Re: [Xen-devel] [PATCH v6 1/4] xen: introduce SYMBOL

[Juergen Gross <jgross@xxxxxxxx>]

On 16/01/2019 00:36, Stefano Stabellini wrote:
> On Tue, 15 Jan 2019, Jan Beulich wrote:
>>> Yes, this instance is only the tip of the
>>> iceberg, we have a long road ahead, but we shouldn't really give up
>>> because it is going to be difficult :-) Stewart's approach would
>>> actually be compliant and help toward reducing reliance on undefined
>>> behavior.
>>>
>>> Would you be OK if I rework the series to follow his approach using
>>> intermediate variables? See the attached patch as a reference, it only
>>> "converts" _start and _end as an example. Fortunately, it will be
>>> textually similar to the previous SYMBOL returning unsigned long version
>>> of the series.
>>
>> Well, I've given reasons why I dislike that, and why (I think) it was
>> done without such intermediate variables. Nevertheless, if this is
>> _the only way_ to achieve compliance, I don't think I could
>> reasonably NAK it.
>>
>> The thing that I don't understand though is how the undefined
>> behavior (if there really is any) goes away: Even if you compare
>> the contents of the variables instead of the original (perhaps
>> casted) pointers, in the end you still compare what C would
>> consider pointers to different objects. It's merely a different
>> way of hiding that fact from C.
> 
> I saw that Stewart wrote a long and detailed reply, but this is my short
> take on this. I don't think so: with this approach there are no dubious
> pointers in C land at all[1]. It is perfectly fine to have addresses as
> integers in C, compare and subtracts addresses as integers, then casting
> one of them to a pointer and accessing a structure with the pointer.
> _start becomes only defined and used outside of C. I think both C and
> MISRAC compliance would be satisfied.
> 
> ([1]: There a catch with the way we use the pointers in alternative.c, both
> x86 and arm, but is easy to fix in a follow-up series. Everything else
> is taken care of.)
> 
> 
>> Undefined behavior would imo
>> go away only if those comparisons/subtractions didn't happen
>> in C anymore. IOW - see my .startof.() / .sizeof.() proposal.
>>
>>> If you are OK with it, do you have any suggestions on how would you like
>>> the intermediate variables to be called? I went with _start/start_ and
>>> _end/end_ but I am open to suggestions. Also to which assembly file you
>>> would like the new variables being added -- I created a new one for the
>>> purpose named var.S in the attached example.
>>
>> First of all we should explore whether the variables could also be
>> linker generated, in particular to avoid the current symbols to be
>> global (thus making it impossible to access them from C files in the
>> first place).
> 
> That would be fantastic. I looked around, I found interesting things
> like PROVIDE, but I don't think what you describe is possible. The
> linker scripts only define symbols, they cannot set or define variables.
> 
> 
>> Failing that, I don't think it matters much where these
>> helper symbols live, and hence your choice is probably fine (I'd
>> prefer though if, just like on Arm, the x86 file didn't live in the
>> boot/ subdirectory; in the end it might even be possible to have
>> some of them in xen/common/var.S).
> 
> OK, I'll move the x86 var.S to xen/arch/x86/x86_64. I cannot share var.S
> because arm32 is using long instead of quad.

Have an architecture specific define ASM_UINTPTR (.quad or .long) for
that purpose?


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel