On Wed, 3 Jan 2018, Michael Cree wrote: > On Tue, Jan 02, 2018 at 02:01:34PM -0500, Mikulas Patocka wrote: > > 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. > > This bug was identified some three years ago; it triggers a failure > in the glibc nptl/tst-eintr3 test. See: > > https://marc.info/?l=linux-alpha&m=140610647213217&w=2 > > and a fix was proposed by RTH, namely: > > https://marc.info/?l=linux-alpha&m=140675667715872&w=2 > > but was never included in the kernel because someone objected to > breaking the ability to run OSF/1 executables. That patch also > deleted the line to set childregs->r20 to 1 which I mark below. > > > > > 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. */ > > This line. Is it not also problematic? If a signal is delivered to the parent process, the incomplete child process is deleted and it is recreated when the syscall is restarted. So, setting "childregs->r20 = 1" shouldn't cause any problems. Mikulas > Cheers > Michael. > > > - 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 > -- 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
