BPF loads with BPF_PROBE_MEM(SX) can load from unsafe pointers and the
JIT adds an exception table entry for the JITed instruction which allows
the exeption handler to set the destination register of the load to zero
and continue execution from the next instruction.

As all arm64 instructions are AARCH64_INSN_SIZE size, the exception
handler can just increment the pc by AARCH64_INSN_SIZE without needing
the exact address of the instruction following the the faulting
instruction.

Simplify the exception table usage in arm64 JIT by only saving the
destination register in ex->fixup and drop everything related to
the fixup_offset. The fault handler is modified to add AARCH64_INSN_SIZE
to the pc.

Signed-off-by: Puranjay Mohan <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Acked-by: Kumar Kartikeya Dwivedi <[email protected]>
Acked-by: Xu Kuohai <[email protected]>

Add selftests for testing the reporting of arena page faults through BPF
streams. Two new bpf programs are added that read and write to an
unmapped arena address and the fault reporting is verified in the
userspace through streams.

The added bpf programs need to access the user_vm_start in struct
bpf_arena, this is done by casting &arena to struct bpf_arena *, but
barrier_var() is used on this ptr before accessing ptr->user_vm_start;
to stop GCC from issuing an out-of-bound access due to the cast from
smaller map struct to larger "struct bpf_arena"

Other tests related to streams have been converted to use __stderr in
progs/stream.c directly and test_stream_errors() in prog_tests/stream.c
has been repurposed to validate the arena fault address printed in the
stderr stream. This can't be directly validated using __stderr because
the address is dynamic.

Signed-off-by: Puranjay Mohan <[email protected]>