[uClinux-dev] _vfork libc implementation on ARM

[David McCullough]

Jivin Shaun Jackman lays it down ...
> Would someone please explain to me how vfork is meant to prevent the
> child from smashing the parent's stack? I am writing an ARM Linux
> implementation for the newlib libc, and vfork has me mystified.
> 
> The system call number is passed in r7 on Thumb. r7 must be preserved.
> So, I push r7 onto the stack.
> 
> _vfork:
> 	push { r7 }
> 	mov r7, #SYS_vfork
> 	swi
> 	pop { r7 }
> 	b _set_errno /* Tail call. */
> 
> _vfork returns for the first time, and the user process calls either
> execve or _exit. Either of these function calls is going to clobber
> the stack, where I've carefully preserved r7.
> 
> My conclusion is that I cannot save r7 on the stack. So, I could
> implement vfork as an assembler macro that clobbers r7, but I'd rather
> it be a linkable function like all the other system calls. Or, save r7
> in some statically allocated piece of memory, but this doesn't sound
> very thread-safe. Although, I'd have to think a bit more about how
> threads and vfork interact. Or, save r7 in one of the clobberable
> registers, such as r1-r3, and depend on the kernel not clobbering that
> register.
> 
> _vfork:
> 	mov r2, r7
> 	mov r7, #SYS_vfork
> 	swi
> 	mov r7, r2
> 	b _set_errno /* Tail call. */
> 
> I'm leaning towards this final idea. How is this usually handled?

Have a look at uClibc/glibc is the best bet.  Looks something like this:

	vfork:
		swi     __NR_vfork
		...

Cheers,
Davidm

-- 
David McCullough,  david_mccullough@securecomputing.com,   Ph:+61 734352815
Secure Computing - SnapGear  http://www.uCdot.org http://www.cyberguard.com