Re: [PATCH v6 1/3] xen/arm: add support for run_in_exception_handler()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.01.2021 10:50, Julien Grall wrote:
> Hi Jan,
> 
> On 21/01/2021 07:55, Jan Beulich wrote:
>> On 20.01.2021 19:20, Julien Grall wrote:
>>> On 16/01/2021 10:33, Juergen Gross wrote:
>>>> Add support to run a function in an exception handler for Arm. Do it
>>>> as on x86 via a bug_frame, but pass the function pointer via a
>>>> register (this needs to be done that way, because inline asm support
>>>> for 32-bit Arm lacks the needed functionality to reference an
>>>> arbitrary function via the bugframe).
>>>
>>> I was going to commit the series, but then realized the commit message
>>> and comment needs some tweaking because technically GCC is supporting
>>> 'i' (I managed to get it working with -fno-pie).
>>>
>>> So how about:
>>>
>>> "This is needed to be done that way because GCC complains about invalid
>>> constraint when using a function pointer with "i" and PIE is enabled
>>> (default on most of GCC shipped with distro). Clang happily accepts it,
>>> so it may be a bug in GCC."
>>
>> May I ask why you think it's a bug in gcc? I'd rather consider it
>> a bug in clang. Taking the address of a symbol doesn't yield a
>> constant in PIC or PIE code, aiui.
> 
> I consider it a bug because we were using it as:
> 
> .pushsection .bug.frame
> 2b:
> .long (%0 - 2b)
> 
> So I expect the compiler to be able to find the displacement in both 
> cases as we don't need to know the exact address.
> 
> I think Clang is just clever enough to figure out we want a displacement.

If they take apart the contents of the asm(), this may be possible,
yes. (Did you try with -no-integrated-as, btw?) But taking apart
the asm() is a very risky game a compiler would play, as it may
easily break the programmer's intentions (or still fail to recognize
whether the use is okay, for the containing construct being too
complex).

> Do you have a suggestion of constraint that could resolve the issue?

No. Don't use -fpie (but I guess that's not an option for other
reasons).

Jan