Re: [RFC PATCH 12/17] gcc-plugins: objtool: Add plugin to detect switch table on arm64

[Julien Thierry <jthierry@redhat.com>]

On 2/3/21 12:01 AM, Nick Desaulniers wrote:
> 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 there is a small confusion. The plugin nor the data it generates 
is not to be used by a kernel unwinder. It's here to allow objtool to 
assess whether the code being checked can be unwound (?) reliably (not 
omitting functions). Part of this is checking that a branch/jump in a 
function does not end up in some code that is not related to the 
function without setting up a call frame.

This is about static validation rather than functionality.

>>> 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.
> 

I'll give it a try at least for the arm64 side.

Thanks,

-- 
Julien Thierry


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