Re: [PATCH 1/3] signal(i386): alternative signal stack wraparoundoccurs

[Shi Weihua]

Mikael Pettersson wrote::
> On Thu, 4 Oct 2007 21:47:30 +0900, KAMEZAWA Hiroyuki wrote:
> > On Thu, 04 Oct 2007 21:33:12 +0900
> > Shi Weihua <shiwh@xxxxxxxxxxxxxx> wrote:
> >
> > > KAMEZAWA Hiroyuki wrote::
> > > > On Thu, 04 Oct 2007 20:56:14 +0900
> > > > Shi Weihua <shiwh@xxxxxxxxxxxxxx> wrote:
> > > >
> > > > > 	stack.ss_sp = addr + pagesize;
> > > > > 	stack.ss_flags = 0;
> > > > > 	stack.ss_size = pagesize;
> > > > Here is bad. 
> > > > stack,ss_sp = addr;
> > > > stack.ss_flags = 0;
> > > > stack.ss_size = pagesize * 2;
> > > [What the test code want to do]
> > > addr+pagesize*2 - addr+pagesize	 -> sigaltstack
> > > addr+pagesize	- addr		 -> protected region
> > > The code want to catch overflow when esp enter the protected region.
> > You have to protect the top of *registered* sigaltstack.
> > The reason of wraparound is %esp will be set to the bottom of sigaltstack
> > if it is not on sigaltstack area when signaled.
> > What you have to do is protect the top of registerd sigaltstack.
> > If %esp is in the range of registerd sigaltstack at SEGV, wraparound
> > will stop.
>
> Exactly right. You mprotect or munmap the end of the altstack,
> not the area beyond it.
So we tell users "Even if you protectted half of mmap's space, but you must to register all space to 
kernel. " ?

The image about my test code's result:
              No patch        Patched
âââââââââââââ
â           ââ 1 â â 3          â 1
â    A      â    â(wraparound)
â           â    â
â           ââ 2 â              â 2
â           â    â
âââââââââââââ    â
ââââââââââââââ 3 â              â 3
âââââBâââââââ                  (caught)
ââprotectedââ
âââââââââââââ
âââââââââââââ
âââââââââââââ
A+B  mmap's space
A    sigaltstack
B    protectted

I agree that if register A+B to kernel, the wraparound will stop.
But if register A to kernel, why not kernel do something?

Thanks
Shi Weihua
> /Mikael

-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/