arm64: bpf: Annotate JITed code for BTI

arm64: bpf: Annotate JITed code for BTI

In order to extend the protection offered by BTI to all code executing in
kernel mode we need to annotate JITed BPF code appropriately for BTI. To
do this we need to add a landing pad to the start of each BPF function and
also immediately after the function prologue if we are emitting a function
which can be tail called. Jumps within BPF functions are all to immediate
offsets and therefore do not require landing pads.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20200506195138.22086-6-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>