libbpf: Provide barrier() and barrier_var() in bpf_helpers.h

Add barrier() and barrier_var() macros into bpf_helpers.h to be used by
end users. While a bit advanced and specialized instruments, they are
sometimes indispensable. Instead of requiring each user to figure out
exact asm volatile incantations for themselves, provide them from
bpf_helpers.h.

Also remove conflicting definitions from selftests. Some tests rely on
barrier_var() definition being nothing, those will still work as libbpf
does the #ifndef/#endif guarding for barrier() and barrier_var(),
allowing users to redefine them, if necessary.

Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/lib/bpf/bpf_helpers.h

···
 #endif
 
 /*
+ * Compiler (optimization) barrier.
+ */
+#ifndef barrier
+#define barrier() asm volatile("" ::: "memory")
+#endif
+
+/* Variable-specific compiler (optimization) barrier. It's a no-op which makes
+ * compiler believe that there is some black box modification of a given
+ * variable and thus prevents compiler from making extra assumption about its
+ * value and potential simplifications and optimizations on this variable.
+ *
+ * E.g., compiler might often delay or even omit 32-bit to 64-bit casting of
+ * a variable, making some code patterns unverifiable. Putting barrier_var()
+ * in place will ensure that cast is performed before the barrier_var()
+ * invocation, because compiler has to pessimistically assume that embedded
+ * asm section might perform some extra operations on that variable.
+ *
+ * This is a variable-specific variant of more global barrier().
+ */
+#ifndef barrier_var
+#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))
+#endif
+
+/*
  * Helper macro to throw a compilation error if __bpf_unreachable() gets
  * built into the resulting code. This works given BPF back end does not
  * implement __builtin_trap(). This is useful to assert that certain paths

tools/testing/selftests/bpf/progs/exhandler_kern.c

···
 #include <bpf/bpf_tracing.h>
 #include <bpf/bpf_core_read.h>
 
-#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))
-
 char _license[] SEC("license") = "GPL";
 
 unsigned int exception_triggered;

tools/testing/selftests/bpf/progs/loop5.c

···
 // Copyright (c) 2019 Facebook
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
-#define barrier() __asm__ __volatile__("": : :"memory")
 
 char _license[] SEC("license") = "GPL";
 

tools/testing/selftests/bpf/progs/profiler1.c

···
 // SPDX-License-Identifier: GPL-2.0
 /* Copyright (c) 2020 Facebook */
-#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))
 #define UNROLL
 #define INLINE __always_inline
 #include "profiler.inc.h"

tools/testing/selftests/bpf/progs/pyperf.h

···
 	bool done;
 };
 
-#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))
-
 static int process_frame_callback(__u32 i, struct process_frame_ctx *ctx)
 {
 	int zero = 0;

tools/testing/selftests/bpf/progs/test_pkt_access.c

···
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_endian.h>
 
-#define barrier() __asm__ __volatile__("": : :"memory")
-
 /* llvm will optimize both subprograms into exactly the same BPF assembly
  *
  * Disassembly of section .text: