Commit c2758baa authored by Dave Marchevsky's avatar Dave Marchevsky Committed by Alexei Starovoitov
Browse files

libbpf: Modify bpf_printk to choose helper based on arg count 



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. Before this patch, attempting to use 4
args to bpf_printk results in a compile-time error. After this patch,
using bpf_printk with 4 args results in a trace_vprintk helper call
being emitted and a load-time failure on older kernels.

Signed-off-by: default avatarDave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210917182911.2426606-5-davemarchevsky@fb.com
parent 10aceb62

tools/lib/bpf/bpf_helpers.h

+37 −8
Original line number Diff line number Diff line
@@ -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));		\

})



#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));		\

})



/* Use __bpf_printk when bpf_printk call has 3 or fewer fmt args

 * Otherwise use __bpf_vprintk

 */

#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 /*3*/, __bpf_printk /*2*/,\

		   __bpf_printk /*1*/, __bpf_printk /*0*/)



/* Helper macro to print out debug messages */

#define bpf_printk(fmt, args...) ___bpf_pick_printk(args)(fmt, ##args)



#endif