RE: Linux process ABI broken in 2.6?

=20

quoted

On Fri, 2005-12-09 at 00:07 +0100, Tjernlund wrote:

quoted

Seems like ppc32 kernel pass the application entry point address
in r7 and MSR in r8 when starting the application. The=20

source might be

quoted

quoted

ret_from_syscall, in entry.S:=20
	...
	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.
=20
Is this on purpose and why?
=20
Secion 8.4.1 in
=20

http://refspecs.freestandards.org/LSB_3.1.0/LSB-Core-PPC32/LSB
-Core-PPC32.html#PROCESSINITIALIZATION

quoted

quoted

=20
says:
"Contrary to what is stated in the Registers part of=20

chapter 3 of the System V Application Binary Interface=20
PowerPC Processor

quoted

quoted

Supplement there are no values set in registers r3, r4,=20

r5, r6 and r7. Instead the values specified to appear in all of those

quoted

quoted

registers except r7 are placed on the stack. The value to=20

be placed into register r7, the termination function pointer=20
is not passed

quoted

quoted

to the process."
=20
How do one not pass a termination function in r7 other=20

than setting

quoted

quoted

r7 to zero?

Just ignore those registers on entry. The semantics of a=20

syscall are to

quoted

clobber all volatile registers and there is no point doing anything
else.

Ben.

=20
[Sorry to break the thread, but I mailed from the wrong=20
account which I can't reach ATM]
=20
Sure, but this syscall is a bit special as it has to prepare=20
the runtime env. for the new process.
Not having any defined value for register at process startup feels ..
=20
An undefined r7 register makes it hard to differ between=20
dynamic and static apps in crt1.
I now understand why glibc has its own non standard=20
__libc_start function and I really don't
want that in uClibc. Is it too much trouble to define r7 to=20
always have a zero
value at process startup?
=20
As far as I can tell 2.4 has all unused registers zeroed.

The only way to pass any info from ld.so to crt is to mess with the
stack, write a NULL word
before argc and set the stack ptr to point to it. Then let crt test for
a null word, if null
then crt will know that this is a dynamic app, if not then it is a
static app with no rtld_fini
function.

I feel the messing with the stack in this way not ideal, so I would like
to see a better
exec ABI where at least one register,r7, will be zero.

The fix is trivial, just swap r7 to r9 in the above assembler code.

Is that feasible?

 Jocke