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 <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220509004148.1801791-8-andrii@kernel.org

6 files changed, 24 insertions, 8 deletions

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index bbae9a057bc8..fb04eaf367f1 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -76,6 +76,30 @@
 #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
diff --git a/tools/testing/selftests/bpf/progs/exhandler_kern.c b/tools/testing/selftests/bpf/progs/exhandler_kern.c
index dd9b30a0f0fc..20d009e2d266 100644
--- a/tools/testing/selftests/bpf/progs/exhandler_kern.c
+++ b/tools/testing/selftests/bpf/progs/exhandler_kern.c
@@ -7,8 +7,6 @@
 #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;
diff --git a/tools/testing/selftests/bpf/progs/loop5.c b/tools/testing/selftests/bpf/progs/loop5.c
index 913791923fa3..1b13f37f85ec 100644
--- a/tools/testing/selftests/bpf/progs/loop5.c
+++ b/tools/testing/selftests/bpf/progs/loop5.c
@@ -2,7 +2,6 @@
 // Copyright (c) 2019 Facebook
 #include <linux/bpf.h>
 #include <bpf/bpf_helpers.h>
-#define barrier() __asm__ __volatile__("": : :"memory")
 
 char _license[] SEC("license") = "GPL";
 
diff --git a/tools/testing/selftests/bpf/progs/profiler1.c b/tools/testing/selftests/bpf/progs/profiler1.c
index 4df9088bfc00..fb6b13522949 100644
--- a/tools/testing/selftests/bpf/progs/profiler1.c
+++ b/tools/testing/selftests/bpf/progs/profiler1.c
@@ -1,6 +1,5 @@
 // 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"
diff --git a/tools/testing/selftests/bpf/progs/pyperf.h b/tools/testing/selftests/bpf/progs/pyperf.h
index 5d3dc4d66d47..6c7b1fb268d6 100644
--- a/tools/testing/selftests/bpf/progs/pyperf.h
+++ b/tools/testing/selftests/bpf/progs/pyperf.h
@@ -171,8 +171,6 @@ struct process_frame_ctx {
 	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;
diff --git a/tools/testing/selftests/bpf/progs/test_pkt_access.c b/tools/testing/selftests/bpf/progs/test_pkt_access.c
index 0558544e1ff0..5cd7c096f62d 100644
--- a/tools/testing/selftests/bpf/progs/test_pkt_access.c
+++ b/tools/testing/selftests/bpf/progs/test_pkt_access.c
@@ -14,8 +14,6 @@
 #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: