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Re: [PATCH] i386: Don't use frame pointer without stack access
On Mon, Aug 7, 2017 at 3:48 PM, Michael Matz <matz@suse.de> wrote:
> Hi,
>
> On Mon, 7 Aug 2017, H.J. Lu wrote:
>
>> >> [hjl@gnu-tools-1 pr81736]$
>> >>
>> >> Does it mean clang is broken?
>> >
>> > In my book, yes.
>>
>> Does GCC do this for all targets or just x86?
>
> No idea. If so I'd say those other targets are broken as well (as long as
> the concept of frame pointer makes sense on them, their ABI defines one
> but leaves it optional and something like an unwinder could make use of
> it).
>
>> I am looking for a run-time test which breaks unwinder.
>
> I don't have one handy. Idea: make two threads, one endlessly looping in
> the "frame-less" function, the other causing a signal to the first thread,
> and the signal handler checking that unwinding up to caller of
> frame_less() is possible via %[er]bp chaining.
OTOH, even if the function doesn't make its own frame, the %[er]bp is
still untouched and points to the calling function frame. So, I think
that even in that case unwinding should be unharmed.
Frame setupsthat HJ presented are really unneeded, so there is a clear benefit.
How about we keep the patch for a while and see if something indeed
breaks? Linux live patching looks quite fragile in this area.
Uros.