[clang] [clang][Analyzer] Move checker 'alpha.unix.Errno' to 'unix.Errno'. (PR #69469)

[via cfe-commits]

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@@ -934,6 +934,76 @@ Check calls to various UNIX/Posix functions: ``open, pthread_once, calloc, mallo
 .. literalinclude:: checkers/unix_api_example.c
     :language: c
 
+.. _unix-Errno:
+
+unix.Errno (C)
+""""""""""""""
+
+Check for improper use of ``errno``.
+This checker implements partially CERT rule
+`ERR30-C. Set errno to zero before calling a library function known to set errno,
+and check errno only after the function returns a value indicating failure
+<https://wiki.sei.cmu.edu/confluence/pages/viewpage.action?pageId=87152351>`_.
+The checker can find the first read of ``errno`` after successful standard
+function calls.
+
+The C and POSIX standards often do not define if a standard library function
+may change value of ``errno`` if the call does not fail.
+Therefore, ``errno`` should only be used if it is known from the return value
+of a function that the call has failed.
+There are exceptions to this rule (for example ``strtol``) but the affected
+functions are not yet supported by the checker.
+The return values for the failure cases are documented in the standard Linux man
+pages of the functions and in the `POSIX standard <https://pubs.opengroup.org/onlinepubs/9699919799/>`_.
+
+.. code-block:: c
+
+ int unsafe_errno_read(int sock, void *data, int data_size) {
+   if (send(sock, data, data_size, 0) != data_size) {
+     // 'send' can be successful even if not all data was sent
+     if (errno == 1) { // An undefined value may be read from 'errno'
+       return 0;
+     }
+   }
+   return 1;
+ }
+
+The checker :ref:`unix-StdCLibraryFunctions` must be turned on to get the
+warnings from this checker. The supported functions are the same as by
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DonatNagyE wrote:

Thanks for the clarification. I think it'd be important to improve this situation (it's very user-hostile to hide an "if you want to use this, you must manually enable another checker" remark in the docs), but this is independent of the currently reviewed commit.

https://github.com/llvm/llvm-project/pull/69469