MISRA C deviations for Xen¶

The following is the list of MISRA C:2012 deviations for the Xen codebase that are not covered by a SAF-x-safe or SAF-x-false-positive-<tool> comment, as specified in docs/misra/documenting-violations.rst; the lack of such comments is usually due to the excessive clutter they would bring to the codebase or the impossibility to express such a deviation (e.g., if it’s composed of several conditions).

Deviations related to MISRA C:2012 Directives:¶

Directive identifier	Justification	Notes
D4.3	Accepted for the ARM64 codebase	Tagged as disapplied for ECLAIR on any other violation report.
D4.3	The inline asm in ‘xen/arch/arm/arm64/lib/bitops.c’ is tightly coupled with the surronding C code that acts as a wrapper, so it has been decided not to add an additional encapsulation layer.	Tagged as deliberate for ECLAIR.

Deviations related to MISRA C:2012 Rules:¶

Rule identifier	Justification	Notes
R2.1	The compiler implementation guarantees that the unreachable code is removed. Constant expressions and unreachable branches of if and switch statements are expected.	Tagged as safe for ECLAIR.
R2.1	Unreachability caused by calls to the following functions or macros is deliberate and there is no risk of code being unexpectedly left out.	Tagged as deliberate for ECLAIR. Such macros are: BUG assert_failed __builtin_unreachable ASSERT_UNREACHABLE
R2.1	Pure declarations, that is, declarations without initializations are not executable, and therefore it is safe for them to be unreachable. The most notable example of such a pattern being used in the codebase is that of a variable declaration that should be available in all the clauses of a switch statement.	ECLAIR has been configured to ignore those statements.
R2.2	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 to undetected violations of Rule 2.2, if any. Testing on time behavior gives us confidence on the fact that, should the program contain dead code that is not removed by the compiler, the resulting slowdown is negligible.	Project-wide deviation, tagged as disapplied for ECLAIR.
R3.1	Comments starting with ‘/*’ and containing hyperlinks are safe as they are not instances of commented-out code.	Tagged as safe for ECLAIR.
R5.3	As specified in rules.rst, shadowing due to macros being used as macro arguments is allowed, as it’s deemed not at risk of causing developer confusion.	Tagged as safe for ECLAIR. So far, the following macros are deviated: READ_SYSREG and WRITE_SYSREG max_{t}? and min_{t}? read_[bwlq] and read_[bwlq]_relaxed per_cpu and this_cpu __emulate_2op and __emulate_2op_nobyte read_debugreg and write_debugreg
R7.2	Violations caused by __HYPERVISOR_VIRT_START are related to the particular use of it done in xen_mk_ulong.	Tagged as deliberate for ECLAIR.
R7.4	Allow pointers of non-character type as long as the pointee is const-qualified.	ECLAIR has been configured to ignore these assignments.
R8.3	The type ret_t is deliberately used and defined as int or long depending on the architecture.	Tagged as deliberate for ECLAIR.
R8.3	Some files are not subject to respect MISRA rules at the moment, but some entity from a file in scope is used; therefore ECLAIR does report a violation, since not all the files involved in the violation are excluded from the analysis.	Tagged as deliberate for ECLAIR. Such excluded files are: xen/arch/x86/time.c xen/arch/x86/acpi/cpu_idle.c xen/arch/x86/mpparse.c xen/common/bunzip2.c xen/common/unlz4.c xen/common/unlzma.c xen/common/unlzo.c xen/common/unxz.c xen/common/unzstd.c
R8.4	The definitions present in the files ‘asm-offsets.c’ for any architecture are used to generate definitions for asm modules, and are not called by C code. Therefore the absence of prior declarations is safe.	Tagged as safe for ECLAIR.
R8.4	The functions defined in the file xen/common/coverage/gcov_base.c are meant to be called from gcc-generated code in a non-release build configuration. Therefore, the absence of prior declarations is safe.	Tagged as safe for ECLAIR.
R8.6	The following variables are compiled in multiple translation units belonging to different executables and therefore are safe. current_stack_pointer bsearch sort	Tagged as safe for ECLAIR.
R8.6	Declarations without definitions are allowed (specifically when the definition is compiled-out or optimized-out by the compiler).	Tagged as deliberate in ECLAIR.
R8.10	The gnu_inline attribute without static is deliberately allowed.	Tagged as deliberate for ECLAIR.
R9.5	The possibility of committing mistakes by specifying an explicit dimension is higher than omitting the dimension, therefore all such instances of violations are deviated.	Project-wide deviation, tagged as deliberate for ECLAIR.
R10.1, R10.3, R10.4	The value-preserving conversions of integer constants are safe.	Tagged as safe for ECLAIR.
R10.1	Shifting non-negative integers to the right is safe.	Tagged as safe for ECLAIR.
R10.1	Shifting non-negative integers to the left is safe if the result is still non-negative.	Tagged as safe for ECLAIR.
R10.1	Bitwise logical operations on non-negative integers are safe.	Tagged as safe for ECLAIR.
R10.1	The implicit conversion to Boolean for logical operator arguments is well-known to all Xen developers to be a comparison with 0.	Tagged as safe for ECLAIR.
R10.1	Xen only supports architectures where signed integers are representend using two’s complement and all the Xen developers are aware of this. For this reason, bitwise operations are safe.	Tagged as safe for ECLAIR.
R10.1	Given the assumptions on the toolchain detailed in docs/misra/C-language-toolchain.rst and the build flags used by the project, it is deemed safe to use bitwise shift operators. See automation/eclair_analysis/deviations.ecl for the full explanation.	Tagged as safe for ECLAIR.
R13.5	All developers and reviewers can be safely assumed to be well aware of the short-circuit evaluation strategy for logical operators.	Project-wide deviation; tagged as disapplied for ECLAIR.
R14.2	The severe restrictions imposed by this rule on the use of ‘for’ statements are not counterbalanced by the presumed facilitation of the peer review activity.	Project-wide deviation; tagged as disapplied for ECLAIR.
R14.3	The Xen team relies on the fact that invariant conditions of ‘if’ statements are deliberate.	Project-wide deviation; tagged as disapplied for ECLAIR.
R20.7	Code violating Rule 20.7 is safe when macro parameters are used: (1) as function arguments; (2) as macro arguments; (3) as array indices; (4) as lhs in assignments.	Tagged as safe for ECLAIR.

Other deviations:¶

Deviation	Justification
do-while-0 loops	The do-while-0 is a well-recognized loop idiom used by the Xen community and can therefore be used, even though it would cause a number of violations in some instances.
while-0 and while-1 loops	while-0 and while-1 are well-recognized loop idioms used by the Xen community and can therefore be used, even though they would cause a number of violations in some instances.