|
| 1 | +# ERR32-C: Do not rely on indeterminate values of errno |
| 2 | + |
| 3 | +This query implements the CERT-C rule ERR32-C: |
| 4 | + |
| 5 | +> Do not rely on indeterminate values of errno |
| 6 | +
|
| 7 | + |
| 8 | +## Description |
| 9 | + |
| 10 | +According to the C Standard \[[ISO/IEC 9899:2011](https://wiki.sei.cmu.edu/confluence/display/c/AA.+Bibliography#AA.Bibliography-ISO-IEC9899-2011)\], the behavior of a program is [undefined](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-undefinedbehavior) when |
| 11 | + |
| 12 | +> the value of `errno` is referred to after a signal occurred other than as the result of calling the `abort` or `raise` function and the corresponding signal handler obtained a `SIG_ERR` return from a call to the `signal` function. |
| 13 | +
|
| 14 | + |
| 15 | +See [undefined behavior 133](https://wiki.sei.cmu.edu/confluence/display/c/CC.+Undefined+Behavior#CC.UndefinedBehavior-ub_133). |
| 16 | + |
| 17 | +A signal handler is allowed to call `signal();` if that fails, `signal()` returns `SIG_ERR` and sets `errno` to a positive value. However, if the event that caused a signal was external (not the result of the program calling `abort()` or `raise()`), the only functions the signal handler may call are `_Exit()` or `abort()`, or it may call `signal()` on the signal currently being handled; if `signal()` fails, the value of `errno` is [indeterminate](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-indeterminatevalue). |
| 18 | + |
| 19 | +This rule is also a special case of [SIG31-C. Do not access shared objects in signal handlers](https://wiki.sei.cmu.edu/confluence/display/c/SIG31-C.+Do+not+access+shared+objects+in+signal+handlers). The object designated by `errno` is of static storage duration and is not a `volatile sig_atomic_t`. As a result, performing any action that would require `errno` to be set would normally cause [undefined behavior](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-undefinedbehavior). The C Standard, 7.14.1.1, paragraph 5, makes a special exception for `errno` in this case, allowing `errno` to take on an indeterminate value but specifying that there is no other [undefined behavior](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-undefinedbehavior). This special exception makes it possible to call `signal()` from within a signal handler without risking [undefined behavior](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-undefinedbehavior), but the handler, and any code executed after the handler returns, must not depend on the value of `errno` being meaningful. |
| 20 | + |
| 21 | +## Noncompliant Code Example |
| 22 | + |
| 23 | +The `handler()` function in this noncompliant code example attempts to restore default handling for the signal indicated by `signum`. If the request to set the signal to default can be honored, the `signal()` function returns the value of the signal handler for the most recent successful call to the `signal()` function for the specified signal. Otherwise, a value of `SIG_ERR` is returned and a positive value is stored in `errno`. Unfortunately, the value of `errno` is indeterminate because the `handler()` function is called when an external signal is raised, so any attempt to read `errno` (for example, by the `perror()` function) is [undefined behavior](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-undefinedbehavior): |
| 24 | + |
| 25 | +```cpp |
| 26 | +#include <signal.h> |
| 27 | +#include <stdlib.h> |
| 28 | +#include <stdio.h> |
| 29 | + |
| 30 | +typedef void (*pfv)(int); |
| 31 | + |
| 32 | +void handler(int signum) { |
| 33 | + pfv old_handler = signal(signum, SIG_DFL); |
| 34 | + if (old_handler == SIG_ERR) { |
| 35 | + perror("SIGINT handler"); /* Undefined behavior */ |
| 36 | + /* Handle error */ |
| 37 | + } |
| 38 | +} |
| 39 | + |
| 40 | +int main(void) { |
| 41 | + pfv old_handler = signal(SIGINT, handler); |
| 42 | + if (old_handler == SIG_ERR) { |
| 43 | + perror("SIGINT handler"); |
| 44 | + /* Handle error */ |
| 45 | + } |
| 46 | + |
| 47 | + /* Main code loop */ |
| 48 | + |
| 49 | + return EXIT_SUCCESS; |
| 50 | +} |
| 51 | + |
| 52 | +``` |
| 53 | +The call to `perror()` from `handler()` also violates [SIG30-C. Call only asynchronous-safe functions within signal handlers](https://wiki.sei.cmu.edu/confluence/display/c/SIG30-C.+Call+only+asynchronous-safe+functions+within+signal+handlers). |
| 54 | +
|
| 55 | +## Compliant Solution |
| 56 | +
|
| 57 | +This compliant solution does not reference `errno` and does not return from the signal handler if the `signal()` call fails: |
| 58 | +
|
| 59 | +```cpp |
| 60 | +#include <signal.h> |
| 61 | +#include <stdlib.h> |
| 62 | +#include <stdio.h> |
| 63 | +
|
| 64 | +typedef void (*pfv)(int); |
| 65 | +
|
| 66 | +void handler(int signum) { |
| 67 | + pfv old_handler = signal(signum, SIG_DFL); |
| 68 | + if (old_handler == SIG_ERR) { |
| 69 | + abort(); |
| 70 | + } |
| 71 | +} |
| 72 | +
|
| 73 | +int main(void) { |
| 74 | + pfv old_handler = signal(SIGINT, handler); |
| 75 | + if (old_handler == SIG_ERR) { |
| 76 | + perror("SIGINT handler"); |
| 77 | + /* Handle error */ |
| 78 | + } |
| 79 | +
|
| 80 | + /* Main code loop */ |
| 81 | +
|
| 82 | + return EXIT_SUCCESS; |
| 83 | +} |
| 84 | +
|
| 85 | +``` |
| 86 | + |
| 87 | +## Noncompliant Code Example (POSIX) |
| 88 | + |
| 89 | +POSIX is less restrictive than C about what applications can do in signal handlers. It has a long list of [asynchronous-safe](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-asynchronous-safe) functions that can be called. (See [SIG30-C. Call only asynchronous-safe functions within signal handlers](https://wiki.sei.cmu.edu/confluence/display/c/SIG30-C.+Call+only+asynchronous-safe+functions+within+signal+handlers).) Many of these functions set `errno` on error, which can lead to a signal handler being executed between a call to a failed function and the subsequent inspection of `errno`. Consequently, the value inspected is not the one set by that function but the one set by a function call in the signal handler. POSIX applications can avoid this problem by ensuring that signal handlers containing code that might alter `errno`; always save the value of `errno` on entry and restore it before returning. |
| 90 | + |
| 91 | +The signal handler in this noncompliant code example alters the value of `errno`. As a result, it can cause incorrect error handling if executed between a failed function call and the subsequent inspection of `errno`: |
| 92 | + |
| 93 | +```cpp |
| 94 | +#include <signal.h> |
| 95 | +#include <stdlib.h> |
| 96 | +#include <errno.h> |
| 97 | +#include <sys/wait.h> |
| 98 | + |
| 99 | +void reaper(int signum) { |
| 100 | + errno = 0; |
| 101 | + for (;;) { |
| 102 | + int rc = waitpid(-1, NULL, WNOHANG); |
| 103 | + if ((0 == rc) || (-1 == rc && EINTR != errno)) { |
| 104 | + break; |
| 105 | + } |
| 106 | + } |
| 107 | + if (ECHILD != errno) { |
| 108 | + /* Handle error */ |
| 109 | + } |
| 110 | +} |
| 111 | + |
| 112 | +int main(void) { |
| 113 | + struct sigaction act; |
| 114 | + act.sa_handler = reaper; |
| 115 | + act.sa_flags = 0; |
| 116 | + if (sigemptyset(&act.sa_mask) != 0) { |
| 117 | + /* Handle error */ |
| 118 | + } |
| 119 | + if (sigaction(SIGCHLD, &act, NULL) != 0) { |
| 120 | + /* Handle error */ |
| 121 | + } |
| 122 | + |
| 123 | + /* ... */ |
| 124 | + |
| 125 | + return EXIT_SUCCESS; |
| 126 | +} |
| 127 | + |
| 128 | +``` |
| 129 | +
|
| 130 | +## Compliant Solution (POSIX) |
| 131 | +
|
| 132 | +This compliant solution saves and restores the value of `errno` in the signal handler: |
| 133 | +
|
| 134 | +```cpp |
| 135 | +#include <signal.h> |
| 136 | +#include <stdlib.h> |
| 137 | +#include <errno.h> |
| 138 | +#include <sys/wait.h> |
| 139 | +
|
| 140 | +void reaper(int signum) { |
| 141 | + errno_t save_errno = errno; |
| 142 | + errno = 0; |
| 143 | + for (;;) { |
| 144 | + int rc = waitpid(-1, NULL, WNOHANG); |
| 145 | + if ((0 == rc) || (-1 == rc && EINTR != errno)) { |
| 146 | + break; |
| 147 | + } |
| 148 | + } |
| 149 | + if (ECHILD != errno) { |
| 150 | + /* Handle error */ |
| 151 | + } |
| 152 | + errno = save_errno; |
| 153 | +} |
| 154 | +
|
| 155 | +int main(void) { |
| 156 | + struct sigaction act; |
| 157 | + act.sa_handler = reaper; |
| 158 | + act.sa_flags = 0; |
| 159 | + if (sigemptyset(&act.sa_mask) != 0) { |
| 160 | + /* Handle error */ |
| 161 | + } |
| 162 | + if (sigaction(SIGCHLD, &act, NULL) != 0) { |
| 163 | + /* Handle error */ |
| 164 | + } |
| 165 | +
|
| 166 | + /* ... */ |
| 167 | +
|
| 168 | + return EXIT_SUCCESS; |
| 169 | +} |
| 170 | +
|
| 171 | +``` |
| 172 | + |
| 173 | +## Risk Assessment |
| 174 | + |
| 175 | +Referencing indeterminate values of `errno` is [undefined behavior](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-undefinedbehavior). |
| 176 | + |
| 177 | +<table> <tbody> <tr> <th> Rule </th> <th> Severity </th> <th> Likelihood </th> <th> Remediation Cost </th> <th> Priority </th> <th> Level </th> </tr> <tr> <td> ERR32-C </td> <td> Low </td> <td> Unlikely </td> <td> Low </td> <td> <strong>P3</strong> </td> <td> <strong>L3</strong> </td> </tr> </tbody> </table> |
| 178 | + |
| 179 | + |
| 180 | +## Automated Detection |
| 181 | + |
| 182 | +<table> <tbody> <tr> <th> Tool </th> <th> Version </th> <th> Checker </th> <th> Description </th> </tr> <tr> <td> <a> Axivion Bauhaus Suite </a> </td> <td> 7.2.0 </td> <td> <strong>CertC-ERR32</strong> </td> <td> </td> </tr> <tr> <td> <a> Compass/ROSE </a> </td> <td> </td> <td> </td> <td> Could detect violations of this rule by looking for signal handlers that themselves call <code>signal()</code> . A violation is reported if the call fails and the handler therefore checks <code>errno</code> . A violation also exists if the signal handler modifies <code>errno</code> without first copying its value elsewhere </td> </tr> <tr> <td> <a> Coverity </a> </td> <td> 2017.07 </td> <td> <strong>MISRA C 2012 Rule 22.8</strong> <strong><strong>MISRA C 2012 Rule 22.9</strong></strong> <strong><strong><strong>MISRA C 2012 Rule 22.10</strong></strong></strong> </td> <td> Implemented </td> </tr> <tr> <td> <a> Helix QAC </a> </td> <td> 2022.3 </td> <td> <strong>C2031, C4781, C4782, C4783</strong> <strong>C++4781, C++4782, C++4783</strong> </td> <td> </td> </tr> <tr> <td> <a> Klocwork </a> </td> <td> 2022.3 </td> <td> <strong>MISRA.INCL.SIGNAL.2012</strong> <strong>MISRA.STDLIB.SIGNAL</strong> </td> <td> </td> </tr> <tr> <td> <a> LDRA tool suite </a> </td> <td> 9.7.1 </td> <td> 44 S </td> <td> Enhanced enforcement </td> </tr> <tr> <td> <a> Parasoft C/C++test </a> </td> <td> 2022.1 </td> <td> <strong>CERT_C-ERR32-a</strong> </td> <td> Properly use errno value </td> </tr> <tr> <td> <a> Polyspace Bug Finder </a> </td> <td> R2022b </td> <td> <a> CERT C: Rule ERR32-C </a> </td> <td> Checks for misuse of errno in a signal handler (rule fully covered) </td> </tr> <tr> <td> <a> PRQA QA-C </a> </td> <td> 9.7 </td> <td> <strong>2031</strong> </td> <td> </td> </tr> </tbody> </table> |
| 183 | + |
| 184 | + |
| 185 | +## Related Vulnerabilities |
| 186 | + |
| 187 | +Search for [vulnerabilities](https://wiki.sei.cmu.edu/confluence/display/c/BB.+Definitions#BB.Definitions-vulnerability) resulting from the violation of this rule on the [CERT website](https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+ERR32-C). |
| 188 | + |
| 189 | +## Related Guidelines |
| 190 | + |
| 191 | +[Key here](https://wiki.sei.cmu.edu/confluence/display/c/How+this+Coding+Standard+is+Organized#HowthisCodingStandardisOrganized-RelatedGuidelines) (explains table format and definitions) |
| 192 | + |
| 193 | +<table> <tbody> <tr> <th> Taxonomy </th> <th> Taxonomy item </th> <th> Relationship </th> </tr> <tr> <td> <a> CERT C Secure Coding Standard </a> </td> <td> <a> SIG30-C. Call only asynchronous-safe functions within signal handlers </a> </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> <tr> <td> <a> CERT C Secure Coding Standard </a> </td> <td> <a> SIG31-C. Do not access shared objects in signal handlers </a> </td> <td> Prior to 2018-01-12: CERT: Unspecified Relationship </td> </tr> </tbody> </table> |
| 194 | + |
| 195 | + |
| 196 | +## Bibliography |
| 197 | + |
| 198 | +<table> <tbody> <tr> <td> \[ <a> ISO/IEC 9899:2011 </a> \] </td> <td> Subclause 7.14.1.1, "The <code>signal</code> Function" </td> </tr> </tbody> </table> |
| 199 | + |
| 200 | + |
| 201 | +## Implementation notes |
| 202 | + |
| 203 | +The rule is enforced in the context of a single function. |
| 204 | + |
| 205 | +## References |
| 206 | + |
| 207 | +* CERT-C: [ERR32-C: Do not rely on indeterminate values of errno](https://wiki.sei.cmu.edu/confluence/display/c) |
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