[RFC][AArch64] function prologue analyzer in linux kernel

[AKASHI Takahiro <takahiro dot akashi at linaro dot org>]

Hi,

I'm the author of ftrace support on arm64(aarch64) linux. As part of
ftrace, we can utilize "stack tracer" which reports the maximum usage
of kernel stack:

---8<---
# cat /sys/kernel/debug/tracing/stack_max_size
4088
# cat /sys/kernel/debug/tracing/stack_trace
        Depth    Size   Location    (49 entries)
        -----    ----   --------
  0)     4088      16   __local_bh_enable_ip+0x18/0xd8
  1)     4072      32   _raw_read_unlock_bh+0x38/0x48
  2)     4040      32   xs_udp_write_space+0x44/0x50
  3)     4008      32   sock_wfree+0x88/0x90
  4)     3976      32   skb_release_head_state+0x70/0xa0
 [snip]
 44)      808      32   load_elf_binary+0x29c/0x10d0
 45)      776     224   search_binary_handler+0xbc/0x208
 46)      552      96   do_execveat_common.isra.15+0x4e4/0x690
 47)      456     112   SyS_execve+0x4c/0x60
 48)      344     344   el0_svc_naked+0x24/0x28
--->8---

Here, "Depth" (and hence "Size") is determined, after scanning a stack,
by saved fp pointer (more precisely + 0x10) in a stack frame instead
of (not saved) stack pointer. (Please note that arm64 kernel is always
compiled with -fno-omit-frame-pointer.)

As fp is updated after branching into a function, and allocates not only
a function's stack frame but also callee's local variables, using this
saved value of fp as "Depth", or sp of a caller function, is not
appropriate for calculating a stack size of a function.

So I'd like to introduce a function prologue analyzer to determine
a size allocated by a function's prologue and deduce it from "Depth".
My implementation of this analyzer has been submitted to
linux-arm-kernel mailing list[1].
I borrowed some ideas from gdb's analyzer[2], especially a loop of
instruction decoding as well as stop of decoding at exiting a basic block,
but implemented my own simplified one because gdb version seems to do
a bit more than what we expect here.
Anyhow, since it is somewhat heuristic (and may not be maintainable for
a long term), could you review it from a broader viewpoint of toolchain,
please?

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-December/393721.html
[2] aarch64_analyze_prologue() in gdb/aarch64-tdep.c


Thanks,
-Takahiro AKASHI