From: Nick Desaulniers <ndesaulniers@google.com>
To: Julien Thierry <jthierry@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
swine@google.com, linux-efi <linux-efi@vger.kernel.org>,
Kees Cook <keescook@chromium.org>,
clang-built-linux <clang-built-linux@googlegroups.com>,
Peter Zijlstra <peterz@infradead.org>,
Catalin Marinas <catalin.marinas@arm.com>,
Masahiro Yamada <masahiroy@kernel.org>,
yonghyun@google.com, LKML <linux-kernel@vger.kernel.org>,
Michal Marek <michal.lkml@markovi.net>,
raphael.gault@arm.com, Mark Brown <broonie@kernel.org>,
linux-hardening@vger.kernel.org,
Josh Poimboeuf <jpoimboe@redhat.com>,
Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Bill Wendling <morbo@google.com>
Subject: Re: [RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect switch table on arm64
Date: Tue, 2 Feb 2021 15:01:22 -0800 [thread overview]
Message-ID: <CAKwvOdkqWyDbAvMJAd6gkc2QAEL7DiZg6_uRJ6NUE4tCip4Jvw@mail.gmail.com> (raw)
In-Reply-To: <671f1aa9-975e-1bda-6768-259adbdc24c8@redhat.com>
On Tue, Feb 2, 2021 at 12:57 AM Julien Thierry <jthierry@redhat.com> wrote:
>
>
>
> On 2/2/21 12:17 AM, Nick Desaulniers wrote:
> > On Mon, Feb 1, 2021 at 1:44 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >>
> >> On Fri, Jan 29, 2021 at 10:10:01AM -0800, Nick Desaulniers wrote:
> >>> On Wed, Jan 27, 2021 at 3:27 PM Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> >>>>
> >>>> On Wed, Jan 27, 2021 at 02:15:57PM -0800, Nick Desaulniers wrote:
> >>>>>> From: Raphael Gault <raphael.gault@arm.com>
> >>>>>>
> >>>>>> This plugins comes into play before the final 2 RTL passes of GCC and
> >>>>>> detects switch-tables that are to be outputed in the ELF and writes
> >>>>>> information in an ".discard.switch_table_info" section which will be
> >>>>>> used by objtool.
> >>>>>>
> >>>>>> Signed-off-by: Raphael Gault <raphael.gault@arm.com>
> >>>>>> [J.T.: Change section name to store switch table information,
> >>>>>> Make plugin Kconfig be selected rather than opt-in by user,
> >>>>>> Add a relocation in the switch_table_info that points to
> >>>>>> the jump operation itself]
> >>>>>> Signed-off-by: Julien Thierry <jthierry@redhat.com>
> >>>>>
> >>>>> Rather than tightly couple this feature to a particular toolchain via
> >>>>> plugin, it might be nice to consider what features could be spec'ed out
> >>>>> for toolchains to implement (perhaps via a -f flag).
> >>>>
> >>>> The problem is being able to detect switch statement jump table vectors.
> >>>>
> >>>> For a given indirect branch (due to a switch statement), what are all
> >>>> the corresponding jump targets?
> >>>>
> >>>> We would need the compiler to annotate that information somehow.
> >>>
> >>> Makes sense, the compiler should have this information. How is this
> >>> problem solved on x86?
> >>
> >> Thus far we've been able to successfully reverse engineer it on x86,
> >> though it hasn't been easy.
> >>
> >> There were some particulars for arm64 which made doing so impossible.
> >> (I don't remember the details.)
>
> The main issue is that the tables for arm64 have more indirection than x86.
I wonder if PAC or BTI also make this slightly more complex? PAC at
least has implications for unwinders, IIUC.
>
> On x86, the dispatching jump instruction fetches the target address from
> a contiguous array of addresses based on a given offset. So the list of
> potential targets of the jump is neatly organized in a table (and sure,
> before link time these are just relocation, but still processable).
>
> On arm64 (with GCC at least), what is stored in a table is an array of
> candidate offsets from the jump instruction. And because arm64 is
> limited to 32bit instructions, the encoding often requires multiple
> instructions to compute the target address:
>
> ldr<*> x_offset, [x_offsets_table, x_index, ...] // load offset
> adr x_dest_base, <addr> // load target branch for offset 0
> add x_dest, x_target_base, x_offset, ... // compute final address
> br x_dest // jump
>
> Where this gets trickier is that (with GCC) the offsets stored in the
> table might or might not be signed constants (and this can be seen in
> GCC intermediate representations, but I do not believe this information
> is output in the final object file). And on top of that, GCC might
> decide to use offsets that are seen as unsigned during intermediate
> representation as signed offset by sign extending them in the add
> instruction.
>
> So, to handle this we'd have to track the different operation done with
> the offset, from the load to the final jump, decoding the instructions
> and deducing the potential target instructions from the table of offsets.
>
> But that is error prone as we don't really know how many instructions
> can be between the ones doing the address computation, and I remember
> some messy case of a jump table inside a jump table where tracking the
> instruction touching one or the other offset would need a lot of corner
> case handling.
>
> And this of course is just for GCC, I haven't looked at what it all
> looks like on Clang's end.
Sure, but this is what production unwinders do, and they don't require
compiler plugins, right? I don't doubt unwinders can be made simpler
with changes to toolchain output; please work with your compiler
vendor on making such changes rather than relying on compiler plugins
to do so.
> > I think the details are pertinent to finding a portable solution. The
> > commit message of this commit in particular doesn't document such
> > details, such as why such an approach is necessary or how the data is
> > laid out for objtool to consume it.
> >
>
> Sorry, I will need to make that clearer. The next patch explains it a
> bit [1]
>
> Basically, for simplicity, the plugin creates a new section containing
Right, this takes a focus on simplicity, at the cost of alienating a toolchain.
Ard's point about 3193c0836f20 relating to -fgcse is that when
presented with tricky cases to unwind, the simplest approach is taken.
There it was disabling a compiler specific compiler optimization, here
it's either a compiler specific compiler plugin (or disabling another
compiler optimization). The pattern seems to be "Objtool isn't smart
enough" ... "compiler optimization disabled" or "compiler plugin
dependency."
> tables (one per jump table) of references to the jump targets, similar
> to what x86 has, except that in this case this table isn't actually used
> by runtime code and is discarded at link time. I only chose this to
> minimize what needed to be changed in objtool and because the format
> seemed simple enough.
>
> But I'm open on some alternative, whether it's a -fjump-table-info
Yes, I think we could spec out something like that. But I would
appreciate revisiting open questions around stack validation (frame
pointers), preventing the generation of jump tables to begin with
(-fno-jump-tables) in place of making objtool more robust, or
generally the need to depend on compiler plugins.
> option added to compilers with a different format to do the links. The
> important requirement is to be able to know all the candidate targets
> for a "br <reg>" instruction.
>
> [1] https://lkml.org/lkml/2021/1/20/910
>
> Thanks,
>
> --
> Julien Thierry
>
--
Thanks,
~Nick Desaulniers
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next prev parent reply other threads:[~2021-02-02 23:02 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-20 17:37 [RFC PATCH 00/17] objtool: add base support for arm64 Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 01/17] tools: Add some generic functions and headers Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 02/17] tools: arm64: Make aarch64 instruction decoder available to tools Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 03/17] tools: bug: Remove duplicate definition Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 04/17] objtool: arm64: Add base definition for arm64 backend Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 05/17] objtool: arm64: Decode add/sub instructions Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 06/17] objtool: arm64: Decode jump and call related instructions Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 07/17] objtool: arm64: Decode other system instructions Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 08/17] objtool: arm64: Decode load/store instructions Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 09/17] objtool: arm64: Decode LDR instructions Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 10/17] objtool: arm64: Accept padding in code sections Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 11/17] efi: libstub: Ignore relocations for .discard sections Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect switch table on arm64 Julien Thierry
2021-01-27 22:15 ` Nick Desaulniers
2021-01-27 23:26 ` Josh Poimboeuf
2021-01-29 18:10 ` Nick Desaulniers
2021-02-01 21:44 ` Josh Poimboeuf
2021-02-01 23:17 ` Nick Desaulniers
2021-02-02 0:02 ` Josh Poimboeuf
2021-02-02 14:24 ` David Laight
2021-02-02 22:33 ` Nick Desaulniers
2021-02-02 23:36 ` Josh Poimboeuf
2021-02-02 23:52 ` Nick Desaulniers
2021-02-02 8:57 ` Julien Thierry
2021-02-02 23:01 ` Nick Desaulniers [this message]
2021-02-03 0:14 ` Josh Poimboeuf
2021-02-03 11:57 ` Peter Zijlstra
2021-02-03 13:04 ` Mark Brown
2021-02-03 13:58 ` Mark Rutland
2021-02-03 8:11 ` Julien Thierry
2021-02-09 16:30 ` Daniel Kiss
2021-01-20 17:37 ` [RFC PATCH 13/17] objtool: arm64: Implement functions to add switch tables alternatives Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 14/17] objtool: arm64: Cache section with switch table information Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 15/17] objtool: arm64: Handle supported relocations in alternatives Julien Thierry
2021-01-20 17:37 ` [RFC PATCH 16/17] objtool: arm64: Ignore replacement section for alternative callback Julien Thierry
2021-01-20 17:38 ` [RFC PATCH 17/17] objtool: arm64: Enable stack validation for arm64 Julien Thierry
2021-01-21 9:03 ` [RFC PATCH 00/17] objtool: add base support " Ard Biesheuvel
2021-01-21 10:26 ` Julien Thierry
2021-01-21 11:08 ` Ard Biesheuvel
2021-01-21 11:23 ` Peter Zijlstra
2021-01-21 11:48 ` Ard Biesheuvel
2021-01-21 18:54 ` Josh Poimboeuf
2021-01-22 17:43 ` Mark Brown
2021-01-22 17:54 ` Ard Biesheuvel
2021-01-28 22:10 ` Madhavan T. Venkataraman
2021-01-29 15:47 ` Mark Brown
2021-01-22 21:15 ` Madhavan T. Venkataraman
2021-01-22 21:43 ` Ard Biesheuvel
2021-01-22 21:44 ` Madhavan T. Venkataraman
2021-01-25 21:19 ` Josh Poimboeuf
2021-01-22 21:16 ` Madhavan T. Venkataraman
2021-01-21 13:23 ` Julien Thierry
2021-01-21 14:23 ` Mark Brown
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