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

[Shi Weihua <shiwh@cn.fujitsu.com>]

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@cn.fujitsu.com> wrote:
>>
>>> KAMEZAWA Hiroyuki wrote::
>>>> On Thu, 04 Oct 2007 20:56:14 +0900
>>>> Shi Weihua <shiwh@cn.fujitsu.com> 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
┌───────────┐
│           │← 1 ┌ ← 3          ← 1
│    A      │    │(wraparound)
│           │    │
│           │← 2 │              ← 2
│           │    │
├───────────┤    │
│▒▒▒▒▒▒▒▒▒▒▒│← 3 ┘              ← 3
│▒▒▒▒B▒▒▒▒▒▒│                  (caught)
│▒protected▒│
│▒▒▒▒▒▒▒▒▒▒▒│
│▒▒▒▒▒▒▒▒▒▒▒│
└───────────┘
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
> 
> 
>