Many functions require the allocation of multiple resources. Failing and returning somewhere in the middle of this function without freeing all of the allocated resources could produce a memory leak. It is a common error to forget to free one (or all) of the resources in this manner, so a goto-chain is the simplest and cleanest way to organize exits when order is preserved.
Noncompliant Code Example
In this noncompliant example, exit code is written for every instance in which the function can terminate prematurely. Notice how failing to allocate obj2 produces a memory leak and fails to close the opened file.
Please note that these examples assume errno_t and NOERR to be defined, as requested by [[DCL09-C. Declare functions that return an errno error code with a return type of errno_t]]. An equivalent compatible example would define errno_t as an int and NOERR as zero.
typedef struct object { // A generic struct -- The contents don't matter
int propertyA, propertyB, propertyC;
} object_t;
errno_t do_something(void){
FILE *fin;
object_t *obj1, *obj2;
errno_t ret_val;
fin = fopen("some_file", "r");
if (fin == NULL){
return errno;
}
obj1 = malloc(sizeof(object_t));
if (obj1 == NULL){
ret_val = errno;
fclose(fin);
return ret_val;
}
obj2 = malloc(sizeof(object_t));
if (obj2 == NULL){
ret_val = errno;
fclose(fin);
return ret_val; // Forgot to free obj1 !!
}
// ... more code ...
fclose(fin);
free(obj1);
free(obj2);
return NOERR;
}
This is also just a small example; in much larger examples, errors like this would be even harder to detect.
Compliant Solution
In this revised version, we have used a goto-chain in replacement of each individual return segment. If there is no error, control flow will fall through to the SUCCESS label, release all of the resources and return NOERR. In the case of an error, the return value will be set to errno, control flow will jump to the proper failure label, and the appropriate resources will be released before returning.
// ... assume the same struct as above ...
errno_t do_something(void){
FILE *fin;
object_t *obj1, *obj2;
errno_t ret_val = NOERR; // Initially assume a successful return value
fin = fopen("some_file", "r");
if (fin == NULL){
ret_val = errno;
goto FAIL_FIN;
}
obj1 = malloc(sizeof(object_t));
if (obj1 == NULL){
ret_val = errno;
goto FAIL_OBJ1;
}
obj2 = malloc(sizeof(object_t));
if (obj2 == NULL){
ret_val = errno;
goto FAIL_OBJ2;
}
// ... more code ...
SUCCESS: // Free everything
free(obj2);
FAIL_OBJ2: // Otherwise, free only the objects we allocated
free(obj1);
FAIL_OBJ1:
fclose(fin);
FAIL_FIN:
return ret_val;
}
The benefits of this method are that the code is cleaner and we prevent the rewriting of similar code upon every function error.
Compliant Solution (Linux kernel)
Many effective examples of goto-chains are quite large. The following compliant code is an excerpt from the Linux kernel. This is the copy_process function from kernel/fork.c from Version 2.6.29 of the kernel.
The function uses 17 goto labels (not all displayed here) in order to perform cleanup code should any internal function yield an error code. If no errors occur, the program returns a pointer to the new process p. If any error occurs, the program diverts control to a particular goto label, which performs cleanup for the sections of the function that have currently been successfully executed, while not performing cleanup on sections of the function that have not been executed yet. Consequently only resources that were successfully opened are actually closed.
All comments in this excerpt were added to indicate additional code in the kernel not displayed here.
static struct task_struct *copy_process(unsigned long clone_flags,
unsigned long stack_start,
struct pt_regs *regs,
unsigned long stack_size,
int __user *child_tidptr,
struct pid *pid,
int trace)
{
int retval;
struct task_struct *p;
int cgroup_callbacks_done = 0;
if ((clone_flags & (CLONE_NEWNS|CLONE_FS)) == (CLONE_NEWNS|CLONE_FS))
return ERR_PTR(-EINVAL);
/* ... */
retval = security_task_create(clone_flags);
if (retval)
goto fork_out;
retval = -ENOMEM;
p = dup_task_struct(current);
if (!p)
goto fork_out;
/* ... */
/* copy all the process information */
if ((retval = copy_semundo(clone_flags, p)))
goto bad_fork_cleanup_audit;
if ((retval = copy_files(clone_flags, p)))
goto bad_fork_cleanup_semundo;
if ((retval = copy_fs(clone_flags, p)))
goto bad_fork_cleanup_files;
if ((retval = copy_sighand(clone_flags, p)))
goto bad_fork_cleanup_fs;
if ((retval = copy_signal(clone_flags, p)))
goto bad_fork_cleanup_sighand;
if ((retval = copy_mm(clone_flags, p)))
goto bad_fork_cleanup_signal;
if ((retval = copy_namespaces(clone_flags, p)))
goto bad_fork_cleanup_mm;
if ((retval = copy_io(clone_flags, p)))
goto bad_fork_cleanup_namespaces;
retval = copy_thread(0, clone_flags, stack_start, stack_size, p, regs);
if (retval)
goto bad_fork_cleanup_io;
/* ... */
return p;
/* ... cleanup code starts here ... */
bad_fork_cleanup_io:
put_io_context(p->io_context);
bad_fork_cleanup_namespaces:
exit_task_namespaces(p);
bad_fork_cleanup_mm:
if (p->mm)
mmput(p->mm);
bad_fork_cleanup_signal:
cleanup_signal(p);
bad_fork_cleanup_sighand:
__cleanup_sighand(p->sighand);
bad_fork_cleanup_fs:
exit_fs(p); /* blocking */
bad_fork_cleanup_files:
exit_files(p); /* blocking */
bad_fork_cleanup_semundo:
exit_sem(p);
bad_fork_cleanup_audit:
audit_free(p);
/* ... more cleanup code ... */
fork_out:
return ERR_PTR(retval);
}
Risk Assessment
Failure to free allocated memory or close opened files results in a memory leak and possibly unexpected results.
Recommendation |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
MEM12-C |
low |
probable |
medium |
P3 |
L3 |
References
[[ISO/IEC 9899:1999]] Section 7.20.3, "Memory management functions"
[[ISO/IEC 9899:1999]] Section 7.19.5, "File access functions"
Linux kernel Sourcecode (v2.6.xx)
2.6.29, kernel/fork.c, the copy_process() function
[[Seacord 05]] Chapter 4, "Dynamic Memory Management"