Adding documentation to CWE-197 as part of ossf#531

Signed-off-by: Helge Wehder <[email protected]>

docs/Secure-Coding-Guide-for-Python/CWE-664/CWE-197/README.md

@@ -0,0 +1,154 @@
+# CWE-197: Numeric Truncation Error
+
+Ensure to have predictable outcomes in loops by using int instead of `float` variables as a counter.
+
+Floating-point arithmetic can only represent a finite subset of real numbers [[IEEE Std 754-2019](https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8766229)], such as `0.555....` represented by `0.5555555555555556` also discussed in [CWE-1339: Insufficient Precision or Accuracy of a Real Number](https://github.com/ossf/wg-best-practices-os-developers/tree/main/docs/Secure-Coding-Guide-for-Python/CWE-682/CWE-1339). Code examples in this rule are based on [Albing and Vossen, 2017].
+
+Side effects of using `float` as a counter is demonstrated in `example01.py` showcasing that calculating `0.1 + 0.2` does not end up as `0.3`.
+
+[*example01.py:*](example01.py)
+
+```py
+""" Code Example """
+
+value = 0.0
+while value <= 1:
+    print(f"{type(value)} {value}")
+    value += 0.1
+```
+
+ **Output of exampl01.py:**
+
+```bash
+<class 'float'> 0.0
+<class 'float'> 0.1
+<class 'float'> 0.2
+<class 'float'> 0.30000000000000004
+<class 'float'> 0.4
+<class 'float'> 0.5
+<class 'float'> 0.6
+<class 'float'> 0.7
+<class 'float'> 0.7999999999999999
+<class 'float'> 0.8999999999999999
+<class 'float'> 0.9999999999999999
+```
+
+## Non-Compliant Code Example
+
+ The `noncompliant01.py` code demonstrates a side effect when a floating point counter is used.
+
+[*noncompliant01.py:*](noncompliant01.py)
+
+```py
+""" Non-compliant Code Example """
+counter = 0.0
+while counter <= 1.0:
+    if counter == 0.8:
+        print("we reached 0.8")  
+        break  # never going to reach this
+    counter += 0.1
+```
+
+The `noncompliant01.py` code will never print "we are at 0.8" due to lack of precision or controlled rounding.
+
+## Compliant Solution
+
+The `compliant01.py` makes use of integer as long as possible and only converts to `float` where needed.
+
+[*compliant01.py:*](compliant01.py)
+
+```py
+""" Compliant Code Example """
+counter = 0
+while counter <= 10:
+    value = counter/10
+    if value == 0.8:
+        print("we reached 0.8")
+        break
+    counter += 1
+```
+
+## Non-Compliant Code Example
+
+The `example02.py` code demonstrates more precision limites in floating numbers.
+
+[*example02.py:*](example02.py)
+
+```py
+""" Code Example """
+print(f"{1.0 + 1e-16:.20f}")
+print(f"{1.0 + 1e-15:.20f}")
+```
+
+**Output of example02.py:**
+
+```bash
+1.00000000000000000000
+1.00000000000000111022
+```
+
+Below `noncompliant02.py` code tries to increment a floating-point `COUNTER` by a too small value causing an infinite loop.
+
+[*noncompliant02.py:*](noncompliant02.py)
+
+```py
+""" Non-compliant Code Example """
+counter = 1.0 + 1e-16
+target = 1.0 + 1e-15
+while counter <= target:  # never ends
+    print(f"counter={counter / 10**16 :.20f}")
+    print(f" target={target / 10**16:.20f}")
+    counter += 1e-16
+
+```
+
+The code will loop forever due to missing precision in the initial calculation of `COUNTER = 1.0 + 1e-16`.
+
+## Compliant Solution
+
+Use of an `int` loop counter that is only converted to `float` when required is demonstrated in `compliant2.py`.
+
+[*compliant02.py:*](compliant02.py)
+
+```py
+""" Compliant Code Example """
+counter = 1
+target = 10
+while counter <= target:
+    print(f"counter={counter / 10**16 :.20f}")
+    print(f" target={target / 10**16:.20f}")
+    counter += 1
+```
+
+## Defnitions
+
+|Definition|Explanation|Reference|
+|:---|:---|:---|
+|Loop Counters|loop counters are variables used to control the iterations of a loop|[Loop counter - Wikipedia](https://en.wikipedia.org/wiki/For_loop#Loop_counters)|
+
+## Automated Detection
+
+|Tool|Version|Checker|Description|
+|:---|:---|:---|:---|
+|Bandit|1.7.4 on Python 3.10.4|Not Available||
+|Flake8|8-4.0.1 on Python 3.10.4|Not Available||
+
+## Related Guidelines
+
+|||
+|:---|:---|
+|[MITRE CWE](http://cwe.mitre.org/)|Pillar [CWE-664: Improper Control of a Resource Through its Lifetime (4.13) (mitre.org)](https://cwe.mitre.org/data/definitions/664.html)|
+|[MITRE CWE](http://cwe.mitre.org/)|Class [CWE-197: Numeric Truncation Error](https://cwe.mitre.org/data/definitions/197.html)|
+|[SEI CERT Coding Standard for Java](https://wiki.sei.cmu.edu/confluence/display/java/SEI+CERT+Oracle+Coding+Standard+for+Java)|[NUM09-J. Do not use floating-point variables as loop counters](https://wiki.sei.cmu.edu/confluence/display/java/NUM09-J.+Do+not+use+floating-point+variables+as+loop+counters)|
+|[SEI CERT C Coding Standard](https://web.archive.org/web/20220511061752/https://wiki.sei.cmu.edu/confluence/display/c/SEI+CERT+C+Coding+Standard)|[FLP30-C. Do not use floating-point variables as loop counters](https://web.archive.org/web/20220511061752/https://wiki.sei.cmu.edu/confluence/display/c/FLP30-C.+Do+not+use+floating-point+variables+as+loop+counters)|
+|[ISO/IEC TR 24772:2010]||
+
+## Biblography
+
+|||
+|:---|:---|
+|[IEEE Std 754-2019](https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8766229)|IEEE Standard for Floating-Point Arithmetic, available from: [https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8766229](https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8766229), [Last  accessed June 2024] |
+|[Wikipedia 2024]|Repeating Decimals, available from:[https://en.wikipedia.org/wiki/Repeating_decimal](https://en.wikipedia.org/wiki/Repeating_decimal), [Last accessed August 2024] |
+|[Albing and Vossen, 2017]|Albin, C. and Vossen, JP (2017) 6.13 Looping with Floating Point Values. In: Bleiel, J., Brown, K. and Head, R. eds. bash Cookbook: Solutions and Examples for bash Users, 2d Edition. Sebastopol: O'Reilly Media, Inc., pp.159-160|
+|[Bloch 2005]|Puzzle 34, "Down for the Count", available from: [https://web.archive.org/web/20220511061752/https://wiki.sei.cmu.edu/confluence/display/java/Rule+AA.+References#RuleAA.References-Bloch05](https://web.archive.org/web/20220511061752/https://wiki.sei.cmu.edu/confluence/display/java/Rule+AA.+References#RuleAA.References-Bloch05), [Last accessed August 2024] |
+

docs/Secure-Coding-Guide-for-Python/CWE-664/CWE-197/compliant01.py

@@ -1,7 +1,11 @@
 # SPDX-FileCopyrightText: OpenSSF project contributors
 # SPDX-License-Identifier: MIT
 """ Compliant Code Example """
-value = 0
-while value <= 9:
-    print(value / 9)
-    value = value + 1
+
+counter = 0
+while counter <= 10:
+    value = counter/10
+    if value == 0.8:
+        print("we reached 0.8")
+        break
+    counter += 1

docs/Secure-Coding-Guide-for-Python/CWE-664/CWE-197/compliant02.py

@@ -1,7 +1,9 @@
 # SPDX-FileCopyrightText: OpenSSF project contributors
 # SPDX-License-Identifier: MIT
 """ Compliant Code Example """
-value = 1
-while value <= 10:
-    print(f"{value / 10 ** 14:.14f}")
-    value = value + 1
+counter = 1
+target = 10
+while counter <= target:
+    print(f"counter={counter / 10**16 :.20f}")
+    print(f" target={target / 10**16:.20f}")
+    counter += 1

docs/Secure-Coding-Guide-for-Python/CWE-664/CWE-197/example01.py

@@ -0,0 +1,8 @@
+# SPDX-FileCopyrightText: OpenSSF project contributors
+# SPDX-License-Identifier: MIT
+""" Code Example """
+
+value = 0.0
+while value <= 1:
+    print(f"{type(value)} {value}")
+    value += 0.1

docs/Secure-Coding-Guide-for-Python/CWE-664/CWE-197/example02.py

@@ -0,0 +1,5 @@
+# SPDX-FileCopyrightText: OpenSSF project contributors
+# SPDX-License-Identifier: MIT
+""" Code Example """
+print(f"{1.0 + 1e-16:.20f}")
+print(f"{1.0 + 1e-15:.20f}")

docs/Secure-Coding-Guide-for-Python/CWE-664/CWE-197/noncompliant01.py

@@ -1,7 +1,9 @@
 # SPDX-FileCopyrightText: OpenSSF project contributors
 # SPDX-License-Identifier: MIT
 """ Non-compliant Code Example """
-value = float(0.0)
-while value <= 1:
-    print(value)
-    value = value + float(1.0/9.0)
+counter = 0.0
+while counter <= 1.0:
+    if counter == 0.8:
+        print("we reached 0.8")  
+        break  # never going to reach this
+    counter += 0.1

docs/Secure-Coding-Guide-for-Python/CWE-664/CWE-197/noncompliant02.py

@@ -1,8 +1,10 @@
 # SPDX-FileCopyrightText: OpenSSF project contributors
 # SPDX-License-Identifier: MIT
 """ Non-compliant Code Example """
-value = float(1.0) + float("1e-18")
-target = float(1.0) + float("1e-17")
-while value <= target:
-    print(value)
-    value = value + float("1e-18")
+
+counter = 1.0 + 1e-16
+target = 1.0 + 1e-15
+while counter <= target:  # never ends
+    print(f"counter={counter / 10**16 :.20f}")
+    print(f" target={target / 10**16:.20f}")
+    counter += 1e-16

docs/Secure-Coding-Guide-for-Python/readme.md

@@ -41,7 +41,7 @@ It is **not production code** and requires code-style or python best practices t
 |[CWE-664: Improper Control of a Resource Through its Lifetime](https://cwe.mitre.org/data/definitions/664.html)|Prominent CVE|
 |:-----------------------------------------------------------------------------------------------------------------------------------------------|:----|
 |[CWE-134: Use of Externally-Controlled Format String](CWE-664/CWE-134/README.md)|[CVE-2022-27177](https://www.cvedetails.com/cve/CVE-2022-27177/),<br/>CVSSv3.1: **9.8**,<br/>EPSS: **00.37** (01.12.2023)|
-|[CWE-197: Numeric Truncation Error](CWE-664/CWE-197/.)||
+|[CWE-197: Numeric Truncation Error](CWE-664/CWE-197/README.md)||
 |[CWE-197: Control rounding when converting to less precise numbers](CWE-664/CWE-197/01/.)||
 |[CWE-400: Uncontrolled Resource Consumption](CWE-664/CWE-400/README.md)||
 |[CWE-409: Improper Handling of Highly Compressed Data (Data Amplification)](CWE-664/CWE-409/.)||