Re: v4.10: kernel stack frame pointer .. has bad value (null)

[Andy Lutomirski]

On Tue, Mar 7, 2017 at 9:52 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Mar 7, 2017 at 9:38 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>>
>> So I'm thinking we should have -maccumulate-outgoing-args always enabled
>> on x86_32 just like we already do on x86_64.
>
> Ugh. I realize we have workarounds for bugs, but I think
> -maccumulate-outgoing-args is nasty. It just generates worse code by
> avoiding the much nicer push/pop sequences, afaik.
>
> On x86-64 it's not such a big deal, because we pass the first six
> arguments in registers anyway, so the arguments on the stack is a
> fairly unusual special case.
>
> But on x86-32, we only have three argument registers, so this
> braindamage is potentially worse.
>
> I guess we already do this in most situations due to the gcc bugs, but
> I do think it's sad that we would do it for our _own_ bugs too.
>

Is it our bug or a gcc bug?  I would have thought
-fno-omit-frame-pointer meant that the call-frame-to-return-address
offset should be constant and -fomit-frame-pointer meant "do
whatever".

Also, maybe I'm missing something, but does gcc's code even allow the
function to return sensibly?  It could do it by a nasty calculation
involving backing out the old esp from edi, but that seems quite
overcomplicated.

--Andy