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? 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
