On Mon, 2009-11-16 at 05:12 -0600, Serge E. Hallyn wrote:
> Quoting Nathan Lynch (ntl@xxxxxxxxx):
> > On Thu, 2009-11-12 at 23:24 -0600, serue@xxxxxxxxxx wrote:
> > > + if (use_clone) {
> > > + int stacksize = 4*getpagesize();
> > > + void *stack = malloc(stacksize);
> > > +
> > > + if (!stack) {
> > > + perror("malloc");
> > > + return -1;
> > > + }
> > > +
> > > + printf("about to clone with %lx\n", flags);
> > > + if (chosen_pid)
> > > + printf("Will choose pid %d\n", chosen_pid);
> > > + flags |= SIGCHLD;
> > > + pid = clone_with_pids(do_child, stack, flags, &pid_set,
> > > + (void *)argv);
> >
> > The stack argument should be adjusted with the usual stack += stacksize
> > - 1 or similar, right?
>
> the clone_with_pids() helper in user-cr/clone_s390x.c (and IIRC the
> x86 one by Suka also) does this implicitly, by doing:
>
> s = child_stack;
> *--s = arg;
> *--s = fn;
> child_stack -= 16
That's setting up arguments for the function to run in the child, and
afaict that code assumes the value of child_stack is the _end_ of the
stack region. The code I quoted above is passing the beginning of the
region (the return value from malloc).
On powerpc the segfaults went away when I made the following change.
diff --git a/nsexeccwp.c b/nsexeccwp.c
index a71d9a4..92eb092 100644
--- a/nsexeccwp.c
+++ b/nsexeccwp.c
@@ -309,8 +309,8 @@ int main(int argc, char *argv[])
if (chosen_pid)
printf("Will choose pid %d\n", chosen_pid);
flags |= SIGCHLD;
- pid = clone_with_pids(do_child, stack, flags, &pid_set,
- (void *)argv);
+ pid = clone_with_pids(do_child, stack + stacksize - 1,
+ flags, &pid_set, (void *)argv);
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