netdev - [PATCH v4 bpf-next 5/9] libbpf: use static const fmt string in __bpf

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Date:   Thu, 2 Sep 2021 10:19:25 -0700
From:   Dave Marchevsky <davemarchevsky@...com>
To:     <bpf@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yonghong Song <yhs@...com>, <netdev@...r.kernel.org>,
        Dave Marchevsky <davemarchevsky@...com>
Subject: [PATCH v4 bpf-next 5/9] libbpf: use static const fmt string in __bpf_printk

The __bpf_printk convenience macro was using a 'char' fmt string holder
as it predates support for globals in libbpf. Move to more efficient
'static const char', but provide a fallback to the old way via
BPF_NO_GLOBAL_DATA so users on old kernels can still use the macro.

Signed-off-by: Dave Marchevsky <davemarchevsky@...com>
---
 tools/lib/bpf/bpf_helpers.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index a7e73be6dac4..7df7d9a23099 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -216,10 +216,16 @@ enum libbpf_tristate {
 		     ___param, sizeof(___param));		\
 })
 
+#ifdef BPF_NO_GLOBAL_DATA
+#define BPF_PRINTK_FMT_MOD
+#else
+#define BPF_PRINTK_FMT_MOD static const
+#endif
+
 /* Helper macro to print out debug messages */
 #define __bpf_printk(fmt, ...)				\
 ({							\
-	char ____fmt[] = fmt;				\
+	BPF_PRINTK_FMT_MOD char ____fmt[] = fmt;	\
 	bpf_trace_printk(____fmt, sizeof(____fmt),	\
 			 ##__VA_ARGS__);		\
 })
-- 
2.30.2