linux-kernel - Re: [PATCH v4 2/2] x86/purgatory: Make sure we fail the build if purgatory.ro has missing symbols

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Date:   Fri, 13 Mar 2020 11:47:41 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>,
        Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] x86/purgatory: Make sure we fail the build if
 purgatory.ro has missing symbols

Hi,

On 3/13/20 5:42 AM, Arvind Sankar wrote:
> On Thu, Mar 12, 2020 at 01:50:39PM +0100, Borislav Petkov wrote:
>> On Thu, Mar 12, 2020 at 12:58:24PM +0100, Hans de Goede wrote:
>>> My version of this patch has already been tested this way. It is
>>
>> Tested with kexec maybe but if the 0day bot keeps finding breakage, that
>> ain't good enough.
>>
>>> 1. Things are already broken, my patch just exposes the brokenness
>>> of some configs, it is not actually breaking things (well it breaks
>>> the build, changing a silent brokenness into an obvious one).
>>
>> As I already explained, that is not good enough.
>>
>>> 2. I send out the first version of this patch on 7 October 2019, it
>>> has not seen any reaction until now. So I'm sending out new versions
>>> quickly now that this issue is finally getting some attention...
>>
>> And that is never the right approach.
>>
>> Maintainers are busy as hell so !urgent stuff gets to wait. Spamming
>> them with more patchsets does not help - fixing stuff properly does.
>>
>> So, to sum up: if Arvind's approach is the better one, then we should do
>> that and s390 should be fixed this way too. And all tested. And we will
>> remove the hurry element from it all since it has not been noticed so
>> far so it is not urgent and we can take our time and fix it properly.
>>
>> Ok?
>>
>> Thx.
>>
>> -- 
>> Regards/Gruss,
>>      Boris.
>>
>> https://people.kernel.org/tglx/notes-about-netiquette
> 
> If I could try to summarize the situation here:
> - the purgatory requires filtering out certain CFLAGS/other settings set
>    for the generic kernel in order to work correctly
> - the patch proposed by Hans de Goede will detect missing filters at
>    build time rather than when kexec is executed
> - the filtering is currently not perfect as demonstrated by issues that
>    0day bot is finding -- but the patchset will find these problems at
>    build time rather than runtime
> - there might be a slight optimization as proposed by me [1] but it
>    might have problems as in [2] even if it seems to work
> 
> I think the patch as of v5 [0] is useful right now, to catch CFLAGS
> additions that aren't currently being filtered correctly. The real
> problem is that there exist CFLAGS that should be used for all source
> files in the kernel, and there are CFLAGS (eg tracing, stack check etc)
> that should only be used for the kernel proper. For special
> compilations, such as boot stubs, vdso's, purgatory we should have the
> generic CFLAGS but not the kernel-proper CFLAGS. The issue currently is
> that these special compilations need to filter out all the flags added
> for kernel-proper, and this is a moving target as more tracing/sanity
> flags get added.  Neither the solution of simply re-initializing CFLAGS
> (which will miss generic CFLAGS) nor trying to filter out CFLAGS (which
> will miss new kernel-proper CFLAGS) works very well. I think ideally
> splitting these into independent variables, i.e. BASE_FLAGS that can be
> used for everything, and KERNEL_FLAGS only to be used for the kernel
> proper is likely eventually the better solution, rather than conflating
> both into KBUILD_CFLAGS.
> 
> But to move forward incrementally, patch v5 is probably the cleanest. My
> suggestion in [1] I'm thinking is changing things significantly for
> kexec, by changing the purgatory from a relocatable object file into an
> actual executable, and might have knock-on implications that need to be
> reviewed and tested carefully before it can be merged, as shown by [2].
> 
> [0] https://lore.kernel.org/lkml/20200312114951.56009-1-hdegoede@redhat.com/
> [1] https://lore.kernel.org/lkml/20200312001006.GA170175@rani.riverdale.lan/
> [2] https://lore.kernel.org/lkml/20200312182322.GA506594@rani.riverdale.lan/

Thank you for taking such a detailed look at this.

Given your concerns about the alternative fix you proposed I agree that
it is best to just move forward with my v5 patch-set for now; and then we
can try to improve the missing-symbols check in the future.

Regards,

Hans