selftests/bpf: Add BPF assertion macros

Add macros implementing an 'assert' statement primitive using macros,
built on top of the BPF exceptions support introduced in previous
patches.

The bpf_assert_*_with variants allow supplying a value which can the be
inspected within the exception handler to signify the assert statement
that led to the program being terminated abruptly, or be returned by the
default exception handler.

Note that only 64-bit scalar values are supported with these assertion
macros, as during testing I found other cases quite unreliable in
presence of compiler shifts/manipulations extracting the value of the
right width from registers scrubbing the verifier's bounds information
and knowledge about the value in the register.

Thus, it is easier to reliably support this feature with only the full
register width, and support both signed and unsigned variants.

The bpf_assert_range is interesting in particular, which clamps the
value in the [begin, end] (both inclusive) range within verifier state,
and emits a check for the same at runtime.
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230912233214.1518551-17-memxor@gmail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/bpf_experimental.h

@@ -207,4 +207,247 @@ extern void bpf_throw(u64 cookie) __ksym;
  */
 #define __exception_cb(name) __attribute__((btf_decl_tag("exception_callback:" #name)))
 
+#define __bpf_assert_signed(x) _Generic((x), \
+    unsigned long: 0,       \
+    unsigned long long: 0,  \
+    signed long: 1,         \
+    signed long long: 1     \
+)
+
+#define __bpf_assert_check(LHS, op, RHS)								 \
+	_Static_assert(sizeof(&(LHS)), "1st argument must be an lvalue expression");			 \
+	_Static_assert(sizeof(LHS) == 8, "Only 8-byte integers are supported\n");			 \
+	_Static_assert(__builtin_constant_p(__bpf_assert_signed(LHS)), "internal static assert");	 \
+	_Static_assert(__builtin_constant_p((RHS)), "2nd argument must be a constant expression")
+
+#define __bpf_assert(LHS, op, cons, RHS, VAL)							\
+	({											\
+		(void)bpf_throw;								\
+		asm volatile ("if %[lhs] " op " %[rhs] goto +2; r1 = %[value]; call bpf_throw"	\
+			       : : [lhs] "r"(LHS), [rhs] cons(RHS), [value] "ri"(VAL) : );	\
+	})
+
+#define __bpf_assert_op_sign(LHS, op, cons, RHS, VAL, supp_sign)			\
+	({										\
+		__bpf_assert_check(LHS, op, RHS);					\
+		if (__bpf_assert_signed(LHS) && !(supp_sign))				\
+			__bpf_assert(LHS, "s" #op, cons, RHS, VAL);			\
+		else									\
+			__bpf_assert(LHS, #op, cons, RHS, VAL);				\
+	 })
+
+#define __bpf_assert_op(LHS, op, RHS, VAL, supp_sign)					\
+	({										\
+		if (sizeof(typeof(RHS)) == 8) {						\
+			const typeof(RHS) rhs_var = (RHS);				\
+			__bpf_assert_op_sign(LHS, op, "r", rhs_var, VAL, supp_sign);	\
+		} else {								\
+			__bpf_assert_op_sign(LHS, op, "i", RHS, VAL, supp_sign);	\
+		}									\
+	 })
+
+/* Description
+ *	Assert that a conditional expression is true.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the value zero when the assertion fails.
+ */
+#define bpf_assert(cond) if (!(cond)) bpf_throw(0);
+
+/* Description
+ *	Assert that a conditional expression is true.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the specified value when the assertion fails.
+ */
+#define bpf_assert_with(cond, value) if (!(cond)) bpf_throw(value);
+
+/* Description
+ *	Assert that LHS is equal to RHS. This statement updates the known value
+ *	of LHS during verification. Note that RHS must be a constant value, and
+ *	must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the value zero when the assertion fails.
+ */
+#define bpf_assert_eq(LHS, RHS)						\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, ==, RHS, 0, true);			\
+	})
+
+/* Description
+ *	Assert that LHS is equal to RHS. This statement updates the known value
+ *	of LHS during verification. Note that RHS must be a constant value, and
+ *	must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the specified value when the assertion fails.
+ */
+#define bpf_assert_eq_with(LHS, RHS, value)				\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, ==, RHS, value, true);		\
+	})
+
+/* Description
+ *	Assert that LHS is less than RHS. This statement updates the known
+ *	bounds of LHS during verification. Note that RHS must be a constant
+ *	value, and must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the value zero when the assertion fails.
+ */
+#define bpf_assert_lt(LHS, RHS)						\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, <, RHS, 0, false);			\
+	})
+
+/* Description
+ *	Assert that LHS is less than RHS. This statement updates the known
+ *	bounds of LHS during verification. Note that RHS must be a constant
+ *	value, and must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the specified value when the assertion fails.
+ */
+#define bpf_assert_lt_with(LHS, RHS, value)				\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, <, RHS, value, false);		\
+	})
+
+/* Description
+ *	Assert that LHS is greater than RHS. This statement updates the known
+ *	bounds of LHS during verification. Note that RHS must be a constant
+ *	value, and must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the value zero when the assertion fails.
+ */
+#define bpf_assert_gt(LHS, RHS)						\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, >, RHS, 0, false);			\
+	})
+
+/* Description
+ *	Assert that LHS is greater than RHS. This statement updates the known
+ *	bounds of LHS during verification. Note that RHS must be a constant
+ *	value, and must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the specified value when the assertion fails.
+ */
+#define bpf_assert_gt_with(LHS, RHS, value)				\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, >, RHS, value, false);		\
+	})
+
+/* Description
+ *	Assert that LHS is less than or equal to RHS. This statement updates the
+ *	known bounds of LHS during verification. Note that RHS must be a
+ *	constant value, and must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the value zero when the assertion fails.
+ */
+#define bpf_assert_le(LHS, RHS)						\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, <=, RHS, 0, false);		\
+	})
+
+/* Description
+ *	Assert that LHS is less than or equal to RHS. This statement updates the
+ *	known bounds of LHS during verification. Note that RHS must be a
+ *	constant value, and must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the specified value when the assertion fails.
+ */
+#define bpf_assert_le_with(LHS, RHS, value)				\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, <=, RHS, value, false);		\
+	})
+
+/* Description
+ *	Assert that LHS is greater than or equal to RHS. This statement updates
+ *	the known bounds of LHS during verification. Note that RHS must be a
+ *	constant value, and must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the value zero when the assertion fails.
+ */
+#define bpf_assert_ge(LHS, RHS)						\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, >=, RHS, 0, false);		\
+	})
+
+/* Description
+ *	Assert that LHS is greater than or equal to RHS. This statement updates
+ *	the known bounds of LHS during verification. Note that RHS must be a
+ *	constant value, and must fit within the data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the specified value when the assertion fails.
+ */
+#define bpf_assert_ge_with(LHS, RHS, value)				\
+	({								\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, >=, RHS, value, false);		\
+	})
+
+/* Description
+ *	Assert that LHS is in the range [BEG, END] (inclusive of both). This
+ *	statement updates the known bounds of LHS during verification. Note
+ *	that both BEG and END must be constant values, and must fit within the
+ *	data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the value zero when the assertion fails.
+ */
+#define bpf_assert_range(LHS, BEG, END)					\
+	({								\
+		_Static_assert(BEG <= END, "BEG must be <= END");	\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, >=, BEG, 0, false);		\
+		__bpf_assert_op(LHS, <=, END, 0, false);		\
+	})
+
+/* Description
+ *	Assert that LHS is in the range [BEG, END] (inclusive of both). This
+ *	statement updates the known bounds of LHS during verification. Note
+ *	that both BEG and END must be constant values, and must fit within the
+ *	data type of LHS.
+ * Returns
+ *	Void.
+ * Throws
+ *	An exception with the specified value when the assertion fails.
+ */
+#define bpf_assert_range_with(LHS, BEG, END, value)			\
+	({								\
+		_Static_assert(BEG <= END, "BEG must be <= END");	\
+		barrier_var(LHS);					\
+		__bpf_assert_op(LHS, >=, BEG, value, false);		\
+		__bpf_assert_op(LHS, <=, END, value, false);		\
+	})
+
 #endif