Re: [Eclair false positive] Re: [PATCH] x86/msr: Rework wrmsr_safe() using asm goto()

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

On 2025-05-21 21:43, Andrew Cooper wrote:
> On 21/05/2025 8:21 pm, Nicola Vetrini wrote:
> > On 2025-05-21 20:00, Andrew Cooper wrote:
> > > On 21/05/2025 3:36 pm, Andrew Cooper wrote:
> > > > diff --git a/xen/arch/x86/include/asm/msr.h
> > > > b/xen/arch/x86/include/asm/msr.h
> > > > index 0d3b1d637488..4c4f18b3a54d 100644
> > > > --- a/xen/arch/x86/include/asm/msr.h
> > > > +++ b/xen/arch/x86/include/asm/msr.h
> > > > @@ -69,20 +69,20 @@ static inline void wrmsr_ns(uint32_t msr,
> > > > uint32_t lo, uint32_t hi)
> > > >  /* wrmsr with exception handling */
> > > >  static inline int wrmsr_safe(unsigned int msr, uint64_t val)
> > > >  {
> > > > -    int rc;
> > > > -    uint32_t lo, hi;
> > > > -    lo = (uint32_t)val;
> > > > -    hi = (uint32_t)(val >> 32);
> > > > -
> > > > -    __asm__ __volatile__(
> > > > -        "1: wrmsr\n2:\n"
> > > > -        ".section .fixup,\"ax\"\n"
> > > > -        "3: movl %5,%0\n; jmp 2b\n"
> > > > -        ".previous\n"
> > > > -        _ASM_EXTABLE(1b, 3b)
> > > > -        : "=&r" (rc)
> > > > -        : "c" (msr), "a" (lo), "d" (hi), "0" (0), "i" (-EFAULT));
> > > > -    return rc;
> > > > +    uint32_t lo = val, hi = val >> 32;
> > > > +
> > > > +    asm_inline goto (
> > > > +        "1: wrmsr\n\t"
> > > > +        _ASM_EXTABLE(1b, %l[fault])
> > > > +        :
> > > > +        : "a" (lo), "c" (msr), "d" (hi)
> > > > +        :
> > > > +        : fault );
> > > > +
> > > > +    return 0;
> > > > +
> > > > + fault:
> > > > +    return -EFAULT;
> > > >  }
> > > It turns out this is the first piece of Eclair-scanned code using asm
> > > goto.
> > >
> > > https://gitlab.com/xen-project/hardware/xen-staging/-/jobs/10108558677
> > > (The run also contained an equivalent change to xsetbv())
> > >
> > > We're getting R1.1 and R2.6 violations.
> > >
> > > R1.1 complains about [STD.adrslabl] "label address" not being valid 
> > > C99.
> > > R2.6 complains about unused labels.
> > >
> > > I expect this means that Eclair doesn't know how to interpret asm 
> > > goto()
> > > yet.  The labels listed are reachable from inside the asm block.
> > >
> > > From a qualification point of view, this allows for some extensive
> > > optimisations dropping emitted code.
> > Hi Andrew,
> >
> > R1.1 is easy to fix, I'll send a patch myself. On R2.6 you are
> > probably right. I suggest marking the rule not clean to unblock while
> > we investigate. It should not be hard to fix this FP.
> I've not committed the patch yet, so staging is still green.
>
> But, it occurs to me that this is not the first asm goto() to be 
> scanned
> by Eclair.  There's wrmsr_amd_safe() in amd.c (c/s b3d8b3e3f3aa from 
> ~6w
> ago) using exactly the same pattern, which has been passing just fine.
>
> Clearly something else is relevant in terms of triggering violations.
>
> ~Andrew
I think the reason it's simply that file being out of scope due to being 
imported from Linux (whether that is still true I don't know), which is 
unfortunate. We ought to revise that list 
(docs/misra/exclude-list.json).
--
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253