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 specific build
configurations (when assertions are enabled, i.e., when `NDEBUG' is not defined).
This is due to calls to `__builtin_unreachable()' in the expansion of the
macro `ASSERT_UNREACHABLE()'.

Conversely, in builds where `NDEBUG' is defined (assertions are disabled),
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 such builds,
avoiding unreachable code.

To account for that in specific builds, 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>
---
Changes in v2:
- updated wording according to the reviewers comments

Link to v1:
https://patchew.org/Xen/e699179c079df36f6cb4fdc7865a73cb9fe79f8c.1753881652.git.dmytro._5Fprokopchuk1@epam.com/
---
 automation/eclair_analysis/ECLAIR/deviations.ecl |  5 +++++
 docs/misra/deviations.rst                        | 12 ++++++++++++
 docs/misra/rules.rst                             |  9 +++++++++
 3 files changed, 26 insertions(+)

diff --git a/automation/eclair_analysis/ECLAIR/deviations.ecl b/automation/eclair_analysis/ECLAIR/deviations.ecl
index 483507e7b9..ceecd0093b 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..73ff7c05b1 100644
--- a/docs/misra/deviations.rst
+++ b/docs/misra/deviations.rst
@@ -86,6 +86,18 @@ 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. This behavior occurs only in configurations where
+       assertions are enabled. To address this, 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.
+       Xen expects developers to ensure code remains safe and reliable in
+       builds, even when debug-only assertions like `ASSERT_UNREACHABLE()
+       are removed.
+
    * - 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..13cb3c6206 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 deemed 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

On 30.07.2025 23:01, 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 specific build
> configurations (when assertions are enabled, i.e., when `NDEBUG' is not defined).
> This is due to calls to `__builtin_unreachable()' in the expansion of the
> macro `ASSERT_UNREACHABLE()'.
> 
> Conversely, in builds where `NDEBUG' is defined (assertions are disabled),
> 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 such builds,
> avoiding unreachable code.
> 
> To account for that in specific builds, 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>

Wording-wise I'm okay now (one further nit below), but I don't feel
capable of ack-ing. I'd like to remind you though that commit messages
want to be limited to 75(?) chars per line, so "git log" output wouldn't
go beyond 80 chars.

> --- a/docs/misra/deviations.rst
> +++ b/docs/misra/deviations.rst
> @@ -86,6 +86,18 @@ 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. This behavior occurs only in configurations where
> +       assertions are enabled. To address this, 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.
> +       Xen expects developers to ensure code remains safe and reliable in
> +       builds, even when debug-only assertions like `ASSERT_UNREACHABLE()
> +       are removed.

Formatting nit: I think it would be nice if lines were wrapped consistently,
such that available space is actually properly used. (Right here this means
the "for" on the 5th line could move up, which likely would call for further
re-wrapping of subsequent text. The "are" on the last line apparently also
would still fit on the earlier line.)

> --- 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 deemed as violations::

I'm not a native speaker, but "deemed as" feels wrong to me; I would expect
the "as" wants simply dropping (or replacing by "to be").

Adjustments could likely be done while committing, assuming the necessary
ack appears.

Jan