[PATCH v2 bpf-next 3/6] libbpf: Modify bpf_printk to choose helper based on arg count

[Dave Marchevsky <davemarchevsky@fb.com>]

Instead of being a thin wrapper which calls into bpf_trace_printk,
libbpf's bpf_printk convenience macro now chooses between
bpf_trace_printk and bpf_trace_vprintk. If the arg count (excluding
format string) is >3, use bpf_trace_vprintk, otherwise use the older
helper.

The motivation behind this added complexity - instead of migrating
entirely to bpf_trace_vprintk - is to maintain good developer experience
for users compiling against new libbpf but running on older kernels.
Users who are passing <=3 args to bpf_printk will see no change in their
bytecode.

__bpf_vprintk functions similarly to BPF_SEQ_PRINTF and BPF_SNPRINTF
macros elsewhere in the file - it allows use of bpf_trace_vprintk
without manual conversion of varargs to u64 array. Previous
implementation of bpf_printk macro is moved to __bpf_printk for use by
the new implementation.

This does change behavior of bpf_printk calls with >3 args in the "new
libbpf, old kernels" scenario. On my system, using a clang built from
recent upstream sources (14.0.0 https://github.com/llvm/llvm-project.git
50b62731452cb83979bbf3c06e828d26a4698dca), attempting to use 4 args to
__bpf_printk (old impl) results in a compile-time error:

  progs/trace_printk.c:21:21: error: too many args to 0x6cdf4b8: i64 = Constant<6>
        trace_printk_ret = __bpf_printk("testing,testing %d %d %d %d\n",

I was able to replicate this behavior with an older clang as well. When
the format string has >3 format specifiers, there is no output to the
trace_pipe in either case.

After this patch, using bpf_printk with 4 args would result in a
trace_vprintk helper call being emitted and a load-time failure on older
kernels.

Signed-off-by: Dave Marchevsky <davemarchevsky@fb.com>
---
 tools/lib/bpf/bpf_helpers.h | 45 ++++++++++++++++++++++++++++++-------
 1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/tools/lib/bpf/bpf_helpers.h b/tools/lib/bpf/bpf_helpers.h
index b9987c3efa3c..5f087306cdfe 100644
--- a/tools/lib/bpf/bpf_helpers.h
+++ b/tools/lib/bpf/bpf_helpers.h
@@ -14,14 +14,6 @@
 #define __type(name, val) typeof(val) *name
 #define __array(name, val) typeof(val) *name[]
 
-/* Helper macro to print out debug messages */
-#define bpf_printk(fmt, ...)				\
-({							\
-	char ____fmt[] = fmt;				\
-	bpf_trace_printk(____fmt, sizeof(____fmt),	\
-			 ##__VA_ARGS__);		\
-})
-
 /*
  * Helper macro to place programs, maps, license in
  * different sections in elf_bpf file. Section names
@@ -224,4 +216,41 @@ enum libbpf_tristate {
 		     ___param, sizeof(___param));		\
 })
 
+/* Helper macro to print out debug messages */
+#define __bpf_printk(fmt, ...)				\
+({							\
+	char ____fmt[] = fmt;				\
+	bpf_trace_printk(____fmt, sizeof(____fmt),	\
+			 ##__VA_ARGS__);		\
+})
+
+/*
+ * __bpf_vprintk wraps the bpf_trace_vprintk helper with variadic arguments
+ * instead of an array of u64.
+ */
+#define __bpf_vprintk(fmt, args...)				\
+({								\
+	static const char ___fmt[] = fmt;			\
+	unsigned long long ___param[___bpf_narg(args)];		\
+								\
+	_Pragma("GCC diagnostic push")				\
+	_Pragma("GCC diagnostic ignored \"-Wint-conversion\"")	\
+	___bpf_fill(___param, args);				\
+	_Pragma("GCC diagnostic pop")				\
+								\
+	bpf_trace_vprintk(___fmt, sizeof(___fmt),		\
+		     ___param, sizeof(___param));		\
+})
+
+#define ___bpf_pick_printk(...) \
+	___bpf_nth(_, ##__VA_ARGS__, __bpf_vprintk, __bpf_vprintk, __bpf_vprintk,	\
+		__bpf_vprintk, __bpf_vprintk, __bpf_vprintk, __bpf_vprintk,		\
+		__bpf_vprintk, __bpf_vprintk, __bpf_printk, __bpf_printk,		\
+		__bpf_printk, __bpf_printk)
+
+#define bpf_printk(fmt, args...)		\
+({						\
+	___bpf_pick_printk(args)(fmt, args);	\
+})
+
 #endif
-- 
2.30.2