Re: [PATCH 7/7] DWARF: add the config option

From	"H.J. Lu" <hjl.tools@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH 7/7] DWARF: add the config option
Date	2017-05-09 18:50 +0200
Message-ID	<tFguJ-4li-5@gated-at.bofh.it> (permalink)
References	(5 earlier) <tF2i5-3ix-7@gated-at.bofh.it> <tF3e9-3Xg-1@gated-at.bofh.it> <tF4ad-4v9-3@gated-at.bofh.it> <tFafD-hL-3@gated-at.bofh.it> <tFeMi-37Z-5@gated-at.bofh.it>
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On Tue, May 9, 2017 at 7:58 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> On Tue, May 09, 2017 at 03:00:45AM -0700, hpa@zytor.com wrote:
>> I'm, ahem, highly skeptical to creating our own unwinding data format
>> unless there is *documented, supported, and tested* way to force the
>> compiler to *automatically* fall back to frame pointers any time there
>> may be complexity involved, which at a very minimum includes any kind
>> of data-dependent manipulation of the stack pointer.
>
> That would be nice.  But isn't falling back to a frame pointer (or
> another callee-saved reg or a stack location) already needed in such
> cases?  Otherwise how could DWARF unwinding work?
>
>> Otherwise you will have to fail the kernel build when your static tool
>> runs into instruction sequences it can't deal with, but the compiler
>> may generate them - boom.
>
> Failing the build is harsh, we could just warn about it and skip the
> data for the affected function(s).
>
> BTW, there is another option.  Instead of generating the data from
> scratch, we could just convert gcc's DWARF CFI to the format we need.
>
> However that wouldn't solve the problems we have with the holes and
> inaccuracies in DWARF from our hand-annotated asm, inline asm, and
> special sections (extable, alternatives, etc).  We'd still have to rely
> on objtool for that, so we'd still be in the same boat of needing
> objtool to be able to follow gcc code paths.

CFI directives are documented in GNU assembler manual.  They
store unwind info in .eh_frame section.  They work well with assembly
codes in glibc.  But I don't know how well they work with kernel unwind.

> So yes, it sucks that objtool needs to work for unwinding to work.  But
> if we want decent DWARF-esque unwinding, I don't see any way around
> that due to the low-level nature of the kernel.
>
>> Worse, your tool will not even recognize the problem and you're in a
>> worse place than when you started.
>
> We could have a runtime NMI-based stack checker which ensures it can
> always unwind to the bottom of the stack.  Over time this would
> hopefully provide full validation of the unwinder data and
> functionality.
>
> --
> Josh



-- 
H.J.

Thread

[PATCH 7/7] DWARF: add the config option Jiri Slaby <jslaby@suse.cz> - 2017-05-05 14:30 +0200
  Re: [PATCH 7/7] DWARF: add the config option Linus Torvalds <torvalds@linux-foundation.org> - 2017-05-05 22:00 +0200
    Re: [PATCH 7/7] DWARF: add the config option Ingo Molnar <mingo@kernel.org> - 2017-05-06 09:30 +0200
      Re: [PATCH 7/7] DWARF: add the config option Jiri Slaby <jslaby@suse.cz> - 2017-05-10 09:50 +0200
    Re: [PATCH 7/7] DWARF: add the config option Jiri Kosina <jikos@kernel.org> - 2017-05-06 16:30 +0200
    Re: [PATCH 7/7] DWARF: add the config option Ingo Molnar <mingo@kernel.org> - 2017-05-07 23:50 +0200
      Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-07 23:50 +0200
        Re: [PATCH 7/7] DWARF: add the config option Vojtech Pavlik <vojtech@suse.com> - 2017-05-08 10:20 +0200
          Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-08 15:20 +0200
      Re: [PATCH 7/7] DWARF: add the config option hpa@zytor.com - 2017-05-08 00:10 +0200
    Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-07 23:50 +0200
      Re: [PATCH 7/7] DWARF: add the config option Andy Lutomirski <luto@amacapital.net> - 2017-05-08 07:40 +0200
        Re: [PATCH 7/7] DWARF: add the config option Ingo Molnar <mingo@kernel.org> - 2017-05-08 08:20 +0200
        Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-08 16:50 +0200
          Re: [PATCH 7/7] DWARF: add the config option hpa@zytor.com - 2017-05-08 21:10 +0200
      Re: [PATCH 7/7] DWARF: add the config option Andy Lutomirski <luto@kernel.org> - 2017-05-09 02:30 +0200
        Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-09 03:40 +0200
          Re: [PATCH 7/7] DWARF: add the config option Andy Lutomirski <luto@kernel.org> - 2017-05-09 04:40 +0200
            Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-09 05:40 +0200
              Re: [PATCH 7/7] DWARF: add the config option hpa@zytor.com - 2017-05-09 12:10 +0200
                Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-09 17:00 +0200
                Re: [PATCH 7/7] DWARF: add the config option "H.J. Lu" <hjl.tools@gmail.com> - 2017-05-09 18:50 +0200
                Re: [PATCH 7/7] DWARF: add the config option Jiri Slaby <jslaby@suse.cz> - 2017-05-10 10:20 +0200
                Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-10 15:10 +0200
                Re: [PATCH 7/7] DWARF: add the config option "H.J. Lu" <hjl.tools@gmail.com> - 2017-05-10 18:30 +0200
      Re: [PATCH 7/7] DWARF: add the config option Jiri Kosina <jikos@kernel.org> - 2017-05-09 20:50 +0200
        Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-09 21:30 +0200
          Re: [PATCH 7/7] DWARF: add the config option Jiri Slaby <jslaby@suse.cz> - 2017-05-10 10:40 +0200
            Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-10 15:20 +0200
    Re: [PATCH 7/7] DWARF: add the config option Jiri Slaby <jslaby@suse.cz> - 2017-05-10 09:50 +0200
      Re: [PATCH 7/7] DWARF: add the config option Josh Poimboeuf <jpoimboe@redhat.com> - 2017-05-10 14:50 +0200
        Re: [PATCH 7/7] DWARF: add the config option Jiri Slaby <jslaby@suse.cz> - 2017-05-10 14:50 +0200
      Re: [PATCH 7/7] DWARF: add the config option Linus Torvalds <torvalds@linux-foundation.org> - 2017-05-10 20:20 +0200

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