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xen/docs/misra/deviations.rst
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Nicola Vetrini 80d28865e1 automation/eclair: add deviations for Rule 20.7 
These deviations deal with the following cases:
- macro arguments used directly as initializer list arguments;
- uses of the __config_enabled macro, that can't be brought
  into compliance without breaking its functionality;
- exclude files that are out of scope (efi headers and cpu_idle);
- uses of alternative_{call,vcall}[0-9] macros.

The existing configuration for R20.7 is reordered so that it matches the
cases listed in its documentation comment.

Signed-off-by: Nicola Vetrini <nicola.vetrini@bugseng.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
2024-04-05 11:15:47 -07:00

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.. SPDX-License-Identifier: CC-BY-4.0
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:
----------------------------------------------
.. list-table::
:header-rows: 1
* - 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:
-----------------------------------------
.. list-table::
:header-rows: 1
* - 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.
* - R2.6
- Labels deliberately marked as unused trough the pseudo-attribute
`__maybe_unused` are either the result of them not being in certain build
configurations, or as a deliberate practice (e.g., `unimplemented_insn`).
Given that the compiler is then entitled to remove them, the presence of
such labels poses no risks.
- Tagged as `deliberate` 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
* - R5.6
- The type ret_t is deliberately defined multiple times depending on the
type of guest to service.
- Tagged as `deliberate` for ECLAIR.
* - R5.6
- On X86, some types are deliberately defined multiple times, depending on
the number of guest paging levels.
- Tagged as `deliberate` for ECLAIR. Such types are:
- guest_intpte_t
- guest_l[12]e_t
* - R5.6
- Some files are not subject to respect MISRA rules at
the moment, but, among these out-of-scope files, there are definitions
of typedef names that are already defined within in-scope files and
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/include/efi/*
- xen/arch/*/include/asm/*/efibind.h
* - R7.1
- It is safe to use certain octal constants the way they are defined
in specifications, manuals, and algorithm descriptions. Such places
are marked safe with a /\* octal-ok \*/ in-code comment.
- Tagged as `safe` for ECLAIR.
* - 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
* - R8.3
- Parameter name "unused" (with an optional numeric suffix) is deliberate
and makes explicit the intention of not using such parameter within the
function.
- Tagged as `deliberate` for ECLAIR.
* - 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.
* - R8.4
- Functions and variables used only by asm modules are marked with
the `asmlinkage` macro. Existing code may use a SAF-1-safe
textual deviation (see safe.json), but new code should not use
it.
- 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.
* - R8.6
- The search procedure for Unix linkers is well defined, see ld(1) manual:
"The linker will search an archive only once, at the location where it
is specified on the command line. If the archive defines a symbol which
was undefined in some object which appeared before the archive on the
command line, the linker will include the appropriate file(s) from the
archive".
In Xen, thanks to the order in which file names appear in the build
commands, if arch-specific definitions are present, they get always
linked in before searching in the lib.a archive resulting from xen/lib.
- Tagged as `deliberate` for 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.
* - R10.1
- The macro ISOLATE_LSB encapsulates the well-known pattern (x & -x)
applied to unsigned integer values on 2's complement architectures
(i.e., all architectures supported by Xen), used to obtain a mask where
just the least significant nonzero bit of x is set.
If no bits are set, 0 is returned.
- Tagged as `safe` for ECLAIR.
* - R11.8
- Violations caused by container_of are due to pointer arithmetic operations
with the provided offset. The resulting pointer is then immediately cast back to its
original type, which preserves the qualifier. This use is deemed safe.
Fixing this violation would require to increase code complexity and lower readability.
- Tagged as `safe` for ECLAIR.
* - R11.9
- __ACCESS_ONCE uses an integer, which happens to be zero, as a
compile time check. The typecheck uses a cast. The usage of zero or other
integers for this purpose is allowed.
- Tagged as `deliberate` 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.
* - R14.4
- The XEN team relies on the fact that the enum is_dying has the
constant with assigned value 0 act as false and the other ones as true,
therefore have the same behavior of a boolean.
- Project-wide deviation; tagged as `deliberate` for ECLAIR.
* - R16.3
- Switch clauses ending with continue, goto, return statements are safe.
- Tagged as `safe` for ECLAIR.
* - R16.3
- Switch clauses ending with a call to a function that does not give
the control back (i.e., a function with attribute noreturn) are safe.
- Tagged as `safe` for ECLAIR.
* - R16.3
- Switch clauses ending with pseudo-keyword \"fallthrough\" are safe.
- Tagged as `safe` for ECLAIR.
* - R16.3
- Switch clauses ending with failure method \"BUG()\" are safe.
- Tagged as `safe` for ECLAIR.
* - R16.3
- Existing switch clauses not ending with the break statement are safe if
an explicit comment indicating the fallthrough intention is present.
However, the use of such comments in new code is deprecated:
the pseudo-keyword "fallthrough" shall be used.
- Tagged as `safe` for ECLAIR. The accepted comments are:
- /\* fall through \*/
- /\* fall through. \*/
- /\* fallthrough \*/
- /\* fallthrough. \*/
- /\* Fall through \*/
- /\* Fall through. \*/
- /\* Fallthrough \*/
- /\* Fallthrough. \*/
* - R16.6
- A switch statement with a single switch clause and no default label may
be used in place of an equivalent if statement if it is considered to
improve readability.
- Tagged as `deliberate` for ECLAIR.
* - R20.4
- The override of the keyword \"inline\" in xen/compiler.h is present so
that section contents checks pass when the compiler chooses not to
inline a particular function.
- Comment-based deviation.
* - 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;
(5) as initializers, possibly designated, in initalizer lists.
- Tagged as `safe` for ECLAIR.
* - R20.7
- Violations due to the use of macros defined in files that are not
in scope for compliance are allowed, as that is imported code.
- Tagged as `safe` for ECLAIR.
* - R20.7
- To avoid compromising readability, the macros `alternative_(v)?call[0-9]`
are allowed not to parenthesize their arguments, as there are already
sanity checks in place.
- Tagged as `safe` for ECLAIR.
* - R20.12
- Variadic macros that use token pasting often employ the gcc extension
`ext_paste_comma`, as detailed in `C-language-toolchain.rst`, which is
not easily replaceable; macros that in addition perform regular argument
expansion on the same argument subject to the # or ## operators violate
the Rule if the argument is a macro.
- Tagged as `deliberate` for ECLAIR.
* - R20.12
- Macros that are used for runtime or build-time assertions contain
deliberate uses of an argument as both a regular argument and a
stringification token, to provide useful diagnostic messages.
- Tagged as `deliberate` for ECLAIR.
* - R20.12
- GENERATE_CASE is a local helper macro that allows some selected switch
statements to be more compact and readable. As such, the risk of
developer confusion in using such macro is deemed negligible. This
construct is deviated only in Translation Units that present a violation
of the Rule due to uses of this macro.
- Tagged as `deliberate` for ECLAIR.
Other deviations:
-----------------
.. list-table::
:header-rows: 1
* - 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.
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