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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: Dave Marchevsky <davemarchevsky@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210917182911.2426606-5-davemarchevsky@fb.com
This commit is contained in:
Dave Marchevsky 2021-09-17 11:29:06 -07:00 committed by Alexei Starovoitov
parent 10aceb629e
commit c2758baa97
1 changed files with 37 additions and 8 deletions
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45
tools/lib/bpf/bpf_helpers.h
View File

@ -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