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[PATCH 17/18] x86, pthread_cond_*wait: Do not depend on %eax not being clobbered
- From: Nix <nix at esperi dot org dot uk>
- To: libc-alpha at sourceware dot org
- Date: Tue, 8 Mar 2016 13:51:03 +0000
- Subject: [PATCH 17/18] x86, pthread_cond_*wait: Do not depend on %eax not being clobbered
- Authentication-results: sourceware.org; auth=none
- References: <1457445064-7107-1-git-send-email-nix at esperi dot org dot uk>
From: Nick Alcock <nick.alcock@oracle.com>
The x86-specific versions of both pthread_cond_wait and
pthread_cond_timedwait have (in their fall-back-to-futex-wait slow
paths) calls to __pthread_mutex_cond_lock_adjust followed by
__pthread_mutex_unlock_usercnt, which load the parameters before the
first call but then assume that the first parameter, in %eax, will
survive unaffected. This happens to have been true before now, but %eax
is a call-clobbered register, and this assumption is not safe: it could
change at any time, at GCC's whim, and indeed the stack-protector canary
checking code clobbers %eax while checking that the canary is
uncorrupted.
So reload %eax before calling __pthread_mutex_unlock_usercnt. (Do this
unconditionally, even when stack-protection is not in use, because it's
the right thing to do, it's a slow path, and anything else is dicing
with death.)
v5: New.
* sysdeps/unix/sysv/linux/i386/pthread_cond_timedwait.S: Reload
call-clobbered %eax on retry path.
* sysdeps/unix/sysv/linux/i386/pthread_cond_wait.S: Likewise.
---
sysdeps/unix/sysv/linux/i386/pthread_cond_timedwait.S | 1 +
sysdeps/unix/sysv/linux/i386/pthread_cond_wait.S | 1 +
2 files changed, 2 insertions(+)
diff --git a/sysdeps/unix/sysv/linux/i386/pthread_cond_timedwait.S b/sysdeps/unix/sysv/linux/i386/pthread_cond_timedwait.S
index 96f8a8d..6256376 100644
--- a/sysdeps/unix/sysv/linux/i386/pthread_cond_timedwait.S
+++ b/sysdeps/unix/sysv/linux/i386/pthread_cond_timedwait.S
@@ -297,6 +297,7 @@ __pthread_cond_timedwait:
correctly. */
movl dep_mutex(%ebx), %eax
call __pthread_mutex_cond_lock_adjust
+ movl dep_mutex(%ebx), %eax
xorl %edx, %edx
call __pthread_mutex_unlock_usercnt
jmp 8b
diff --git a/sysdeps/unix/sysv/linux/i386/pthread_cond_wait.S b/sysdeps/unix/sysv/linux/i386/pthread_cond_wait.S
index 94302b0..5016718 100644
--- a/sysdeps/unix/sysv/linux/i386/pthread_cond_wait.S
+++ b/sysdeps/unix/sysv/linux/i386/pthread_cond_wait.S
@@ -309,6 +309,7 @@ __pthread_cond_wait:
correctly. */
movl dep_mutex(%ebx), %eax
call __pthread_mutex_cond_lock_adjust
+ movl dep_mutex(%ebx), %eax
xorl %edx, %edx
call __pthread_mutex_unlock_usercnt
jmp 8b
--
2.7.0.198.g6dd47b6