[clang] [BoundsSafety] Initial documentation for -fbounds-safety (PR #70749)

[Dan Liew via cfe-commits]

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+==============================================
+-fbounds-safety: Enforcing bounds safety for C
+==============================================
+
+.. contents::
+   :local:
+
+Overview
+========
+
+-fbounds-safety is a C extension to enforce bounds safety to prevent out-of-bounds (OOB) memory accesses, which remain a major source of security vulnerabilities in C. -fbounds-safety aims to eliminate this class of bugs by turning OOB accesses into deterministic traps.
+
+The -fbounds-safety extension offers bounds annotations that programmers can use to attach bounds to pointers. For example, programmers can add the __counted_by(N) annotation to parameter ptr, indicating that the pointer has N valid elements:
+
+.. code-block:: c
+
+   void foo(int *__counted_by(N) ptr, size_t N);
+
+Using this bounds information, the compiler inserts bounds checks on every pointer dereference, ensuring that the program does not access memory outside the specified bounds. The compiler requires programmers to provide enough bounds information so that the accesses can be checked at either run time or compile time — and it rejects code if it cannot.
+
+The most important contribution of “-fbounds-safety” is how it reduces the programmer’s annotation burden by reconciling bounds annotations at ABI boundaries with the use of implicit wide pointers (a.k.a. “fat” pointers) that carry bounds information on local variables without the need for annotations. We designed this model so that it preserves ABI compatibility with C while minimizing adoption effort.
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delcypher wrote:

Did you mean to use open and close quotes (`“` and `”`) rather than symmetrical quote `"`? I would prefer to use the symmetrical quote because it makes edits to the document easier.

https://github.com/llvm/llvm-project/pull/70749