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 <mpatocka@xxxxxxxxxx> Cc: stable@xxxxxxxxxxxxxxx --- 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; -- To unsubscribe from this list: send the line "unsubscribe linux-alpha" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
