3.2.102-rc1 review patch. If anyone has any objections, please let me know.
------------------ From: Mikulas Patocka <mpato...@redhat.com> commit 21ffceda1c8b3807615c40d440d7815e0c85d366 upstream. 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> Signed-off-by: Matt Turner <matts...@gmail.com> [bwh: Backported to 3.2: - Remove the settls variable, which was done upstream in commit 25906730ec01 "alpha: reorganize copy_process(), prepare to saner fork_idle()"] - Adjust context] Signed-off-by: Ben Hutchings <b...@decadent.org.uk> --- --- a/arch/alpha/kernel/process.c +++ b/arch/alpha/kernel/process.c @@ -279,7 +279,7 @@ copy_thread(unsigned long clone_flags, u struct thread_info *childti = task_thread_info(p); struct pt_regs * childregs; struct switch_stack * childstack, *stack; - unsigned long stack_offset, settls; + unsigned long stack_offset; stack_offset = PAGE_SIZE - sizeof(struct pt_regs); if (!(regs->ps & 8)) @@ -288,11 +288,9 @@ copy_thread(unsigned long clone_flags, u (stack_offset + PAGE_SIZE + task_stack_page(p)); *childregs = *regs; - settls = regs->r20; 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 = ((struct switch_stack *) childregs) - 1; *childstack = *stack; @@ -302,16 +300,16 @@ copy_thread(unsigned long clone_flags, u childti->pcb.flags = 1; /* set FEN, clear everything else */ /* Set a new TLS for the child thread? Peek back into the - syscall arguments that we saved on syscall entry. Oops, - except we'd have clobbered it with the parent/child set - of r20. Read the saved copy. */ + syscall arguments that we saved on syscall entry. */ /* Note: if CLONE_SETTLS is not set, then we must inherit the value from the parent, which will have been set by the block copy in dup_task_struct. This is non-intuitive, but is required for proper operation in the case of a threaded application calling fork. */ if (clone_flags & CLONE_SETTLS) - childti->pcb.unique = settls; + childti->pcb.unique = regs->r20; + else + regs->r20 = 0; /* OSF/1 has some strange fork() semantics. */ return 0; }
