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Date:      Wed, 24 Apr 2002 17:01:00 -0400 (EDT)
From:      John Baldwin <jhb@FreeBSD.org>
To:        Kenneth Culver <culverk@alpha.yumyumyum.org>
Cc:        FreeBSD-CURRENT List <freebsd-current@FreeBSD.ORG>, freebsd-hackers@FreeBSD.ORG, Brandon S Allbery KF8NH <allbery@ece.cmu.edu>, Andrew Gallatin <gallatin@cs.duke.edu>
Subject:   Re: implementing linux mmap2 syscall
Message-ID:  <XFMail.20020424170100.jhb@FreeBSD.org>
In-Reply-To: <20020424161828.R38664-100000@alpha.yumyumyum.org>

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On 24-Apr-2002 Kenneth Culver wrote:
>>
>> Brandon S Allbery KF8NH writes:
>>  > On Wed, 2002-04-24 at 10:41, Andrew Gallatin wrote:
>>  > > Maybe the argument isn't where you expect it to be, but is there.
>>  > > Can you make a test program which calls mmap2 with its 6th arg as
>>  > > something unique like 0xdeadbeef?  Then print out (in hex :) the
>>  > > trapframe
>>  > > from the linux prepsyscall routine & see if you can find the deadbeef.
>>  >
>>  > My recollection is that beyond 5 arguments, a pointer to the remaining
>>  > ones is passed.  (But my recollection may be wrong and I don't wish to
>>  > subject myself to the source cesspool at the moment....)
>>  >
>>
>> I think that's how it used to work.  Apparently, they've changed it
>> recently and they now pass 6 args in registers.  Eg, in the linux
>> kernel sources, old_mmap() fetches its args via copy_from_user(),
>> whereas the newer sys_mmap2() doesn't.
>>
>> Drew
>>
> Yep, according to the docs and source code I looked at in the linux
> kernel, the sixth arg is in the ebp register. I've looked all over the
> FreeBSD kernel and the linux emulator for the place that actually sets
> these, and unfortunately, due to my lack of asm coding knowledge, I can't
> find anything.

libc sets it before it enters the kernel.  Then on kernel entry we save ebp
in the trapframe.

-- 

John Baldwin <jhb@FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/

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