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

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