arm64: bpf: make BPF prologue and epilogue align with ARM64 AAPCS
authorYang Shi <yang.shi@linaro.org>
Mon, 16 Nov 2015 22:35:35 +0000 (14:35 -0800)
committerDavid S. Miller <davem@davemloft.net>
Tue, 17 Nov 2015 19:44:39 +0000 (14:44 -0500)
commitec0738db8d7a9bd6653d9f9d81b201a519144d63
treeab7bd5c2ac7e695b3c463ee4fb21a816be409d50
parente639b8d8a7a728f0b05ef2df6cb6b45dc3d4e556
arm64: bpf: make BPF prologue and epilogue align with ARM64 AAPCS

Save and restore FP/LR in BPF prog prologue and epilogue, save SP to FP
in prologue in order to get the correct stack backtrace.

However, ARM64 JIT used FP (x29) as eBPF fp register, FP is subjected to
change during function call so it may cause the BPF prog stack base address
change too.

Use x25 to replace FP as BPF stack base register (fp). Since x25 is callee
saved register, so it will keep intact during function call.
It is initialized in BPF prog prologue when BPF prog is started to run
everytime. Save and restore x25/x26 in BPF prologue and epilogue to keep
them intact for the outside of BPF. Actually, x26 is unnecessary, but SP
requires 16 bytes alignment.

So, the BPF stack layout looks like:

                                 high
         original A64_SP =>   0:+-----+ BPF prologue
                                |FP/LR|
         current A64_FP =>  -16:+-----+
                                | ... | callee saved registers
                                +-----+
                                |     | x25/x26
         BPF fp register => -80:+-----+
                                |     |
                                | ... | BPF prog stack
                                |     |
                                |     |
         current A64_SP =>      +-----+
                                |     |
                                | ... | Function call stack
                                |     |
                                +-----+
                                  low

CC: Zi Shen Lim <zlim.lnx@gmail.com>
CC: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Yang Shi <yang.shi@linaro.org>
Acked-by: Zi Shen Lim <zlim.lnx@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
arch/arm64/net/bpf_jit_comp.c