Re: [PATCH v1 2/3] xen: x86: irq: initialize irq desc in create_irq()

[Stewart Hildebrand <stewart.hildebrand@amd.com>]

On 3/27/25 05:00, Nicola Vetrini wrote:
> On 2025-03-27 09:37, Nicola Vetrini wrote:
>> On 2025-03-27 09:03, Jan Beulich wrote:
>>> On 27.03.2025 01:40, Volodymyr Babchuk wrote:
>>>> While building xen with GCC 14.2.1 with "-fcondition-coverage" option,
>>>> the compiler produces a false positive warning:
>>>>
>>>>   arch/x86/irq.c: In function ‘create_irq’:
>>>>   arch/x86/irq.c:281:11: error: ‘desc’ may be used uninitialized [-Werror=maybe-uninitialized]
>>>>     281 |     ret = init_one_irq_desc(desc);
>>>>         |           ^~~~~~~~~~~~~~~~~~~~~~~
>>>>   arch/x86/irq.c:269:22: note: ‘desc’ was declared here
>>>>     269 |     struct irq_desc *desc;
>>>>         |                      ^~~~
>>>>   cc1: all warnings being treated as errors
>>>>   make[2]: *** [Rules.mk:252: arch/x86/irq.o] Error 1
>>>>
>>>> While we have signed/unsigned comparison both in "for" loop and in
>>>> "if" statement, this still can't lead to use of uninitialized "desc",
>>>> as either loop will be executed at least once, or the function will
>>>> return early. So this is a clearly false positive warning. Anyways,
>>>> initialize "desc" with NULL to make GCC happy.
>>>>
>>>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
>>>
>>> Hmm, this puts us in an interesting conflict, I think. Misra, aiui, will ...
>>>
>>>> --- a/xen/arch/x86/irq.c
>>>> +++ b/xen/arch/x86/irq.c
>>>> @@ -265,7 +265,7 @@ void __init clear_irq_vector(int irq)
>>>>  int create_irq(nodeid_t node, bool grant_access)
>>>>  {
>>>>      int irq, ret;
>>>> -    struct irq_desc *desc;
>>>> +    struct irq_desc *desc = NULL;
>>>
>>> ... consider such an assignment useless (and hence potentially confusing)
>>> code. I'm curious what BugsEng folks are going to say here.
>>>
> 
> Just to mention it: having a "do { } while" loop instead of a for (just out of context) probably avoid tripping gcc's false positive and also help with MISRA Rule 9.1 without needing an explicit initializer.
> 
>>
>> It is quite odd to see this only in coverage builds, but the side effects of coverage options might trigger some of gcc's internal analyzer thresholds. Anyway, since there are no concerns about dead code (see https://gitlab.com/xen-project/xen/-/blob/staging/docs/misra/deviations.rst: R2.2, "There shall be no dead code", is globally deviated) and that this might actually be beneficial to remove some caution reports for R9.1 ("The value of an object with automatic storage duration shall not be read before it has been set") I think the overall effect is positive.

I tried running an "-Og default for debug builds" change through CI, and
I ran into *almost* the same error with -Og and certain version(s) of
GCC:

arch/x86/irq.c: In function 'create_irq':
arch/x86/irq.c:298:25: error: 'desc' may be used uninitialized [-Werror=maybe-uninitialized]
  298 |         desc->arch.used = IRQ_UNUSED;
arch/x86/irq.c:268:22: note: 'desc' was declared here
  268 |     struct irq_desc *desc;
      |                      ^~~~

The do { } while loop Nicola suggested indeed fixes it in the case of
-Og:

diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
index dd8d921f18f6..812f9eb91453 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -267,12 +267,18 @@ int create_irq(nodeid_t node, bool grant_access)
     int irq, ret;
     struct irq_desc *desc;
 
-    for (irq = nr_irqs_gsi; irq < nr_irqs; irq++)
+    irq = nr_irqs_gsi;
+
+    if ( irq >= nr_irqs )
+        return -1;
+
+    do
     {
         desc = irq_to_desc(irq);
         if (cmpxchg(&desc->arch.used, IRQ_UNUSED, IRQ_RESERVED) == IRQ_UNUSED)
            break;
-    }
+        irq++;
+    } while ( irq < nr_irqs );
 
     if (irq >= nr_irqs)
          return -ENOSPC;