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 5.5	Required	Identifiers shall be distinct from macro names	Macros expanding to their own name are allowed, e.g.: #define x x Clashes between names of function-like macros and identifiers of non-callable entities are allowed. Callable entities having an identifier that is the same of the name of a function-like macro are not allowed. Example (not allowed): #define f(x, y) f(x, y) void f(int x, int y);
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 16.4	Required	Every switch statement shall have a default label	Switch statements with enums as controlling expression don’t need a default label as gcc -Wall enables -Wswitch which warns (and breaks the build as we use -Werror) if one of the enum labels is missing from the switch. Switch statements with integer types as controlling expression should have a default label: if the switch is expected to handle all possible cases explicitly, then a default label shall be added to handle unexpected error conditions, using BUG(), ASSERT(), WARN(), domain_crash(), or other appropriate methods; if the switch is expected to handle a subset of all possible cases, then an empty default label shall be added with an in-code comment on top of the default label with a reason and when possible a more detailed explanation. Example: default: /* Notifier pattern */ break;
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 21.6	Required	The Standard Library input/output routines shall not be used	Xen doesn’t provide, use, or link against a Standard Library [1]
Rule 21.9	Required	The library functions bsearch and qsort of <stdlib.h> shall not be used	Xen doesn’t provide, use, or link against a Standard Library [1]
Rule 21.10	Required	The Standard Library time and date routines shall not be used	Xen doesn’t provide, use, or link against a Standard Library [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 21.14	Required	The Standard Library function memcmp shall not be used to compare null terminated strings
Rule 21.15	Required	The pointer arguments to the Standard Library functions memcpy, memmove and memcmp shall be pointers to qualified or unqualified versions of compatible types
Rule 21.16	Required	The pointer arguments to the Standard Library function memcmp shall point to either a pointer type, an essentially signed type, an essentially unsigned type, an essentially Boolean type or an essentially enum type	void* arguments are allowed
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

Terms & Definitions

A switch clause can be defined as: “the non-empty list of statements which follows a non-empty list of case/default labels”. A formal definition is available within the amplification of MISRA C:2012 Rule 16.1.

Footnotes

[1] (1,2,3)

Xen implements itself a few functions with names that match the corresponding function names of the Standard Library for developers’ convenience. These functions are part of the Xen code and subject to analysis.