Accessing or modifying shared objects in signal handlers can result in race conditions that can leave data in an inconsistent state. The two exceptions (C Standard, 5.1.2.3, paragraph 5) to this rule are the ability to read from and write to lock-free atomic objects and variables of type volatile sig_atomic_t
. Accessing any other type of object from a signal handler is undefined behavior. (See undefined behavior 131.)
The need for the volatile
keyword is described in DCL22-C. Use volatile for data that cannot be cached.
The type sig_atomic_t
is the integer type of an object that can be accessed as an atomic entity even in the presence of asynchronous interrupts. The type of sig_atomic_t
is implementation-defined, though it provides some guarantees. Integer values ranging from SIG_ATOMIC_MIN
through SIG_ATOMIC_MAX
, inclusive, may be safely stored to a variable of the type. In addition, when sig_atomic_t
is a signed integer type, SIG_ATOMIC_MIN
must be no greater than −127
and SIG_ATOMIC_MAX
no less than 127
. Otherwise, SIG_ATOMIC_MIN
must be 0
and SIG_ATOMIC_MAX
must be no less than 255
. The macros SIG_ATOMIC_MIN
and SIG_ATOMIC_MAX
are defined in the header <stdint.h>
.
According to the C99 Rationale [C99 Rationale 2003], other than calling a limited, prescribed set of library functions,
the C89 Committee concluded that about the only thing a strictly conforming program can do in a signal handler is to assign a value to a
volatile static
variable which can be written uninterruptedly and promptly return.
However, this issue was discussed at the April 2008 meeting of ISO/IEC WG14, and it was agreed that there are no known implementations in which it would be an error to read a value from a volatile sig_atomic_t
variable, and the original intent of the committee was that both reading and writing variables of volatile sig_atomic_t
would be strictly conforming.
The signal handler may also call a handful of functions, including abort().
(See SIG30-C. Call only asynchronous-safe functions within signal handlers for more information.)
Noncompliant Code Example
In this noncompliant code example, err_msg
is updated to indicate that the SIGINT
signal was delivered. The err_msg
variable is a character pointer and not a variable of type volatile sig_atomic_t
.
#include <signal.h> #include <stdlib.h> #include <string.h> enum { MAX_MSG_SIZE = 24 }; char *err_msg; void handler(int signum) { strcpy(err_msg, "SIGINT encountered."); } int main(void) { signal(SIGINT, handler); err_msg = (char *)malloc(MAX_MSG_SIZE); if (err_msg == NULL) { /* Handle error */ } strcpy(err_msg, "No errors yet."); /* Main code loop */ return 0; }
Compliant Solution (Writing volatile sig_atomic_t
)
For maximum portability, signal handlers should only unconditionally set a variable of type volatile sig_atomic_t
and return, as in this compliant solution:
#include <signal.h> #include <stdlib.h> #include <string.h> enum { MAX_MSG_SIZE = 24 }; volatile sig_atomic_t e_flag = 0; void handler(int signum) { e_flag = 1; } int main(void) { char *err_msg = (char *)malloc(MAX_MSG_SIZE); if (err_msg == NULL) { /* Handle error */ } signal(SIGINT, handler); strcpy(err_msg, "No errors yet."); /* Main code loop */ if (e_flag) { strcpy(err_msg, "SIGINT received."); } return 0; }
Compliant Solution (Lock-Free Atomic Access)
Signal handlers can refer to objects with static or thread storage durations that are lock-free atomic objects, as in this compliant solution:
#include <signal.h> #include <stdlib.h> #include <string.h> #include <stdatomic.h> #ifdef __STDC_NO_ATOMICS__ #error "Atomics are not supported" #elif ATOMIC_INT_LOCK_FREE == 0 #error "int is never lock-free" #endif atomic_int e_flag = ATOMIC_VAR_INIT(0); void handler(int signum) { e_flag = 1; } int main(void) { enum { MAX_MSG_SIZE = 24 }; char err_msg[MAX_MSG_SIZE]; #if ATOMIC_INT_LOCK_FREE == 1 if (!atomic_is_lock_free(&e_flag)) { return EXIT_FAILURE; } #endif if (signal(SIGINT, handler) == SIG_ERR) { return EXIT_FAILURE; } strcpy(err_msg, "No errors yet."); /* Main code loop */ if (e_flag) { strcpy(err_msg, "SIGINT received."); } return EXIT_SUCCESS; }
Exceptions
SIG31-C-EX1: The C Standard, 7.14.1.1 paragraph 5 [ISO/IEC 9899:2011], makes a special exception for errno
when a valid call to the signal()
function results in a SIG_ERR
return, allowing errno
to take an indeterminate value. (See ERR32-C. Do not rely on indeterminate values of errno.)
Risk Assessment
Accessing or modifying shared objects in signal handlers can result in accessing data in an inconsistent state. Michal Zalewski's paper "Delivering Signals for Fun and Profit" [Zalewski 2001] provides some examples of vulnerabilities that can result from violating this and other signal-handling rules.
Rule | Severity | Likelihood | Remediation Cost | Priority | Level |
---|---|---|---|---|---|
SIG31-C | High | Likely | High | P9 | L2 |
Automated Detection
Tool | Version | Checker | Description |
---|---|---|---|
Astrée | 24.04 | signal-handler-shared-access | Partially checked |
Axivion Bauhaus Suite | 7.2.0 | CertC-SIG31 | |
CodeSonar | 8.1p0 | CONCURRENCY.DATARACE | Data race |
Compass/ROSE | Can detect violations of this rule for single-file programs | ||
Cppcheck Premium | 24.9.0 | premium-cert-sig31-c | Partially implemented |
Helix QAC | 2024.2 | C2029, C2030 C++3854, C++3855 | |
LDRA tool suite | 9.7.1 | 87 D | Fully implemented |
Parasoft C/C++test | 2023.1 | CERT_C-SIG31-a | Properly define signal handlers |
PC-lint Plus | 1.4 | 2765 | Fully supported |
R2024a | CERT C: Rule SIG31-C | Checks for shared data access within signal handler (rule partially covered) | |
RuleChecker | 24.04 | signal-handler-shared-access | Partially checked |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
Key here (explains table format and definitions)
Taxonomy | Taxonomy item | Relationship |
---|---|---|
ISO/IEC TS 17961:2013 | Accessing shared objects in signal handlers [accsig] | Prior to 2018-01-12: CERT: Unspecified Relationship |
CWE 2.11 | CWE-662, Improper Synchronization | 2017-07-10: CERT: Rule subset of CWE |
CWE 2.11 | CWE-828, Signal Handler with Functionality that is not Asynchronous-Safe | 2017-10-30:MITRE:Unspecified Relationship 2018-10-19:CERT:Rule subset of CWE |
CERT-CWE Mapping Notes
Key here for mapping notes
CWE-662 and SIG31-C
CWE-662 = Union( SIG31-C, list) where list =
- Improper synchronization of shared objects between threads
- Improper synchronization of files between programs (enabling TOCTOU race conditions
CWE-828 and SIG31-C
CWE-828 = SIG31-C + non-async-safe things besides shared objects.
Bibliography
[C99 Rationale 2003] | 5.2.3, "Signals and Interrupts" |
[ISO/IEC 9899:2011] | Subclause 7.14.1.1, "The signal Function" |
[Zalewski 2001] |
8 Comments
David Keaton
Yes, that's right. A signal handler can store into a
volatile sig_atomic_t
of static storage duration but cannot read it. SIG00-A has one such access and it documents its violation of SIG31-C. If an NCCE surfaces that doesn't violate SIG31-C, so much the better.Signal handlers can access all of their parameters and local variables. Only objects of static storage duration are off limits. For the purposes of this restriction, objects in the heap count as having static storage duration. The purpose is to allow all accesses in the signal handler's stack frame, and disallow everything but a store to a
volatile sig_atomic_t
everywhere else.Of course, this is just for maximally portable code. Most systems allow their own set of additional actions.
David Svoboda
Text and compliant example now both get & set static volatile sig_atomic_t values
Robert Seacord
Mitigation Strategies
Static Analysis
Compliance with this rule can be checked using structural static analysis checkers using the following algorithm:
signal(int, void (*f)(int))
.signal(int, void (*f)(int))
get the second argument from the argument list. To make sure that this is not an overloaded function the function type signature is evaluated and/or the location of the declaration of the function is verified to be from the correct file (because this is not a link-time analysis it is not possible to test the library implementation). Any definition forsignal()
in the application is suspicious, because it should be in a library.sig_atomic_t
.Douglas A. Gwyn
It should be mentioned that _Exit (and probably abort) are not advisable, because atexit-registered handlers should be allowed to perform their intended functions before the program is terminated.
Also, there seems to be no real reason for a signal handler to want to read a flag; setting it is sufficient.
David Svoboda
re: _Exit()/abort(). Didn't we have this conversation on SIG30-C?
re: handlers reading flags. True, the examples didn't use to read flags. SIG00-C has example code that reads flags (implementing a finite state machine with them). The examples here do illustrate that reading sig_atomic_t flags is perfectly safe (if not particularly useful
Alex Volkovitsky
should we remove the note about "This may be necessary to ensure that the handler persists" since the cited recommendation tells us not to do this because of the race condition?
David Svoboda
Yes, I updated that paragraph, mainly deferring to SIG30, which discusses safe functions to call in signal handlers.
David Svoboda
Why is the Rem Cost high here? It should be easy to inspect a signal handler source code to see if it references any static data, and whether that data is volatile sig_atomic_t. It would still be hard to repair. So the Remediation Cost should be Medium.