perf bpf: Add struct bpf_map struct

A helper structure used by eBPF C program to describe map attributes to
elf_bpf loader, to be used initially by the special __bpf_stdout__ map
used to print strings into the perf ring buffer in BPF scripts, e.g.:

Using the upcoming stdio.h and puts() macros to use the __bpf_stdout__
map to add strings to the ring buffer:

# cat tools/perf/examples/bpf/hello.c
#include <stdio.h>

int syscall_enter(openat)(void *args)
{
puts("Hello, world\n");
return 0;
}

license(GPL);
#
# cat ~/.perfconfig
[llvm]
dump-obj = true
# perf trace -e openat,tools/perf/examples/bpf/hello.c/call-graph=dwarf/ cat /etc/passwd > /dev/null
LLVM: dumping tools/perf/examples/bpf/hello.o
0.016 ( ): __bpf_stdout__:Hello, world
0.018 ( 0.010 ms): cat/9079 openat(dfd: CWD, filename: /etc/ld.so.cache, flags: CLOEXEC ) = 3
0.057 ( ): __bpf_stdout__:Hello, world
0.059 ( 0.011 ms): cat/9079 openat(dfd: CWD, filename: /lib64/libc.so.6, flags: CLOEXEC ) = 3
0.417 ( ): __bpf_stdout__:Hello, world
0.419 ( 0.009 ms): cat/9079 openat(dfd: CWD, filename: /etc/passwd ) = 3
#
# file tools/perf/examples/bpf/hello.o
tools/perf/examples/bpf/hello.o: ELF 64-bit LSB relocatable, *unknown arch 0xf7* version 1 (SYSV), not stripped
# readelf -SW tools/perf/examples/bpf/hello.o
There are 10 section headers, starting at offset 0x208:

Section Headers:
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 0] NULL 0000000000000000 000000 000000 00 0 0 0
[ 1] .strtab STRTAB 0000000000000000 000188 00007f 00 0 0 1
[ 2] .text PROGBITS 0000000000000000 000040 000000 00 AX 0 0 4
[ 3] syscalls:sys_enter_openat PROGBITS 0000000000000000 000040 000088 00 AX 0 0 8
[ 4] .relsyscalls:sys_enter_openat REL 0000000000000000 000178 000010 10 9 3 8
[ 5] maps PROGBITS 0000000000000000 0000c8 00001c 00 WA 0 0 4
[ 6] .rodata.str1.1 PROGBITS 0000000000000000 0000e4 00000e 01 AMS 0 0 1
[ 7] license PROGBITS 0000000000000000 0000f2 000004 00 WA 0 0 1
[ 8] version PROGBITS 0000000000000000 0000f8 000004 00 WA 0 0 4
[ 9] .symtab SYMTAB 0000000000000000 000100 000078 18 1 1 8
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
p (processor specific)
# readelf -s tools/perf/examples/bpf/hello.o

Symbol table '.symtab' contains 5 entries:
Num: Value Size Type Bind Vis Ndx Name
0: 0000000000000000 0 NOTYPE LOCAL DEFAULT UND
1: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 5 __bpf_stdout__
2: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 7 _license
3: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 8 _version
4: 0000000000000000 0 NOTYPE GLOBAL DEFAULT 3 syscall_enter_openat
#

Cc: Adrian Hunter <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Wang Nan <[e-mail]>
Link: https://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/include/bpf/bpf.h

···
 
 #include <uapi/linux/bpf.h>
 
+/*
+ * A helper structure used by eBPF C program to describe map attributes to
+ * elf_bpf loader, taken from tools/testing/selftests/bpf/bpf_helpers.h:
+ */
+struct bpf_map {
+        unsigned int type;
+        unsigned int key_size;
+        unsigned int value_size;
+        unsigned int max_entries;
+        unsigned int map_flags;
+        unsigned int inner_map_idx;
+        unsigned int numa_node;
+};
+
 #define SEC(NAME) __attribute__((section(NAME),  used))
 
 #define probe(function, vars) \