver.tar.gz
 	3001-make-real-lastlog-h.patch
 	handle-aux-at_base.patch
 	fgetspent_r.patch
+	threads_minus_1.patch
 
 	ldconfig
 	getent.c
@@ -121,6 +122,7 @@ f01ab92b9d385c15369c0bb7d95e1bc06a009c8851e363517d0ba1bae3fc2647af69fc2f363b5d96
 88ae443dbb8e0a4368235bdc3a1c5c7b718495afa75e06deb8e01becc76cb1f0d6964589e2204fc749c9c1b3190b8b9ac1ae2c0099cab8e2ce3ec877103d4332  3001-make-real-lastlog-h.patch
 6a7ff16d95b5d1be77e0a0fbb245491817db192176496a57b22ab037637d97a185ea0b0d19da687da66c2a2f5578e4343d230f399d49fe377d8f008410974238  handle-aux-at_base.patch
 ded41235148930f8cf781538f7d63ecb0c65ea4e8ce792565f3649ee2523592a76b2a166785f0b145fc79f5852fd1fb1729a7a09110b3b8f85cba3912e790807  fgetspent_r.patch
+68830961e297d9a499f3b609be84848ad5d3326a1af56e9e54a40ecd972c48da11532c51da572d45e0df3574d63191e7ae0d3a1b84a029365f8d00691de96952  threads_minus_1.patch
 cb71d29a87f334c75ecbc911becde7be825ab30d8f39fa6d64cb53812a7c9abaf91d9804c72540e5be3ddd3c84cfe7fd9632274309005cb8bcdf9a9b09b4b923  ldconfig
 378d70e65bcc65bb4e1415354cecfa54b0c1146dfb24474b69e418cdbf7ad730472cd09f6f103e1c99ba6c324c9560bccdf287f5889bbc3ef0bdf0e08da47413  getent.c
 9d42d66fb1facce2b85dad919be5be819ee290bd26ca2db00982b2f8e055a0196290a008711cbe2b18ec9eee8d2270e3b3a4692c5a1b807013baa5c2b70a2bbf  iconv.c"
diff --git a/system/musl/threads_minus_1.patch b/system/musl/threads_minus_1.patch
new file mode 100644
index 000000000..05307d97c
--- /dev/null
+++ b/system/musl/threads_minus_1.patch
@@ -0,0 +1,267 @@
+https://www.openwall.com/lists/musl/2020/05/22/10
+
+From 4d5aa20a94a2d3fae3e69289dc23ecafbd0c16c4 Mon Sep 17 00:00:00 2001
+From: Rich Felker <dalias@aerifal.cx>
+Date: Fri, 22 May 2020 17:35:14 -0400
+Subject: [PATCH 1/4] reorder thread list unlink in pthread_exit after all
+ locks
+
+since the backend for LOCK() skips locking if single-threaded, it's
+unsafe to make the process appear single-threaded before the last use
+of lock.
+
+this fixes potential unsynchronized access to a linked list via
+__dl_thread_cleanup.
+---
+ src/thread/pthread_create.c | 19 +++++++++++--------
+ 1 file changed, 11 insertions(+), 8 deletions(-)
+
+diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
+index 5f491092..6a3b0c21 100644
+--- a/src/thread/pthread_create.c
++++ b/src/thread/pthread_create.c
+@@ -90,14 +90,7 @@ _Noreturn void __pthread_exit(void *result)
+ 		exit(0);
+ 	}
+ 
+-	/* At this point we are committed to thread termination. Unlink
+-	 * the thread from the list. This change will not be visible
+-	 * until the lock is released, which only happens after SYS_exit
+-	 * has been called, via the exit futex address pointing at the lock. */
+-	libc.threads_minus_1--;
+-	self->next->prev = self->prev;
+-	self->prev->next = self->next;
+-	self->prev = self->next = self;
++	/* At this point we are committed to thread termination. */
+ 
+ 	/* Process robust list in userspace to handle non-pshared mutexes
+ 	 * and the detached thread case where the robust list head will
+@@ -121,6 +114,16 @@ _Noreturn void __pthread_exit(void *result)
+ 	__do_orphaned_stdio_locks();
+ 	__dl_thread_cleanup();
+ 
++	/* Last, unlink thread from the list. This change will not be visible
++	 * until the lock is released, which only happens after SYS_exit
++	 * has been called, via the exit futex address pointing at the lock.
++	 * This needs to happen after any possible calls to LOCK() that might
++	 * skip locking if libc.threads_minus_1 is zero. */
++	libc.threads_minus_1--;
++	self->next->prev = self->prev;
++	self->prev->next = self->next;
++	self->prev = self->next = self;
++
+ 	/* This atomic potentially competes with a concurrent pthread_detach
+ 	 * call; the loser is responsible for freeing thread resources. */
+ 	int state = a_cas(&self->detach_state, DT_JOINABLE, DT_EXITING);
+-- 
+2.21.0
+
+From e01b5939b38aea5ecbe41670643199825874b26c Mon Sep 17 00:00:00 2001
+From: Rich Felker <dalias@aerifal.cx>
+Date: Thu, 21 May 2020 23:32:45 -0400
+Subject: [PATCH 2/4] don't use libc.threads_minus_1 as relaxed atomic for
+ skipping locks
+
+after all but the last thread exits, the next thread to observe
+libc.threads_minus_1==0 and conclude that it can skip locking fails to
+synchronize with any changes to memory that were made by the
+last-exiting thread. this can produce data races.
+
+on some archs, at least x86, memory synchronization is unlikely to be
+a problem; however, with the inline locks in malloc, skipping the lock
+also eliminated the compiler barrier, and caused code that needed to
+re-check chunk in-use bits after obtaining the lock to reuse a stale
+value, possibly from before the process became single-threaded. this
+in turn produced corruption of the heap state.
+
+some uses of libc.threads_minus_1 remain, especially for allocation of
+new TLS in the dynamic linker; otherwise, it could be removed
+entirely. it's made non-volatile to reflect that the remaining
+accesses are only made under lock on the thread list.
+
+instead of libc.threads_minus_1, libc.threaded is now used for
+skipping locks. the difference is that libc.threaded is permanently
+true once an additional thread has been created. this will produce
+some performance regression in processes that are mostly
+single-threaded but occasionally creating threads. in the future it
+may be possible to bring back the full lock-skipping, but more care
+needs to be taken to produce a safe design.
+---
+ src/internal/libc.h | 2 +-
+ src/malloc/malloc.c | 2 +-
+ src/thread/__lock.c | 2 +-
+ 3 files changed, 3 insertions(+), 3 deletions(-)
+
+diff --git a/src/internal/libc.h b/src/internal/libc.h
+index ac97dc7e..c0614852 100644
+--- a/src/internal/libc.h
++++ b/src/internal/libc.h
+@@ -21,7 +21,7 @@ struct __libc {
+ 	int can_do_threads;
+ 	int threaded;
+ 	int secure;
+-	volatile int threads_minus_1;
++	int threads_minus_1;
+ 	size_t *auxv;
+ 	struct tls_module *tls_head;
+ 	size_t tls_size, tls_align, tls_cnt;
+diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
+index 96982596..2553a62e 100644
+--- a/src/malloc/malloc.c
++++ b/src/malloc/malloc.c
+@@ -26,7 +26,7 @@ int __malloc_replaced;
+ 
+ static inline void lock(volatile int *lk)
+ {
+-	if (libc.threads_minus_1)
++	if (libc.threaded)
+ 		while(a_swap(lk, 1)) __wait(lk, lk+1, 1, 1);
+ }
+ 
+diff --git a/src/thread/__lock.c b/src/thread/__lock.c
+index 45557c88..5b9b144e 100644
+--- a/src/thread/__lock.c
++++ b/src/thread/__lock.c
+@@ -18,7 +18,7 @@
+ 
+ void __lock(volatile int *l)
+ {
+-	if (!libc.threads_minus_1) return;
++	if (!libc.threaded) return;
+ 	/* fast path: INT_MIN for the lock, +1 for the congestion */
+ 	int current = a_cas(l, 0, INT_MIN + 1);
+ 	if (!current) return;
+-- 
+2.21.0
+
+From f12888e9eb9eed60cc266b899dcafecb4752964a Mon Sep 17 00:00:00 2001
+From: Rich Felker <dalias@aerifal.cx>
+Date: Fri, 22 May 2020 17:25:38 -0400
+Subject: [PATCH 3/4] cut down size of some libc struct members
+
+these are all flags that can be single-byte values.
+---
+ src/internal/libc.h | 6 +++---
+ 1 file changed, 3 insertions(+), 3 deletions(-)
+
+diff --git a/src/internal/libc.h b/src/internal/libc.h
+index c0614852..d47f58e0 100644
+--- a/src/internal/libc.h
++++ b/src/internal/libc.h
+@@ -18,9 +18,9 @@ struct tls_module {
+ };
+ 
+ struct __libc {
+-	int can_do_threads;
+-	int threaded;
+-	int secure;
++	char can_do_threads;
++	char threaded;
++	char secure;
+ 	int threads_minus_1;
+ 	size_t *auxv;
+ 	struct tls_module *tls_head;
+-- 
+2.21.0
+
+From 8d81ba8c0bc6fe31136cb15c9c82ef4c24965040 Mon Sep 17 00:00:00 2001
+From: Rich Felker <dalias@aerifal.cx>
+Date: Fri, 22 May 2020 17:45:47 -0400
+Subject: [PATCH 4/4] restore lock-skipping for processes that return to
+ single-threaded state
+
+the design used here relies on the barrier provided by the first lock
+operation after the process returns to single-threaded state to
+synchronize with actions by the last thread that exited. by storing
+the intent to change modes in the same object used to detect whether
+locking is needed, it's possible to avoid an extra (possibly costly)
+memory load after the lock is taken.
+---
+ src/internal/libc.h         | 1 +
+ src/malloc/malloc.c         | 5 ++++-
+ src/thread/__lock.c         | 4 +++-
+ src/thread/pthread_create.c | 8 ++++----
+ 4 files changed, 12 insertions(+), 6 deletions(-)
+
+diff --git a/src/internal/libc.h b/src/internal/libc.h
+index d47f58e0..619bba86 100644
+--- a/src/internal/libc.h
++++ b/src/internal/libc.h
+@@ -21,6 +21,7 @@ struct __libc {
+ 	char can_do_threads;
+ 	char threaded;
+ 	char secure;
++	volatile signed char need_locks;
+ 	int threads_minus_1;
+ 	size_t *auxv;
+ 	struct tls_module *tls_head;
+diff --git a/src/malloc/malloc.c b/src/malloc/malloc.c
+index 2553a62e..a803d4c9 100644
+--- a/src/malloc/malloc.c
++++ b/src/malloc/malloc.c
+@@ -26,8 +26,11 @@ int __malloc_replaced;
+ 
+ static inline void lock(volatile int *lk)
+ {
+-	if (libc.threaded)
++	int need_locks = libc.need_locks;
++	if (need_locks) {
+ 		while(a_swap(lk, 1)) __wait(lk, lk+1, 1, 1);
++		if (need_locks < 0) libc.need_locks = 0;
++	}
+ }
+ 
+ static inline void unlock(volatile int *lk)
+diff --git a/src/thread/__lock.c b/src/thread/__lock.c
+index 5b9b144e..60eece49 100644
+--- a/src/thread/__lock.c
++++ b/src/thread/__lock.c
+@@ -18,9 +18,11 @@
+ 
+ void __lock(volatile int *l)
+ {
+-	if (!libc.threaded) return;
++	int need_locks = libc.need_locks;
++	if (!need_locks) return;
+ 	/* fast path: INT_MIN for the lock, +1 for the congestion */
+ 	int current = a_cas(l, 0, INT_MIN + 1);
++	if (need_locks < 0) libc.need_locks = 0;
+ 	if (!current) return;
+ 	/* A first spin loop, for medium congestion. */
+ 	for (unsigned i = 0; i < 10; ++i) {
+diff --git a/src/thread/pthread_create.c b/src/thread/pthread_create.c
+index 6a3b0c21..6bdfb44f 100644
+--- a/src/thread/pthread_create.c
++++ b/src/thread/pthread_create.c
+@@ -118,8 +118,8 @@ _Noreturn void __pthread_exit(void *result)
+ 	 * until the lock is released, which only happens after SYS_exit
+ 	 * has been called, via the exit futex address pointing at the lock.
+ 	 * This needs to happen after any possible calls to LOCK() that might
+-	 * skip locking if libc.threads_minus_1 is zero. */
+-	libc.threads_minus_1--;
++	 * skip locking if process appears single-threaded. */
++	if (!--libc.threads_minus_1) libc.need_locks = -1;
+ 	self->next->prev = self->prev;
+ 	self->prev->next = self->next;
+ 	self->prev = self->next = self;
+@@ -339,7 +339,7 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
+ 		~(1UL<<((SIGCANCEL-1)%(8*sizeof(long))));
+ 
+ 	__tl_lock();
+-	libc.threads_minus_1++;
++	if (!libc.threads_minus_1++) libc.need_locks = 1;
+ 	ret = __clone((c11 ? start_c11 : start), stack, flags, args, &new->tid, TP_ADJ(new), &__thread_list_lock);
+ 
+ 	/* All clone failures translate to EAGAIN. If explicit scheduling
+@@ -363,7 +363,7 @@ int __pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict att
+ 		new->next->prev = new;
+ 		new->prev->next = new;
+ 	} else {
+-		libc.threads_minus_1--;
++		if (!--libc.threads_minus_1) libc.need_locks = 0;
+ 	}
+ 	__tl_unlock();
+ 	__restore_sigs(&set);
+-- 
+2.21.0
+