From: Daniel Borkmann <dan...@iogearbox.net>
Date: Thu, 11 May 2017 01:53:15 +0200
> Shubham was recently asking on netdev why in arm64 JIT we don't multiply
> the index for accessing the tail call map by 8. That led me into testing
> out arm64 JIT wrt tail calls and it turned out I got a NULL pointer
> dereference on the tail call.
...
> Fix this by emitting the following instead:
>
> [...]
> 00000060: d2800e0a mov x10, #0x70 // #112
> 00000064: 8b0a002a add x10, x1, x10
> 00000068: d37df04b lsl x11, x2, #3
> 0000006c: f86b694b ldr x11, [x10,x11]
> 00000070: b40000ab cbz x11, 0x00000084
> [...]
>
> This basically adds the offset to ptrs to the base address of the bpf
> array we got and we later on access the map with an index * 8 offset
> relative to that. The tail call map itself is basically one large area
> with meta data at the head followed by the array of prog pointers.
> This makes tail calls working again, tested on Cavium ThunderX ARMv8.
>
> Fixes: ddb55992b04d ("arm64: bpf: implement bpf_tail_call() helper")
> Reported-by: Shubham Bansal <illusionist....@gmail.com>
> Signed-off-by: Daniel Borkmann <dan...@iogearbox.net>
Applied and queued up for -stable, thanks.
