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When accessing a bit-field, a thread may inadvertently access a separate bit-field in adjacent memory. This is because compilers are required to store multiple adjacent bit-fields in one storage unit whenever they fit. Consequently, data races may exist not just on a bit-field accessed by multiple threads but also on other bit-fields sharing the same byte or word.  A similar problem is discussed in CON43-C. Do not allow data races in multithreaded code, but the issue described by this rule can be harder to diagnose because it may not be obvious that the same memory location is being modified by multiple threads.

One approach for preventing data races in concurrent programming is to use a mutex. When properly observed by all threads, a mutex can provide safe and secure access to a shared object. However, mutexes provide no guarantees with regard to other objects that might be accessed when the mutex is not controlled by the accessing thread. Unfortunately, there is no portable way to determine which adjacent bit-fields may be stored along with the desired bit-field.

Another approach is to insert a non-bit-field member between any two bit-fields to ensure that each bit-field is the only one accessed within its storage unit. This technique effectively guarantees that no two bit-fields are accessed simultaneously.

Noncompliant Code Example (Bit-field)

Adjacent bit-fields may be stored in a single memory location. Consequently, modifying adjacent bit-fields in different threads is undefined behavior, as shown in this noncompliant code example:

Bit-fields allow for a simple way to break portions of a struct into portions that only use up a certain specified number of bits. If multiple threads are accessing or making modifications to different bit-fields, a race condition may be present because the architecture may not be able to modify only the bits to which the currently being modified member may refer. Therefore, a mutex protecting all bit-fields at the same time must be used.

Non-Compliant Code Example

In the following non-compliant code, two threads presumed to be running simultaneously access two separate members of a global struct.

struct multi_threaded_flags {
  unsigned int flag1 : 2;
  unsigned int flag2 : 2;
};

struct multi_threaded_flags flags;

voidint thread1(void *arg) {
  flags.flag1 = 1;
  return 0;
}

voidint thread2(void *arg) {
  flags.flag2 = 2;
  return 0;
}

Although this appears to be harmless, it is possible (and likely) that the architecture that this is running on has aligned flag1 and flag2 on the same byte. If both assignments occur on a thread scheduling interleaving which ends with the both stores occurring after one another, it is possible that only one of the flags will be set as intended and the other flag will equal its previous value due to the fact that both of the bit-fields fell on the same byte, which was the smallest unit the processor could work on.

Compliant Solution

The C Standard, 3.17, paragraph 3 [ISO/IEC 9899:2024], states

Note 2 to entry: A bit-field and an adjacent non-bit-field member are in separate memory locations. The same applies to two bit-fields, if one is declared inside a nested structure declaration and the other is not, or if the two are separated by a zero-length bit-field declaration, or if they are separated by a non-bit-field member declaration. It is not safe to concurrently update two non-atomic bit-fields in the same structure if all members declared between them are also (nonzero-length) bit-fields, no matter what the sizes of those intervening bit-fields happen to be.

For example, the following instruction sequence is possible:

Thread 1: register 0 = flags
Thread 1: register 0 &= ~mask(flag1)
Thread 2: register 0 = flags
Thread 2: register 0 &= ~mask(flag2)
Thread 1: register 0 |= 1 << shift(flag1)
Thread 1: flags = register 0
Thread 2: register 0 |= 2 << shift(flag2)
Thread 2: flags = register 0

Compliant Solution (Bit-field, C11, Mutex)

This compliant solution protects all accesses This compliant solution protects all usage of the flags with a mutex, preventing an unfortunate thread scheduling interleaving from being able to occur.thereby preventing any data races:

#include <threads.h>
 
struct multi_threaded_flags {
  unsigned int flag1 : 2;
  unsigned int flag2 : 2;
};

struct  pthread_mutexmtf_mutex {
  struct multi_threaded_flags s;
  mtx_t mutex;
};

struct multimtf_threaded_flagsmutex flags;

voidint thread1(void *arg) {
  pthread_mutexif (thrd_success != mtx_lock(&flags.mutex);) {
    /* Handle error */
  }
  flags.s.flag1 = 1;
  if pthread_mutex(thrd_success != mtx_unlock(&flags.mutex)) {
    /* Handle error */
  }
  return 0;
}
 
voidint thread2(void *arg) {
  pthread_mutexif (thrd_success != mtx_lock(&flags.mutex);) {
    /* Handle error */
  }
  flags.s.flag2 = 2;
  if pthread_mutex(thrd_success != mtx_unlock(&flags.mutex)) {
    /* Handle error */
  }
  return 0;
}

Compliant Solution (C11)

In this compliant solution, two threads simultaneously modify two distinct non-bit-field members of a structure. Because the members occupy different bytes in memory, no concurrency protection is required.

struct multi_threaded_flags {
  unsigned char flag1;
  unsigned char flag2;
};
 
struct multi_threaded_flags flags;
 
int thread1(void *arg) {
  flags.flag1 = 1;
  return 0;
}

int thread2(void *arg) {
  flags.flag2 = 2;
  return 0;
}

Unlike C99, C11 and C23 explicitly define a memory location and provides the following note in subclause 3.17 paragraph 2 [ISO/IEC 9899:2024]:

Note 1 to entry: Two threads of execution can update and access separate memory locations without interfering with each other.

It is almost certain that flag1 and flag2 are stored in the same word. Using a compiler that conforms to C99 or earlier, if both assignments occur on a thread-scheduling interleaving that ends with both stores occurring after one another, it is possible that only one of the flags will be set as intended. The other flag will contain its previous value because both members are represented by the same word, which is the smallest unit the processor can work on. Before the changes were made to the C Standard for C11, there were no guarantees that these flags could be modified concurrently.

Risk Assessment

Although the race window is narrow, having an assignment or an expression can evaluate improperly due to because of misinterpreted data can possibly result resulting in a corrupted running state or unintended information disclosure.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

Bibliography

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Image Added Image Added Image Added 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.2.1, "Structure and union specifiers"