MISRA C rules for Xen

Note

IMPORTANT All MISRA C rules, text, and examples are copyrighted by the MISRA Consortium Limited and used with permission.

Please refer to https://www.misra.org.uk/ to obtain a copy of MISRA C, or for licensing options for other use of the rules.

The following is the list of MISRA C rules that apply to the Xen hypervisor.

It is possible that in specific circumstances it is best not to follow a rule because it is not possible or because the alternative leads to better code quality. Those cases are called "deviations". They are permissible as long as they are documented. For details, please refer to docs/misra/documenting-violations.rst and docs/misra/deviations.rst

Other documentation mechanisms are work-in-progress.

The existing codebase is not 100% compliant with the rules. Some of the violations are meant to be documented as deviations, while some others should be fixed. Both compliance and documenting deviations on the existing codebase are work-in-progress.

The list below might need to be updated over time. Reach out to THE REST maintainers if you want to suggest a change.

Dir number Severity Summary Notes
Dir 1.1 Required Any implementation-defined behaviour on which the output of the program depends shall be documented and understood
Dir 2.1 Required All source files shall compile without any compilation errors
Dir 4.1 Required Run-time failures shall be minimized The strategies adopted by Xen to prevent certain classes of runtime failures is documented by C-runtime-failures.rst
Dir 4.7 Required If a function returns error information then that error information shall be tested
Dir 4.10 Required Precautions shall be taken in order to prevent the contents of a header file being included more than once Files that are intended to be included more than once do not need to conform to the directive
Dir 4.11 Required The validity of values passed to library functions shall be checked We do not have libraries in Xen (libfdt and others are not considered libraries from MISRA C point of view as they are imported in source form)
Dir 4.14 Required The validity of values received from external sources shall be checked
Rule number Severity Summary Notes
Rule 1.1 Required The program shall contain no violations of the standard C syntax and constraints, and shall not exceed the implementation's translation limits We make use of several compiler extensions as documented by C-language-toolchain.rst
Rule 1.3 Required There shall be no occurrence of undefined or critical unspecified behaviour
Rule 1.4 Required Emergent language features shall not be used Emergent language features, such as C11 features, should not be confused with similar compiler extensions, which we use. When the time comes to adopt C11, this rule will be revisited.
Rule 2.1 Required A project shall not contain unreachable code
The following are allowed:
  • Invariantly constant conditions, e.g. if(IS_ENABLED(CONFIG_HVM)) { S; }
  • Switch with a controlling value statically determined not to match one or more case statements
  • Functions that are intended to be referenced only from assembly code (e.g. 'do_trap_fiq')
  • asm-offsets.c, as they are not linked deliberately, because they are used to generate definitions for asm modules
  • Declarations without initializer are safe, as they are not executed
Rule 2.6 Advisory A function should not contain unused label declarations
Rule 3.1 Required The character sequences /* and // shall not be used within a comment Comments containing hyperlinks inside C-style block comments are safe
Rule 3.2 Required Line-splicing shall not be used in // comments
Rule 4.1 Required Octal and hexadecimal escape sequences shall be terminated
Rule 4.2 Advisory Trigraphs should not be used
Rule 5.1 Required External identifiers shall be distinct The Xen characters limit for identifiers is 40. Public headers (xen/include/public/) are allowed to retain longer identifiers for backward compatibility.
Rule 5.2 Required Identifiers declared in the same scope and name space shall be distinct The Xen characters limit for identifiers is 40. Public headers (xen/include/public/) are allowed to retain longer identifiers for backward compatibility.
Rule 5.3 Required An identifier declared in an inner scope shall not hide an identifier declared in an outer scope Using macros as macro parameters at invocation time is allowed even if both macros use identically named local variables, e.g. max(var0, min(var1, var2))
Rule 5.4 Required Macro identifiers shall be distinct The Xen characters limit for macro identifiers is 40. Public headers (xen/include/public/) are allowed to retain longer identifiers for backward compatibility.
Rule 5.6 Required A typedef name shall be a unique identifier
Rule 6.1 Required Bit-fields shall only be declared with an appropriate type In addition to the C99 types, we also consider appropriate types enum and all explicitly signed / unsigned integer types.
Rule 6.2 Required Single-bit named bit fields shall not be of a signed type
Rule 7.1 Required Octal constants shall not be used
Rule 7.2 Required A "u" or "U" suffix shall be applied to all integer constants that are represented in an unsigned type

The rule asks that any integer literal that is implicitly unsigned is made explicitly unsigned by using one of the indicated suffixes. As an example, on a machine where the int type is 32-bit wide, 0x77777777 is signed whereas 0x80000000 is (implicitly) unsigned. In order to comply with the rule, the latter should be rewritten as either 0x80000000u or 0x80000000U. Consistency considerations may suggest using the same suffix even when not required by the rule. For instance, if one has:

Original: f(0x77777777); f(0x80000000);

one should do

Solution 1: f(0x77777777U); f(0x80000000U);

over

Solution 2: f(0x77777777); f(0x80000000U);

after having ascertained that "Solution 1" is compatible with the intended semantics.

Rule 7.3 Required The lowercase character l shall not be used in a literal suffix
Rule 7.4 Required A string literal shall not be assigned to an object unless the object type is pointer to const-qualified char All "character types" are permitted, as long as the string element type and the character type match. (There should be no casts.) Assigning a string literal to any object with type "pointer to const-qualified void" is allowed.
Rule 8.1 Required Types shall be explicitly specified
Rule 8.2 Required Function types shall be in prototype form with named parameters Clarification: both function and function pointers types shall have named parameters.
Rule 8.3 Required All declarations of an object or function shall use the same names and type qualifiers The type ret_t maybe be deliberately used and defined as int or long depending on the type of guest to service
Rule 8.4 Required A compatible declaration shall be visible when an object or function with external linkage is defined Allowed exceptions: asm-offsets.c, definitions for asm modules not called from C code, gcov_base.c
Rule 8.5 Required An external object or function shall be declared once in one and only one file
Rule 8.6 Required An identifier with external linkage shall have exactly one external definition Declarations without definitions are allowed (specifically when the definition is compiled-out or optimized-out by the compiler)
Rule 8.8 Required The static storage class specifier shall be used in all declarations of objects and functions that have internal linkage
Rule 8.10 Required An inline function shall be declared with the static storage class gnu_inline (without static) is allowed.
Rule 8.12 Required Within an enumerator list the value of an implicitly-specified enumeration constant shall be unique
Rule 8.14 Required The restrict type qualifier shall not be used
Rule 9.1 Mandatory The value of an object with automatic storage duration shall not be read before it has been set Rule clarification: do not use variables before they are initialized. An explicit initializer is not necessarily required. Try reducing the scope of the variable. If an explicit initializer is added, consider initializing the variable to a poison value.
Rule 9.2 Required The initializer for an aggregate or union shall be enclosed in braces
Rule 9.3 Required Arrays shall not be partially initialized {} is also allowed to specify explicit zero-initialization
Rule 9.4 Required An element of an object shall not be initialized more than once
Rule 10.1 Required Operands shall not be of an inappropriate essential type
The following are allowed:
  • Value-preserving conversions of integer constants
  • Bitwise and, or, xor, one's complement, bitwise and assignment, bitwise or assignment, bitwise xor assignment (bitwise and, or, xor are safe on non-negative integers; also Xen assumes two's complement representation)
  • Left shift, right shift, left shift assignment, right shift assignment (see C-language-toolchain.rst for uses of compilers' extensions)
  • Implicit conversions to boolean for conditionals (?: if while for) and logical operators (! || &&)
  • The essential type model allows the constants defined by anonymous enums (e.g., enum { A, B, C }) to be used as operands to arithmetic operators, as they have a signed essential type.
Rule 10.2 Required Expressions of essentially character type shall not be used inappropriately in addition and subtraction operations
Rule 10.3 Required The value of an expression shall not be assigned to an object with a narrower essential type or of a different essential type category

Please beware that this rule has many violations in the Xen codebase today, and its adoption is aspirational. However, when submitting new patches please try to decrease the number of violations when possible.

gcc has a helpful warning that can help you spot and remove violations of this kind: conversion. For instance, you can use it as follows:

CFLAGS="-Wconversion -Wno-error=sign-conversion -Wno-error=conversion" make -C xen

Rule 10.4 Required Both operands of an operator in which the usual arithmetic conversions are performed shall have the same essential type category

Please beware that this rule has many violations in the Xen codebase today, and its adoption is aspirational. However, when submitting new patches please try to decrease the number of violations when possible.

gcc has a helpful warning that can help you spot and remove violations of this kind: arith-conversion. For instance, you can use it as follows:

CFLAGS="-Warith-conversion -Wno-error=arith-conversion" make -C xen

Rule 11.1 Required Conversions shall not be performed between a pointer to a function and any other type All conversions to integer types are permitted if the destination type has enough bits to hold the entire value. Conversions to bool and void* are permitted.
Rule 11.2 Required Conversions shall not be performed between a pointer to an incomplete type and any other type All conversions to integer types are permitted if the destination type has enough bits to hold the entire value. Conversions to bool and void* are permitted.
Rule 11.3 Required A cast shall not be performed between a pointer to object type and a pointer to a different object type
Rule 11.6 Required A cast shall not be performed between pointer to void and an arithmetic type All conversions to integer types are permitted if the destination type has enough bits to hold the entire value. Conversions to bool are permitted.
Rule 11.7 Required A cast shall not be performed between pointer to object and a noninteger arithmetic type
Rule 11.8 Required A cast shall not remove any const or volatile qualification from the type pointed to by a pointer
Rule 11.9 Required The macro NULL shall be the only permitted form of null pointer constant
Rule 12.5 Mandatory The sizeof operator shall not have an operand which is a function parameter declared as "array of type"
Rule 13.1 Required Initializer lists shall not contain persistent side effects
Rule 13.6 Required The operand of the sizeof operator shall not contain any expression which has potential side effects In addition to sizeof, we also want to apply the rule to typeof and alignof
Rule 14.1 Required A loop counter shall not have essentially floating type
Rule 14.3 Required Controlling expressions shall not be invariant

Due to the extensive usage of IS_ENABLED, sizeof compile-time checks, and other constructs that are detected as errors by MISRA C scanners, managing the configuration of a MISRA C scanner for this rule would be unmanageable. Thus, this rule is adopted with a project-wide deviation on if, ?:, switch(sizeof(...)), and switch(offsetof(...)) statements.

while(0) and while(1) and alike are allowed.

Rule 14.4 Required The controlling expression of an if-statement and the controlling expression of an iteration-statement shall have essentially Boolean type Automatic conversions of integer types to bool are permitted. Automatic conversions of pointer types to bool are permitted. This rule still applies to enum types.
Rule 16.3 Required An unconditional break statement shall terminate every switch-clause In addition to break, also other unconditional flow control statements such as continue, return, goto are allowed.
Rule 16.2 Required A switch label shall only be used when the most closely-enclosing compound statement is the body of a switch statement The x86 emulator (xen/arch/x86/x86_emulate*) is exempt from compliance with this rule. Efforts to make the x86 emulator adhere to Rule 16.2 would result in increased complexity and maintenance difficulty, and could potentially introduce bugs.
Rule 16.6 Required Every switch statement shall have at least two switch-clauses Single-clause switches are allowed when they do not involve a default label.
Rule 16.7 Required A switch-expression shall not have essentially Boolean type
Rule 17.1 Required The features of <stdarg.h> shall not be used
Rule 17.3 Mandatory A function shall not be declared implicitly
Rule 17.4 Mandatory All exit paths from a function with non-void return type shall have an explicit return statement with an expression
Rule 17.5 Advisory The function argument corresponding to a parameter declared to have an array type shall have an appropriate number of elements
Rule 17.6 Mandatory The declaration of an array parameter shall not contain the static keyword between the [ ]
Rule 17.7 Required The value returned by a function having non-void return type shall be used Please beware that this rule has many violations in the Xen codebase today, and its adoption is aspirational. However, when submitting new patches please try to decrease the number of violations when possible.
Rule 18.3 Required The relational operators > >= < and <= shall not be applied to objects of pointer type except where they point into the same object
Rule 19.1 Mandatory An object shall not be assigned or copied to an overlapping object Be aware that the static analysis tool Eclair might report several findings for Rule 19.1 of type "caution". These are instances where Eclair is unable to verify that the code is valid in regard to Rule 19.1. Caution reports are not violations.
Rule 20.4 Required A macro shall not be defined with the same name as a keyword
Rule 20.7 Required Expressions resulting from the expansion of macro parameters shall be enclosed in parentheses Extra parentheses are not required when macro parameters are used as function arguments, as macro arguments, array indices, lhs in assignments or as initializers in initalizer lists.
Rule 20.9 Required All identifiers used in the controlling expression of #if or #elif preprocessing directives shall be #define'd before evaluation
Rule 20.12 Required A macro parameter used as an operand to the # or ## operators, which is itself subject to further macro replacement, shall only be used as an operand to these operators Variadic macros are allowed to violate the rule.
Rule 20.13 Required A line whose first token is # shall be a valid preprocessing directive
Rule 20.14 Required All #else #elif and #endif preprocessor directives shall reside in the same file as the #if #ifdef or #ifndef directive to which they are related
Rule 21.1 Required #define and #undef shall not be used on a reserved identifier or reserved macro name Identifiers starting with an underscore followed by another underscore or an upper-case letter are reserved. Today Xen uses many, such as header guards and bitwise manipulation functions. Upon analysis it turns out Xen identifiers do not clash with the identifiers used by modern GCC, but that is not a guarantee that there won't be a naming clash in the future or with another compiler. For these reasons we discourage the introduction of new reserved identifiers in Xen, and we see it as positive the reduction of reserved identifiers. At the same time, certain identifiers starting with two underscores are also commonly used in Linux (e.g. __set_bit) and we don't think it would be an improvement to rename them.
Rule 21.2 Required A reserved identifier or reserved macro name shall not be declared See comment for Rule 21.1
Rule 21.13 Mandatory Any value passed to a function in <ctype.h> shall be representable as an unsigned char or be the value EOF
Rule 21.17 Mandatory Use of the string handling functions from <string.h> shall not result in accesses beyond the bounds of the objects referenced by their pointer parameters
Rule 21.18 Mandatory The size_t argument passed to any function in <string.h> shall have an appropriate value
Rule 21.19 Mandatory The pointers returned by the Standard Library functions localeconv, getenv, setlocale or, strerror shall only be used as if they have pointer to const-qualified type
Rule 21.20 Mandatory The pointer returned by the Standard Library functions asctime ctime gmtime localtime localeconv getenv setlocale or strerror shall not be used following a subsequent call to the same function
Rule 21.21 Required The Standard Library function system of <stdlib.h> shall not be used
Rule 22.2 Mandatory A block of memory shall only be freed if it was allocated by means of a Standard Library function
Rule 22.4 Mandatory There shall be no attempt to write to a stream which has been opened as read-only
Rule 22.5 Mandatory A pointer to a FILE object shall not be dereferenced
Rule 22.6 Mandatory The value of a pointer to a FILE shall not be used after the associated stream has been closed