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SubjectRe: [PATCH] alpha: fix crash if pthread_create races with signal delivery


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@redhat.com>
> > Cc: stable@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. */
>
> 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@vger.kernel.org
> > More majordomo info at http://vger.kernel.org/majordomo-info.html
>

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