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Date: Mon, 31 Mar 2008 19:23:11 +0200
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To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
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Subject: Re: [9fans] getcallerpc on arm7
Topicbox-Message-UUID: 85cd73b2-ead3-11e9-9d60-3106f5b1d025

> On architectures that don't push the return address on the
> stack during the call instruction, the caller of getcallerpc
> will have saved the desired return address somewhere in
> its stack frame, and getcallerpc must root it out.

It appears the implementation below
works well with arm-elf-gcc on arm7tdmi:

	/*
	 * Functions generated by arm-elf-gcc start
	 * like this (always?):
	 * 	mov	ip, sp
	 *	stmdb	sp!, {fp, ip, lr, pc}
	 *
	 * (The first argument, which is still in r0,
	 * is pushed later, when its address is taken so
	 * that it can be provided to getcallerpc.)
	 *
	 * To get the value of `lr', the return address,
	 * walk up the stack, starting with x, until a
	 * value *u is found which is the previous stack
	 * pointer, i.e. which equals the address u+3.
	 * In that case, the return address is expected
	 * to be in u[1].
	 *
	 * If no such address is found, return an invalid value.
	 */
	ulong
	getcallerpc(void *x)
	{
		ulong *u;
		int	i;

		u = (ulong*)x;
		for (i=0; i<128; i++, u++)
			if (*u == (ulong)(u+3))
				return u[1];
		return ~0;
	}

A helpful document was http://www.cems.uwe.ac.uk/~cduffy/es/5.ppt,
to get an idea of how function arguments are provided
and the stack is managed by the gcc on arm.

If the stack contains a value which matches the
condition for `ip', but which is not ip, getcallerpc
will find it before the real ip/lr, and report a
garbage address.

Michael