mirror of
git://git.openwrt.org/openwrt/openwrt.git
synced 2024-12-13 10:24:51 +00:00
10c211031c
Import proposed upstream fix [2] for the critical locking synchronization bug recently found in musl [1]. This affects all programs that are temporarily multithreaded, but then return to single-threaded operation. [1] https://www.openwall.com/lists/musl/2020/05/22/3 [2] https://www.openwall.com/lists/musl/2020/05/22/10 Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
70 lines
2.5 KiB
Diff
70 lines
2.5 KiB
Diff
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(-)
|
|
|
|
--- 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;
|
|
--- 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);
|
|
}
|
|
|
|
--- 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;
|