[llvm-dev] Mapping InlineAsm parameters to ConstraintInfoVector elements

[Alexander Potapenko via llvm-dev]

Sorry, I've stated the problem incorrectly.
Turns out that I'm seeing false positives exactly because MSan fails to see
that the inline assembly routine initializes its outputs.
E.g. for functions like __test_and_set_bit() in the kernel, there's a local
variable holding the return value:

static __always_inline
<https://elixir.bootlin.com/linux/latest/ident/__always_inline> bool
__test_and_set_bit
<https://elixir.bootlin.com/linux/latest/ident/__test_and_set_bit>(long
nr <https://elixir.bootlin.com/linux/latest/ident/nr>, volatile
<https://elixir.bootlin.com/linux/latest/ident/volatile> unsigned long
*addr){
	bool oldbit;

	asm <https://elixir.bootlin.com/linux/latest/ident/asm>("bts %2,%1"
	    CC_SET <https://elixir.bootlin.com/linux/latest/ident/CC_SET>(c)
	    : CC_OUT <https://elixir.bootlin.com/linux/latest/ident/CC_OUT>(c)
(oldbit), ADDR <https://elixir.bootlin.com/linux/latest/ident/ADDR>
	    : "Ir" (nr <https://elixir.bootlin.com/linux/latest/ident/nr>));
	return oldbit;}

, which the tool treats as uninitialized, unless we explicitly set it to
false before calling asm().


On Fri, Mar 16, 2018 at 11:43 PM Evgenii Stepanov <eugeni.stepanov at gmail.com>
wrote:

> Could you provide an example where MSan checks an output parameter?
>
> On Fri, Mar 16, 2018 at 9:53 AM, Alexander Potapenko via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hi all,
> >
> > I'm trying to figure out which parameters of a given InlineAsm
> instruction
> > are its inputs, and which are the outputs (rationale: make sure MSan
> > doesn't check the output parameters of an asm() statement).
> >
> > As far as I understand, this information is only available through the
> > ConstraintInfoVector for the InlineAsm. However there's no exact match
> > between the constraints and the InlineAsm params. In addition to clobber
> > constraints, which should be safe to ignore, there can be multiple
> > occurrences  of a single parameter having different constraints. Also,
> for
> > InlineAsm instructions returning non-void values there's one additional
> > constraint, but it's unclear whether it always goes first.
> >
> > My current implementation is quite ad-hoc, so I'm wondering if there's
> any
> > code that already does what I'm trying to achieve.
> >
> > Thanks in advance,
> > Alexander
> >
> > --
> > Alexander Potapenko
> > Software Engineer
> >
> > Google Germany GmbH
> > Erika-Mann-Straße, 33
> > 80636 München
> >
> > Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
> > Registergericht und -nummer: Hamburg, HRB 86891
> > Sitz der Gesellschaft: Hamburg
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>


-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Halimah DeLaine Prado
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20180319/bd25d825/attachment.html>