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


On 01/09/2016 12:53 AM, Will Deacon wrote:
On Fri, Jan 08, 2016 at 02:36:32PM +0900, AKASHI Takahiro wrote:
On 01/07/2016 11:56 PM, Richard Earnshaw (lists) wrote:
On 07/01/16 14:22, Will Deacon wrote:
On Thu, Dec 24, 2015 at 04:57:54PM +0900, AKASHI Takahiro wrote:
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,

My main issue with this is that we cannot rely on the frame layout
generated by the compiler and there's little point in asking for
commitment here. Therefore, the heuristics will need updating as and
when we identify new frames that we can't handle. That's pretty fragile
and puts us on the back foot when faced with newer compilers. This might
be sustainable if we don't expect to encounter much variation, but even
that would require some sort of "buy-in" from the various toolchain

GCC already has an option (-fstack-usage) to determine the stack usage
on a per-function basis and produce a report at build time. Why can't
we use that to provide the information we need, rather than attempt to
compute it at runtime based on your analyser?

If -fstack-usage is not sufficient, understanding why might allow us to
propose a better option.

Can you not use the dwarf frame unwind data?  That's always sufficient
to recover the CFA (canonical frame address - the value in SP when
executing the first instruction in a function).  It seems to me it's
unlikely you're going to need something that's an exceedingly high
performance operation.

Thank you for your comment.
Yeah, but we need some utility routines to handle unwind data(.debug_frame).
In fact, some guy has already attempted to merge (part of) libunwind into
the kernel[1], but it was rejected by the kernel community (including Linus
if I correctly remember). It seems that they thought the code was still buggy.

The ARC guys seem to have sneaked something in for their architecture:

so it might not be impossible if we don't require all the bells and
whistles of libunwind.

Thanks. I didn't notice this code.

That is one of reasons that I wanted to implement my own analyzer.

I still don't understand why you can't use fstack-usage. Can you please
tell me why that doesn't work? Am I missing something?

I don't know how gcc calculates the usage here, but I guess it would be more
robust than my analyzer.

The issues, that come up to my mind, are
- -fstack-usage generates a separate output file, *.su and so we have to
  manage them to be incorporated in the kernel binary.
  This implies that (common) kernel makefiles might have to be a bit changed.
- more worse, what if kernel module case? We will have no way to let the kernel
  know the stack usage without adding an extra step at loading.

If we need to put some information about stack usage in the kernel, that should
not be much different from dwarf frame data (.eh_frame).

-Takahiro AKASHI


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