CODEWITHSUNDEEP
csecuritymacros
user405369
user405369

Reputation: 436

How is the __func__ identifier a security risk?

I saw in a comment that using __func__ is a security risk. I need to understand how is that the case?

Upvotes: 5

Views: 241

Answers (4)

askmish
askmish

Reputation: 6684

__func__ is the C99-standard predefined identifier that expands into a character array variable containing the function name when it is used inside of a function. From C99 6.4.2.2/1 describes:

The identifier __func__ is implicitly declared by the translator as if, immediately following the opening brace of each function definition, the declaration

static const char __func__[] = "function-name"; appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function.

Note that it is not a macro and it has no special meaning during preprocessing.

Look at this link for more information about __func__ at:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2004/n1642.html

If the solution relies on the expansion involved in the predefined context-sensitive constant __func__, it cannot be implemented in a function safely and provide the same convenience.

__func__ is textually expanded at the point of its invocation, hence, it could easily affect the flow of control of the invoking code by evaluating break, continue, or return statements or failing to properly terminate if statements. The effects of invoking it can be surprising and lead to subtle flaws.

Prefer inline or static functions to function-like macros or __func__.

However, in cases where defining a __func__ is unavoidable, the definition should avoid statements that change the control flow of the invoking code.And ideally, it should be a single expression.

More idea and example codes:
https://www.securecoding.cert.org/confluence/display/seccode/PRE13-C.+Avoid+changing+control+flow+in+macro+definitions

Upvotes: 0

dreamlax
dreamlax

Reputation: 95355

If your application's security relies on its function names remaining a secret then it is a security risk to use __func__, as the compiler will need to store the function name somewhere in the compiled binary.

Upvotes: 0

CodesInChaos
CodesInChaos

Reputation: 108830

It makes reverse engineering easier. So if you want to keep the way your application works secret, then it makes an attacker's life easier.

That's relevant for DRM features, or if you want to make it harder to imitate your algorithms in competing applications.

But it does not affect security, since an application where reversing shows vulnerabilities was insecure in the first place.

Upvotes: 0

MByD
MByD

Reputation: 137382

Using __func__ reveals the function name in the binary, which ease the work of an attacker that has an access to the binary.

Upvotes: 4

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