Re: [PATCH net] bpf, arm64: fix faulty emission of map access in tail calls

From: Daniel Borkmann <daniel@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 <redacted>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Applied and queued up for -stable, thanks.