[PATCH v3] misra: deviate intentionally unreachable code

[Dmytro Prokopchuk1 <dmytro_prokopchuk1@xxxxxxxx>]

From: Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>

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@xxxxxxxxxxx>
Signed-off-by: Dmytro Prokopchuk <dmytro_prokopchuk1@xxxxxxxx>
---
Changes in v3:
- in subject "automation/eclair" replaced with "misra" (to fit in 50 chars)
- fixed formatting (aligned length chars per line)
- "deemed as" replaced with "considered"

Link to v2:
https://patchew.org/Xen/6e47d071ffdb236642c1e9a70118a86d41487aa0.1753909082.git.dmytro._5Fprokopchuk1@xxxxxxxx/
---
 automation/eclair_analysis/ECLAIR/deviations.ecl |  5 +++++
 docs/misra/deviations.rst                        | 11 +++++++++++
 docs/misra/rules.rst                             |  9 +++++++++
 3 files changed, 25 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..af7a17f96c 100644
--- a/docs/misra/deviations.rst
+++ b/docs/misra/deviations.rst
@@ -86,6 +86,17 @@ 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..d9fd92160b 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 no-return due to calls to the 
`ASSERT_UNREACHABLE()'
+           macro in debug build configurations are not considered 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