Re: Need help in understanding x86 syscall

linux-os (Dick Johnson)
2005-08-11T15:28:57Z

On Thu, 11 Aug 2005, Coywolf Qi Hunt wrote:

> On 8/11/05, Steven Rostedt <rostedt@goodmis.org> wrote:
>> On Thu, 2005-08-11 at 10:04 -0400, linux-os (Dick Johnson) wrote:
>>> Every interrupt software, or hardware, results in the branched
>>> procedure being executed with the interrupts OFF. That's why
>>> one of the first instructions in the kernel entry for a syscall
>>> is 'sti' to turn them back on. Look at entry.S, line 182. This
>>> occurs any time a trap occurs as well (Page 26-168, i486
>>> Programmer's reference manual). FYI, this is helpful when
>>> designing/debugging complex interrupt-service routines since
>>> you can execute the interrupt with a software 'INT' instruction
>>> (with the correct offset from the IRQ you are using). The software
>>> doesn't 'know' where the interrupt came from, HW or SW.
>>
>> I'm looking at 2.6.13-rc6-git1 line 182 of entry.S and I don't see it.
>> Must be a different kernel.
>>
>> According to the documentation that I was looking at, a trap in x86 does
>> _not_ turn off interrupts.
>>
> ...
>>
>> I don't see a sti here.
>

Search for sysenter_entry. This is where the stack is switched
to the kernel stack. Then the code falls through past the
next label, sysenter_past_esp. The very next instruction
after the kernel stack has been set is 'sti'. Clear as day.

>
>> -- Steve
>
>
> He is RBJ, Richard B. Johnson, the LKML defacto official troll.
>
> --
> Coywolf Qi Hunt
> http://ahbl.org/~coywolf/
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.12 on an i686 machine (5537.79 BogoMips).
Warning : 98.36% of all statistics are fiction.
.
I apologize for the following. I tried to kill it with the above dot :

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