On alpha, a process will crash if it attempts to start a thread and a
signal is delivered at the same time. The crash can be reproduced with
this program: https://cygwin.com/ml/cygwin/2014-11/msg00473.html
The reason for the crash is this:
* we call the clone syscall
* we go to the function copy_process
* copy process calls copy_thread_tls, it is a wrapper around copy_thread
* copy_thread sets the tls pointer: childti->pcb.unique = regs->r20
* copy_thread sets regs->r20 to zero
* we go back to copy_process
* copy process checks "if (signal_pending(current))" and returns
-ERESTARTNOINTR
* the clone syscall is restarted, but this time, regs->r20 is zero, so
the new thread is created with zero tls pointer
* the new thread crashes in start_thread when attempting to access tls
The comment in the code says that setting the register r20 is some
compatibility with OSF/1. But OSF/1 doesn't use the CLONE_SETTLS flag, so
we don't have to zero r20 if CLONE_SETTLS is set. This patch fixes the bug
by zeroing regs->r20 only if CLONE_SETTLS is not set.
Signed-off-by: Mikulas Patocka <mpato...@redhat.com>
Cc: sta...@vger.kernel.org
---
arch/alpha/kernel/process.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
Index: linux-stable/arch/alpha/kernel/process.c
===================================================================
--- linux-stable.orig/arch/alpha/kernel/process.c 2017-12-31
17:42:12.000000000 +0100
+++ linux-stable/arch/alpha/kernel/process.c 2018-01-02 18:06:24.000000000
+0100
@@ -265,12 +265,13 @@ copy_thread(unsigned long clone_flags, u
application calling fork. */
if (clone_flags & CLONE_SETTLS)
childti->pcb.unique = regs->r20;
+ else
+ regs->r20 = 0; /* OSF/1 has some strange fork() semantics. */
childti->pcb.usp = usp ?: rdusp();
*childregs = *regs;
childregs->r0 = 0;
childregs->r19 = 0;
childregs->r20 = 1; /* OSF/1 has some strange fork() semantics. */
- regs->r20 = 0;
stack = ((struct switch_stack *) regs) - 1;
*childstack = *stack;
childstack->r26 = (unsigned long) ret_from_fork;
