Platform dependencies may be introduced to improve performance on a particular platform. This can be a dangerous practice, particularly if these dependencies are not appropriately documented during development and addressed during porting. Platform dependencies that have no performance or other benefits should consequently be avoided because they may introduce errors during porting.
The C99 standard identifies four different kinds of non-portable behavior. Each section of Annex J of the C99 standard enumerates distinct instances of behaviors of each kind.
Nonportable Behavior |
Definition |
Annex J Section |
|---|---|---|
Behavior for which the standard provides two or more possibilities and imposes no further requirements on which is chosen in any instance. |
J.1 |
|
Behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which the standard imposes no requirements. An example of undefined behavior is the behavior on integer overflow. |
||
Unspecified behavior whereby each implementation documents how the choice is made. |
J.3 |
|
Behavior that depends on local conventions of nationality, culture, and language that each implementation documents. |
J.4 |
An example of undefined behavior is passing a NULL char* pointer as an argument to the printf function corresponding to the %s format specification. While some implementations (such as the GNU C Library) provide well-defined semantics for this case, others do not and cause programs that rely on this behavior to fail abnormally.
An example of unspecified behavior is the order in which the arguments to a function are evaluated.
An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right.
Most legitimate platform dependencies can and should be isolated in separate modules that expose portable, platform agnostic interfaces to platform-specific implementations. Portable applications that cannot avoid relying on platform-specific details should always provide a generic, portable, standards-based solution as a fallback mechanism for the platform-specific alternative. That way, such application can be more easily ported to new platforms, without an excessive risk of security flaws caused by assumptions that do not hold in the new environment.
Noncompliant Code Example
This noncompliant code example uses the complement operator in the test for unsigned integer overflow.
unsigned int ui1, ui2, sum;
if (~ui1 < ui2) {
/* handle error condition */
}
sum = ui1 + ui2;
This code assumes that the implementation uses two's complement representation. This assumption is commonly true, but not guaranteed by the standard.
This code sample also violates INT14-C. Avoid performing bitwise and arithmetic operations on the same data.
Compliant Solution
This compliant solution implements a strictly conforming test for unsigned overflow.
unsigned int ui1, ui2, sum;
if (UINT_MAX - ui1 < ui2) {
/* handle error condition */
}
sum = ui1 + ui2;
If the noncompliant form of this test is truly faster, talk to your compiler vendor, because if these tests are equivalent, optimization should occur. If both forms have the same performance, prefer the portable form.
Risk Assessment
Unnecessary platform dependencies are, by definition, unnecessary. Avoiding these dependencies can eliminate porting errors resulting from invalidated assumptions.
Recommendation |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
MSC14-C |
low |
unlikely |
medium |
P2 |
L3 |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Other Languages
This rule appears in the C++ Secure Coding Standard as MSC14-CPP. Do not introduce unnecessary platform dependencies.
References
[[Dowd 06]] Chapter 6, "C Language Issues" (Arithmetic Boundary Conditions, pp. 211-223)
[[ISO/IEC 9899:1999]] Section 3.4.1, "implementation-defined behavior," Section 3.4.4, "unspecified behavior," Appendix J.1, "Unspecified behavior," and Appendix J.3, "Implementation-defined behavior"
[[ISO/IEC PDTR 24772]] "BQF Unspecified Behaviour"
[[Seacord 05a]] Chapter 5, "Integers"