From owner-freebsd-hackers  Wed Apr 24 19: 4: 5 2002
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Date: Wed, 24 Apr 2002 22:02:12 -0400 (EDT)
From: Kenneth Culver <culverk@alpha.yumyumyum.org>
To: Peter Wemm <peter@wemm.org>
Cc: John Baldwin <jhb@FreeBSD.ORG>,
	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 
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Alright, so I got tired of trying to figure out if glibc is doing
something wierd or wrong so I downloaded the source for it, and I'm
looking at it now... (for version 2.2.2 which is what we have on FreeBSD's
linux_base-7) and here's what I'm seeing:

pushl %ebp
	pushl %ebx
	pushl %esi
	pushl %edi

	movl OFFLO(%esp), %edx
	movl OFFHI(%esp), %ecx
	testl $0xfff, %edx
	jne L(einval)
	shrdl $12, %ecx, %edx		/* mmap2 takes the offset in
pages.  */
	shrl $12, %ecx
	jne L(einval)
	movl %edx, %ebp

So above I'm seeing the offset arg get into register %ebp, which is what
we expect...

	movl ADDR(%esp), %ebx
	movl LEN(%esp), %ecx
	movl PROT(%esp), %edx
	movl FLAGS(%esp), %esi
	movl FD(%esp), %edi

Then I'm seeing all the other args getting put into the registers they
belong in... (which matches up with our linux_prepsyscall() function)

	movl $SYS_ify(mmap2), %eax	/* System call number in %eax.  */

	/* Do the system call trap.  */
L(do_syscall):
	int $0x80

Now I'm seeing the int 0x80 trap....

	/* Restore registers.  */
	popl %edi
	popl %esi
	popl %ebx
	popl %ebp


So, as far as I can tell, this version of glibc is doing the Right Thing,
and the ebp register is getting messed up somewhere along the line in
either the assembly code that handles the 0x80 trap in FreeBSD, or in
syscall2 (I think it's probably the asm that handles the 0x80 trap)...

Can anyone confirm this?

Ken


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