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

>>> On 14.01.19 at 22:18, <sstabellini@xxxxxxxxxx> wrote:
> Hi Jan,
> One question below to make a decision on the way forward.
> On Mon, 14 Jan 2019, Jan Beulich wrote:
>> >>> On 14.01.19 at 04:45, <Stewart.Hildebrand@xxxxxxxxxxxxxxx> wrote:
>> > So let's keep the linker-accessible variable as a type that works for the
>> > linker (which really could be anything as long as you use the address, not
>> > the value), but name it something else - a name that screams "DON'T USE ME
>> > UNLESS YOU KNOW WHAT YOU'RE DOING". And then before the first use, copy
>> > that value to "uintptr_t _start;".
>> > 
>> > The following is a quick proof of concept for aarch64. I changed the type
>> > of _start and _end, and added code to copy the linker-assigned value to
>> > _start and _end. Upon booting, I see the correct values:
>> Global symbols starting with underscores should already be shouting
>> enough. But what's worse - the whole idea if using array types is to
>> avoid the intermediate variables.
>> > --- a/xen/arch/arm/setup.c
>> > +++ b/xen/arch/arm/setup.c
>> > @@ -726,6 +726,12 @@ static void __init setup_mm(unsigned long dtb_paddr, 
> size_t dtb_size)
>> >  
>> >  size_t __read_mostly dcache_line_bytes;
>> >  
>> > +typedef char TYPE_DOESNT_MATTER;
>> > +extern TYPE_DOESNT_MATTER _start_linker_assigned_dont_use_me,
>> > +                          _end_linker_assigned_dont_use_me;
>> This and ...
>> > @@ -770,10 +776,17 @@ void __init start_xen(unsigned long boot_phys_offset,
>> >      printk("Command line: %s\n", cmdline);
>> >      cmdline_parse(cmdline);
>> >  
>> > +    _start = (uintptr_t)&_start_linker_assigned_dont_use_me;
>> > +    _end = (uintptr_t)&_end_linker_assigned_dont_use_me;
>> ... this violates what the symbol names say. And if you want to
>> avoid issues, you'd want to keep out of C files uses of those
>> symbols altogether anyway, and you easily can: In any
>> assembly file, have
>> _start:      .long _start_linker_assigned_dont_use_me
>> _end:        .long _end_linker_assigned_dont_use_me
>> In particular, they don't need to be runtime initialized, saving
>> you from needing to set them before first use. But as said -
>> things are the way they are precisely to avoid such variables.
>> >> But, instead of converting _start to unsigned long via SYMBOL_HIDE, we
>> >> could convert it to uintptr_t instead, it would be a trivial change on
>> >> top of the existing unsigned long series. Not sure if it is beneficial.
>> > 
>> > The difference would be whether we want to rely on implementation-defined
>> > behavior or not.
>> Why not? Simply specify that compilers with implementation defined
>> behavior not matching our expectations are unsuitable. And btw, I
>> suppose this is just the tiny tip of the iceberg of our reliance on
>> implementation defined behavior.
> The reason is that relying on undefined behavior is not reliable, it is
> not C compliant, it is not allowed by MISRA-C, and not guaranteed to
> work with any compiler.

"undefined behavior" != "implementation defined behavior"

> 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. 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). 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).


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