[v1] automation/eclair: deviate intentionally unreachable code

[PATCH] automation/eclair: deviate intentionally unreachable code

Posted by Dmytro Prokopchuk1 3 months ago

From: Nicola Vetrini <nicola.vetrini@bugseng.com>

MISRA C Rule 2.1 states: "A project shall not contain unreachable code".
Functions that are non-returning and are not explicitly annotated with
the `noreturn' attribute are considered a violation of this rule.

In certain cases, some functions might be non-returning in debug build
configuration (when `NDEBUG' is not defined), due to calls to
`__builtin_unreachable' in the expansion of the macro `ASSERT_UNREACHABLE()'.

Conversely, in non-debug (release) builds (when `NDEBUG' is defined),
the macro `ASSERT_UNREACHABLE()' expands to an empty construct
(`do { } while (0)'), which does not affect the execution flow.
This allows such functions to return normally in release builds,
avoiding unreachable code.

To account for that in debug build, the `noreturn` property of
`__builtin_unreachable` is overridden in the ECLAIR configuration
to deviate these violations.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@epam.com>
---
Test CI pipeline:
https://gitlab.com/xen-project/people/dimaprkp4k/xen/-/pipelines/1957211653
---
 automation/eclair_analysis/ECLAIR/deviations.ecl | 5 +++++
 docs/misra/deviations.rst                        | 8 ++++++++
 docs/misra/rules.rst                             | 9 +++++++++
 3 files changed, 22 insertions(+)

diff --git a/automation/eclair_analysis/ECLAIR/deviations.ecl b/automation/eclair_analysis/ECLAIR/deviations.ecl
index 483507e7b9..8a05e17dac 100644
--- a/automation/eclair_analysis/ECLAIR/deviations.ecl
+++ b/automation/eclair_analysis/ECLAIR/deviations.ecl
@@ -36,6 +36,11 @@ not executable, and therefore it is safe for them to be unreachable."
 -config=MC3A2.R2.1,reports+={deliberate, "any_area(any_loc(file(C_runtime_failures)))"}
 -doc_end
 
+-doc_begin="Calls to function `__builtin_unreachable' in the expansion of macro
+`ASSERT_UNREACHABLE()' are not considered to have the `noreturn' property."
+-call_properties+={"name(__builtin_unreachable)&&stmt(begin(any_exp(macro(name(ASSERT_UNREACHABLE)))))", {"noreturn(false)"}}
+-doc_end
+
 -doc_begin="Proving compliance with respect to Rule 2.2 is generally impossible:
 see https://arxiv.org/abs/2212.13933 for details. Moreover, peer review gives us
 confidence that no evidence of errors in the program's logic has been missed due
diff --git a/docs/misra/deviations.rst b/docs/misra/deviations.rst
index e78179fcb8..fba75be2ee 100644
--- a/docs/misra/deviations.rst
+++ b/docs/misra/deviations.rst
@@ -86,6 +86,14 @@ Deviations related to MISRA C:2012 Rules:
        generate definitions for asm modules.
      - Tagged as `deliberate` for ECLAIR.
 
+   * - R2.1
+     - Calls to the `__builtin_unreachable` function inside the expansion of
+       the `ASSERT_UNREACHABLE()` macro may cause a function to be marked as
+       non-returning. Since this only happens in debug configurations,
+       the `noreturn` property for `__builtin_unreachable` is overridden in
+       these contexts, resulting in the absence of reports that do not have
+       an impact on safety, despite being true positives.
+
    * - R2.2
      - Proving compliance with respect to Rule 2.2 is generally impossible:
        see `<https://arxiv.org/abs/2212.13933>`_ for details. Moreover, peer
diff --git a/docs/misra/rules.rst b/docs/misra/rules.rst
index 3e014a6298..74badcb616 100644
--- a/docs/misra/rules.rst
+++ b/docs/misra/rules.rst
@@ -124,6 +124,15 @@ maintainers if you want to suggest a change.
            they are used to generate definitions for asm modules
          - Declarations without initializer are safe, as they are not
            executed
+         - Functions that are noreturn due to calls to `ASSERT_UNREACHABLE`
+           macro in debug build configurations are not reported as violations::
+
+              static inline bool
+              arch_vcpu_ioreq_completion(enum vio_completion completion)
+              {
+                  ASSERT_UNREACHABLE();
+                  return false;
+              }
 
    * - `Rule 2.6 <https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_02_06.c>`_
      - Advisory
-- 
2.43.0

Re: [PATCH] automation/eclair: deviate intentionally unreachable code

Posted by Jan Beulich 3 months ago

On 30.07.2025 16:06, Dmytro Prokopchuk1 wrote:
> From: Nicola Vetrini <nicola.vetrini@bugseng.com>
> 
> MISRA C Rule 2.1 states: "A project shall not contain unreachable code".
> Functions that are non-returning and are not explicitly annotated with
> the `noreturn' attribute are considered a violation of this rule.
> 
> In certain cases, some functions might be non-returning in debug build
> configuration (when `NDEBUG' is not defined), due to calls to
> `__builtin_unreachable' in the expansion of the macro `ASSERT_UNREACHABLE()'.
> 
> Conversely, in non-debug (release) builds (when `NDEBUG' is defined),
> the macro `ASSERT_UNREACHABLE()' expands to an empty construct
> (`do { } while (0)'), which does not affect the execution flow.
> This allows such functions to return normally in release builds,
> avoiding unreachable code.

While this way of putting it is technically correct as long as all we have
is

#ifndef CONFIG_DEBUG
#define NDEBUG
#endif

in xen/config.h, but I think it would be better if "debug builds"
(controlled by CONFIG_DEBUG) were properly separated from assertions
being active (NDEBUG). For quite some time there has been the plan to
decouple the two.

Similarly, throughout: You write ASSERT_UNREACHABLE() (i.e. including
the parentheses), yet oddly you then don't similarly include them when
referring to __builtin_unreachable().

> --- a/docs/misra/rules.rst
> +++ b/docs/misra/rules.rst
> @@ -124,6 +124,15 @@ maintainers if you want to suggest a change.
>             they are used to generate definitions for asm modules
>           - Declarations without initializer are safe, as they are not
>             executed
> +         - Functions that are noreturn due to calls to `ASSERT_UNREACHABLE`
> +           macro in debug build configurations are not reported as violations::

I find "reported" odd to appear in this file. My take is that here we
describe our intentions, not what some tool may be doing. I'd suggest
"deemed" as replacement.

Jan

> +              static inline bool
> +              arch_vcpu_ioreq_completion(enum vio_completion completion)
> +              {
> +                  ASSERT_UNREACHABLE();
> +                  return false;
> +              }
>  
>     * - `Rule 2.6 <https://gitlab.com/MISRA/MISRA-C/MISRA-C-2012/Example-Suite/-/blob/master/R_02_06.c>`_
>       - Advisory

Re: [PATCH] automation/eclair: deviate intentionally unreachable code

Posted by Andrew Cooper 3 months ago

On 30/07/2025 3:06 pm, Dmytro Prokopchuk1 wrote:
> diff --git a/docs/misra/deviations.rst b/docs/misra/deviations.rst
> index e78179fcb8..fba75be2ee 100644
> --- a/docs/misra/deviations.rst
> +++ b/docs/misra/deviations.rst
> @@ -86,6 +86,14 @@ Deviations related to MISRA C:2012 Rules:
>         generate definitions for asm modules.
>       - Tagged as `deliberate` for ECLAIR.
>  
> +   * - R2.1
> +     - Calls to the `__builtin_unreachable` function inside the expansion of
> +       the `ASSERT_UNREACHABLE()` macro may cause a function to be marked as
> +       non-returning. Since this only happens in debug configurations,
> +       the `noreturn` property for `__builtin_unreachable` is overridden in
> +       these contexts, resulting in the absence of reports that do not have
> +       an impact on safety, despite being true positives.
> +

I'm not sure how best to phrase this, but it's probably worth saying
that Xen expects developers to write code which would fail safe in a
release build when the assertion was removed.

I.e. it's more than just "there may be code there".  It's expected that
there is.

~Andrew

Re: [PATCH] automation/eclair: deviate intentionally unreachable code

Posted by Nicola Vetrini 3 months ago

On 2025-07-30 16:15, Andrew Cooper wrote:
> On 30/07/2025 3:06 pm, Dmytro Prokopchuk1 wrote:
>> diff --git a/docs/misra/deviations.rst b/docs/misra/deviations.rst
>> index e78179fcb8..fba75be2ee 100644
>> --- a/docs/misra/deviations.rst
>> +++ b/docs/misra/deviations.rst
>> @@ -86,6 +86,14 @@ Deviations related to MISRA C:2012 Rules:
>>         generate definitions for asm modules.
>>       - Tagged as `deliberate` for ECLAIR.
>> 
>> +   * - R2.1
>> +     - Calls to the `__builtin_unreachable` function inside the 
>> expansion of
>> +       the `ASSERT_UNREACHABLE()` macro may cause a function to be 
>> marked as
>> +       non-returning. Since this only happens in debug 
>> configurations,
>> +       the `noreturn` property for `__builtin_unreachable` is 
>> overridden in
>> +       these contexts, resulting in the absence of reports that do 
>> not have
>> +       an impact on safety, despite being true positives.
>> +
> 
> I'm not sure how best to phrase this, but it's probably worth saying
> that Xen expects developers to write code which would fail safe in a
> release build when the assertion was removed.
> 
> I.e. it's more than just "there may be code there".  It's expected that
> there is.
> 

Yes, I had some trouble finding the proper wording here, so feel free to 
improve it. It's just to highlight that the code would be truly 
unreachable in debug builds, so an assessor expecting violations there 
won't find them because of this configuration.

> ~Andrew

-- 
Nicola Vetrini, B.Sc.
Software Engineer
BUGSENG (https://bugseng.com)
LinkedIn: https://www.linkedin.com/in/nicola-vetrini-a42471253