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From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Tjernlund <tjernlund@tjernlund.se>
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Date: Fri, 09 Dec 2005 13:05:00 +1100
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Cc: Linuxppc-Dev <linuxppc-dev@ozlabs.org>, 'Linh Dang' <linhd@nortel.com>,
	uclibc@uclibc.org
Subject: Re: Linux process ABI broken in 2.6?
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On Fri, 2005-12-09 at 00:07 +0100, Tjernlund wrote:
> Seems like ppc32 kernel pass the application entry point address
> in r7 and MSR in r8 when starting the application. The source might be
> ret_from_syscall, in entry.S: 
> 	...
> 	lwz	r7,_NIP(r1)
>  	lwz	r8,_MSR(r1)
>  	FIX_SRR1(r8, r0)
>  	lwz	r2,GPR2(r1)
> 	lwz	r1,GPR1(r1)
> 	mtspr	SPRN_SRR0,r7
> 	mtspr	SPRN_SRR1,r8
> 	SYNC
> 	RFI
> I am not convinced this is the source, but a non zero r7
> breaks static apps in uClibc.
> 
> Is this on purpose and why?
> 
> Secion 8.4.1 in
> http://refspecs.freestandards.org/LSB_3.1.0/LSB-Core-PPC32/LSB-Core-PPC32.html#PROCESSINITIALIZATION
> 
> says:
> "Contrary to what is stated in the Registers part of chapter 3 of the System V Application Binary Interface PowerPC Processor
> Supplement there are no values set in registers r3, r4, r5, r6 and r7. Instead the values specified to appear in all of those
> registers except r7 are placed on the stack. The value to be placed into register r7, the termination function pointer is not passed
> to the process."
> 
> How do one not pass a termination function in r7 other than setting
> r7 to zero?

Just ignore those registers on entry. The semantics of a syscall are to
clobber all volatile registers and there is no point doing anything
else.

Ben.