Subject: Re: [PATCH 3/4] powerpc/boot: Fix stack corruption in epapr entry point

From: Benjamin Herrenschmidt <[email-masked]>
Date: Wed, 11 Feb 2015 13:11:57 +1100

On Tue, 2015-02-10 at 20:15 +0800, Jeremy Kerr wrote:
> Currently, a 64-bit little-endian zImage.epapr won't boot in epapr mode,
> as as we never return from platform_init.
> 
> Before entering C, we initialise our stack by setting r1 16 bytes below
> the end of the _bss_stack:
> 
>   stwu	r0,-16(r1)	/* establish a stack frame */
> 
> However, the called function will save the caller's lr in the caller's
> frame's lr save area, at -16(r1) to -32(r1).
> 
> This means that writes to the fdt variable corrupt the saved link
> register:
> 
>  0000000020c06018 l     O .bss   0000000000001000 _bss_stack
>  0000000020c07018 l     O .bss   0000000000000008 fdt
> 
> This change allocates the minimum of 32 bytes for the base of the stack
> instead.

Is that sufficient for ABI v1 ? It feels like it's broken for BE as
well...

Ben.

> Signed-off-by: Jeremy Kerr <[email-masked]>
> 
> ---
>  arch/powerpc/boot/crt0.S |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/boot/crt0.S b/arch/powerpc/boot/crt0.S
> index 14de4f8..bf8eeee 100644
> --- a/arch/powerpc/boot/crt0.S
> +++ b/arch/powerpc/boot/crt0.S
> @@ -218,7 +218,7 @@ p_base:	mflr	r10		/* r10 now points to runtime addr of p_base */
>  	beq	6f
>  	ld	r1,0(r8)
>  	li	r0,0
> -	stdu	r0,-16(r1)	/* establish a stack frame */
> +	stdu	r0,-32(r1)	/* establish a stack frame */
>  6:
>  #endif  /* __powerpc64__ */
>  	/* Call platform_init() */
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