linux-kernel - [PATCH bpf-next 3/5] libbpf: Initialize the bpf_seq

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Message-Id: <20210310220211.1454516-4-revest@chromium.org>
Date:   Wed, 10 Mar 2021 23:02:09 +0100
From:   Florent Revest <revest@...omium.org>
To:     bpf@...r.kernel.org
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        yhs@...com, kpsingh@...nel.org, jackmanb@...omium.org,
        linux-kernel@...r.kernel.org, Florent Revest <revest@...omium.org>
Subject: [PATCH bpf-next 3/5] libbpf: Initialize the bpf_seq_printf parameters array field by field

When initializing the __param array with a one liner, if all args are
const, the initial array value will be placed in the rodata section but
because libbpf does not support relocation in the rodata section, any
pointer in this array will stay NULL.

This is a workaround, ideally the rodata relocation should be supported
by libbpf but this would require a disproportionate amount of work given
the actual usecases. (it is very unlikely that one uses a const array of
relocated addresses)

Signed-off-by: Florent Revest <revest@...omium.org>
---
 tools/lib/bpf/bpf_tracing.h | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/bpf_tracing.h b/tools/lib/bpf/bpf_tracing.h
index f9ef37707888..f6a2deb3cd5b 100644
--- a/tools/lib/bpf/bpf_tracing.h
+++ b/tools/lib/bpf/bpf_tracing.h
@@ -413,6 +413,34 @@ typeof(name(0)) name(struct pt_regs *ctx)				    \
 }									    \
 static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
 
+#define ___bpf_build_param0(narg, x)
+#define ___bpf_build_param1(narg, x) ___param[narg - 1] = x
+#define ___bpf_build_param2(narg, x, args...) ___param[narg - 2] = x; \
+					      ___bpf_build_param1(narg, args)
+#define ___bpf_build_param3(narg, x, args...) ___param[narg - 3] = x; \
+					      ___bpf_build_param2(narg, args)
+#define ___bpf_build_param4(narg, x, args...) ___param[narg - 4] = x; \
+					      ___bpf_build_param3(narg, args)
+#define ___bpf_build_param5(narg, x, args...) ___param[narg - 5] = x; \
+					      ___bpf_build_param4(narg, args)
+#define ___bpf_build_param6(narg, x, args...) ___param[narg - 6] = x; \
+					      ___bpf_build_param5(narg, args)
+#define ___bpf_build_param7(narg, x, args...) ___param[narg - 7] = x; \
+					      ___bpf_build_param6(narg, args)
+#define ___bpf_build_param8(narg, x, args...) ___param[narg - 8] = x; \
+					      ___bpf_build_param7(narg, args)
+#define ___bpf_build_param9(narg, x, args...) ___param[narg - 9] = x; \
+					      ___bpf_build_param8(narg, args)
+#define ___bpf_build_param10(narg, x, args...) ___param[narg - 10] = x; \
+					       ___bpf_build_param9(narg, args)
+#define ___bpf_build_param11(narg, x, args...) ___param[narg - 11] = x; \
+					       ___bpf_build_param10(narg, args)
+#define ___bpf_build_param12(narg, x, args...) ___param[narg - 12] = x; \
+					       ___bpf_build_param11(narg, args)
+#define ___bpf_build_param(args...) \
+	unsigned long long ___param[___bpf_narg(args)];			\
+	___bpf_apply(___bpf_build_param, ___bpf_narg(args))(___bpf_narg(args), args)
+
 /*
  * BPF_SEQ_PRINTF to wrap bpf_seq_printf to-be-printed values
  * in a structure.
@@ -422,7 +450,7 @@ static __always_inline typeof(name(0)) ____##name(struct pt_regs *ctx, ##args)
 		_Pragma("GCC diagnostic push")				    \
 		_Pragma("GCC diagnostic ignored \"-Wint-conversion\"")	    \
 		static const char ___fmt[] = fmt;			    \
-		unsigned long long ___param[] = { args };		    \
+		___bpf_build_param(args);				    \
 		_Pragma("GCC diagnostic pop")				    \
 		int ___ret = bpf_seq_printf(seq, ___fmt, sizeof(___fmt),    \
 					    ___param, sizeof(___param));    \
-- 
2.30.1.766.gb4fecdf3b7-goog