Re: 4k stacks

[linux-os (Dick Johnson)]

On Sat, 24 Dec 2005, Denis Vlasenko wrote:

> On Thursday 22 December 2005 23:53, linux-os (Dick Johnson) wrote:
>>
>> Yesterday I sent a patch to add stack-poison so the stack usage
>> could be observed.
>>
>> Today I wrote a small program and tested the stack usage. Both
>> the program and the patch is attached. The result is:
>>
>> Offset : 2ec8f000	Available Stack bytes = 3104
>> Offset : 2ecb1000	Available Stack bytes = 3104
>> Offset : 2ee5f000	Available Stack bytes = 20
>> Offset : 2f36d000	Available Stack bytes = 3104
>> Offset : 2fd09000	Available Stack bytes = 3012
>> Offset : 2fd0b000	Available Stack bytes = 3312
>> Offset : 2fd0f000	Available Stack bytes = 2132
>> Offset : 2fd2f000	Available Stack bytes = 2744
>> Offset : 2fd57000	Available Stack bytes = 2900
>> Offset : 2fdd5000	Available Stack bytes = 1400
>> Offset : 2fe35000	Available Stack bytes = 2832
>> Offset : 2ff3f000	Available Stack bytes = 776
>> Offset : 2ff45000	Available Stack bytes = 3188
>>
>> This, after compiling the kernel. I did not have 4k stacks
>> enabled for this test so any crashing of the stack beyond
>> one page will not hurt the system. This was on linux-2.6.13.4.
>>
>> Anyway, I tried to enable 4k stacks and the machine would
>> not boot past trying to install the first module. It just
>> stopped with the interrupts disabled. So, I am now rebuilding
>> the kernel back as I write this. That's why I am using 2.6.13
>> at the moment.
>>
>> Anyway, getting down to 20 bytes of stack-space available
>> seems to be pretty scary.
>
> +       movl    %esp, %edi
> +       movl    %edi, %ecx
> +       andl    $~0x1000, %edi
> +       subl    %edi, %ecx
>
> ecx will be equal to ?

Whatever the stack was minus that value ANDed with NOT 0x1000,
i.e. 0x1000 minus the stack already in use. The code assumes
that the stack starts and ends on a 0x1000 (page) boundary.
If that's not true, then all bets are off.

>
> +       movb    $'Q', %al
> +       rep     stosb
> --
> vda
>

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5591.11 BogoMips).
Warning : 98.36% of all statistics are fiction.
.

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