Subject: Re: [PATCH net] bpf, arm64: use separate register for state in stxr

From: Will Deacon <[email-masked]>
Date: Wed, 7 Jun 2017 12:51:31 +0100

Hi Daniel,

On Wed, Jun 07, 2017 at 01:45:37PM +0200, Daniel Borkmann wrote:
> Will reported that in BPF_XADD we must use a different register in stxr
> instruction for the status flag due to otherwise CONSTRAINED UNPREDICTABLE
> behavior per architecture. Reference manual says [1]:
> 
>   If s == t, then one of the following behaviors must occur:
> 
>    * The instruction is UNDEFINED.
>    * The instruction executes as a NOP.
>    * The instruction performs the store to the specified address, but
>      the value stored is UNKNOWN.
> 
> Thus, use a different temporary register for the status flag to fix it.
> 
> Disassembly extract from test 226/STX_XADD_DW from test_bpf.ko:
> 
>   [...]
>   0000003c:  c85f7d4b  ldxr x11, [x10]
>   00000040:  8b07016b  add x11, x11, x7
>   00000044:  c80c7d4b  stxr w12, x11, [x10]
>   00000048:  35ffffac  cbnz w12, 0x0000003c
>   [...]
> 
>   [1] https://static.docs.arm.com/ddi0487/b/DDI0487B_a_armv8_arm.pdf, p.6132
> 
> Fixes: 85f68fe89832 ("bpf, arm64: implement jiting of BPF_XADD")
> Reported-by: Will Deacon <[email-masked]>
> Signed-off-by: Daniel Borkmann <[email-masked]>
> ---
>  arch/arm64/net/bpf_jit_comp.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)

Cheers for fixing this up:

Acked-by: Will Deacon <[email-masked]>

Will