Re: [PATCH net-next] ebpf: Allow dereferences of PTR_TO_STACK registers

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Thu, Jul 23, 2015 at 02:24:40PM -0700, Alex Gartrell wrote:
>         mov %rsp, %r1           ; r1 = rsp
>         add $-8, %r1            ; r1 = rsp - 8
>         store_q $123, -8(%rsp)  ; *(u64*)r1 = 123  <- valid
>         store_q $123, (%r1)     ; *(u64*)r1 = 123  <- previously invalid
>         mov $0, %r0
>         exit                    ; Always need to exit
> 
> And we'd get the following error:
> 
> 	0: (bf) r1 = r10
> 	1: (07) r1 += -8
> 	2: (7a) *(u64 *)(r10 -8) = 999
> 	3: (7a) *(u64 *)(r1 +0) = 999
> 	R1 invalid mem access 'fp'
> 
> 	Unable to load program
> 
> We already know that a register is a stack address and the appropriate
> offset, so we should be able to validate those references as well.
> 
> Signed-off-by: Alex Gartrell <agartrell@fb.com>
> ---
>  kernel/bpf/verifier.c       |  6 ++++-
>  samples/bpf/test_verifier.c | 59 +++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 64 insertions(+), 1 deletion(-)

Looks good.
Acked-by: Alexei Starovoitov <ast@plumgrid.com>

> +			BPF_ST_MEM(BPF_DW, BPF_REG_1, 2, 0xfaceb00c),

nice constants :)