linux-kernel - Re: [RFC PATCH 3/3] objtool/mcount: Add powerpc specific functions

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Message-ID: <87mth9ezml.fsf@mpe.ellerman.id.au>
Date:   Tue, 29 Mar 2022 23:01:38 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "aik@...abs.ru" <aik@...abs.ru>,
        Sathvika Vasireddy <sv@...ux.ibm.com>,
        "naveen.n.rao@...ux.vnet.ibm.com" <naveen.n.rao@...ux.vnet.ibm.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 3/3] objtool/mcount: Add powerpc specific functions

Josh Poimboeuf <jpoimboe@...hat.com> writes:
> On Sun, Mar 27, 2022 at 09:09:20AM +0000, Christophe Leroy wrote:
>> Second point is the endianess and 32/64 selection, especially when 
>> crossbuilding. There is already some stuff regarding endianess based on 
>> bswap_if_needed() but that's based on constant selection at build time 
>> and I couldn't find an easy way to set it conditionaly based on the 
>> target being built.
>>
>> Regarding 32/64 selection, there is almost nothing, it's based on using 
>> type 'long' which means that at the time being the target and the build 
>> platform must both be 32 bits or 64 bits.
>> 
>> For both cases (endianess and 32/64) I think the solution should 
>> probably be to start with the fileformat of the object file being 
>> reworked by objtool.
>
> Do we really need to detect the endianness/bitness at runtime?  Objtool
> is built with the kernel, why not just build-in the same target
> assumptions as the kernel itself?

I don't think we need runtime detection. But it will need to support
basically most combinations of objtool running as 32-bit/64-bit LE/BE
while the kernel it's analysing is 32-bit/64-bit LE/BE.

>> What are current works in progress on objtool ? Should I wait Josh's 
>> changes before starting looking at all this ? Should I wait for anything 
>> else ?
>
> I'm not making any major changes to the code, just shuffling things
> around to make the interface more modular.  I hope to have something
> soon (this week).  Peter recently added a big feature (Intel IBT) which
> is already in -next.
>
> Contributions are welcome, with the understanding that you'll help
> maintain it ;-)
>
> Some years ago Kamalesh Babulal had a prototype of objtool for ppc64le
> which did the full stack validation.  I'm not sure what ever became of
> that.

>From memory he was starting to clean the patches up in late 2019, but I
guess that probably got derailed by COVID. AFAIK he never posted
anything. Maybe someone at IBM has a copy internally (Naveen?).

> FWIW, there have been some objtool patches for arm64 stack validation,
> but the arm64 maintainers have been hesitant to get on board with
> objtool, as it brings a certain maintenance burden.  Especially for the
> full stack validation and ORC unwinder.  But if you only want inline
> static calls and/or mcount then it'd probably be much easier to
> maintain.

I would like to have the stack validation, but I am also worried about
the maintenance burden.

I guess we start with mcount, which looks pretty minimal judging by this
series, and see how we go from there.

cheers