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TOMOYO Linux Cross Reference
Linux/samples/bpf/bpf_load.c

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  1 // SPDX-License-Identifier: GPL-2.0
  2 #include <stdio.h>
  3 #include <sys/types.h>
  4 #include <sys/stat.h>
  5 #include <fcntl.h>
  6 #include <libelf.h>
  7 #include <gelf.h>
  8 #include <errno.h>
  9 #include <unistd.h>
 10 #include <string.h>
 11 #include <stdbool.h>
 12 #include <stdlib.h>
 13 #include <linux/bpf.h>
 14 #include <linux/filter.h>
 15 #include <linux/perf_event.h>
 16 #include <linux/netlink.h>
 17 #include <linux/rtnetlink.h>
 18 #include <linux/types.h>
 19 #include <sys/socket.h>
 20 #include <sys/syscall.h>
 21 #include <sys/ioctl.h>
 22 #include <sys/mman.h>
 23 #include <poll.h>
 24 #include <ctype.h>
 25 #include <assert.h>
 26 #include <bpf/bpf.h>
 27 #include "bpf_load.h"
 28 #include "perf-sys.h"
 29 
 30 #define DEBUGFS "/sys/kernel/debug/tracing/"
 31 
 32 static char license[128];
 33 static int kern_version;
 34 static bool processed_sec[128];
 35 char bpf_log_buf[BPF_LOG_BUF_SIZE];
 36 int map_fd[MAX_MAPS];
 37 int prog_fd[MAX_PROGS];
 38 int event_fd[MAX_PROGS];
 39 int prog_cnt;
 40 int prog_array_fd = -1;
 41 
 42 struct bpf_map_data map_data[MAX_MAPS];
 43 int map_data_count = 0;
 44 
 45 static int populate_prog_array(const char *event, int prog_fd)
 46 {
 47         int ind = atoi(event), err;
 48 
 49         err = bpf_map_update_elem(prog_array_fd, &ind, &prog_fd, BPF_ANY);
 50         if (err < 0) {
 51                 printf("failed to store prog_fd in prog_array\n");
 52                 return -1;
 53         }
 54         return 0;
 55 }
 56 
 57 static int write_kprobe_events(const char *val)
 58 {
 59         int fd, ret, flags;
 60 
 61         if (val == NULL)
 62                 return -1;
 63         else if (val[0] == '\0')
 64                 flags = O_WRONLY | O_TRUNC;
 65         else
 66                 flags = O_WRONLY | O_APPEND;
 67 
 68         fd = open("/sys/kernel/debug/tracing/kprobe_events", flags);
 69 
 70         ret = write(fd, val, strlen(val));
 71         close(fd);
 72 
 73         return ret;
 74 }
 75 
 76 static int load_and_attach(const char *event, struct bpf_insn *prog, int size)
 77 {
 78         bool is_socket = strncmp(event, "socket", 6) == 0;
 79         bool is_kprobe = strncmp(event, "kprobe/", 7) == 0;
 80         bool is_kretprobe = strncmp(event, "kretprobe/", 10) == 0;
 81         bool is_tracepoint = strncmp(event, "tracepoint/", 11) == 0;
 82         bool is_raw_tracepoint = strncmp(event, "raw_tracepoint/", 15) == 0;
 83         bool is_xdp = strncmp(event, "xdp", 3) == 0;
 84         bool is_perf_event = strncmp(event, "perf_event", 10) == 0;
 85         bool is_cgroup_skb = strncmp(event, "cgroup/skb", 10) == 0;
 86         bool is_cgroup_sk = strncmp(event, "cgroup/sock", 11) == 0;
 87         bool is_sockops = strncmp(event, "sockops", 7) == 0;
 88         bool is_sk_skb = strncmp(event, "sk_skb", 6) == 0;
 89         bool is_sk_msg = strncmp(event, "sk_msg", 6) == 0;
 90         size_t insns_cnt = size / sizeof(struct bpf_insn);
 91         enum bpf_prog_type prog_type;
 92         char buf[256];
 93         int fd, efd, err, id;
 94         struct perf_event_attr attr = {};
 95 
 96         attr.type = PERF_TYPE_TRACEPOINT;
 97         attr.sample_type = PERF_SAMPLE_RAW;
 98         attr.sample_period = 1;
 99         attr.wakeup_events = 1;
100 
101         if (is_socket) {
102                 prog_type = BPF_PROG_TYPE_SOCKET_FILTER;
103         } else if (is_kprobe || is_kretprobe) {
104                 prog_type = BPF_PROG_TYPE_KPROBE;
105         } else if (is_tracepoint) {
106                 prog_type = BPF_PROG_TYPE_TRACEPOINT;
107         } else if (is_raw_tracepoint) {
108                 prog_type = BPF_PROG_TYPE_RAW_TRACEPOINT;
109         } else if (is_xdp) {
110                 prog_type = BPF_PROG_TYPE_XDP;
111         } else if (is_perf_event) {
112                 prog_type = BPF_PROG_TYPE_PERF_EVENT;
113         } else if (is_cgroup_skb) {
114                 prog_type = BPF_PROG_TYPE_CGROUP_SKB;
115         } else if (is_cgroup_sk) {
116                 prog_type = BPF_PROG_TYPE_CGROUP_SOCK;
117         } else if (is_sockops) {
118                 prog_type = BPF_PROG_TYPE_SOCK_OPS;
119         } else if (is_sk_skb) {
120                 prog_type = BPF_PROG_TYPE_SK_SKB;
121         } else if (is_sk_msg) {
122                 prog_type = BPF_PROG_TYPE_SK_MSG;
123         } else {
124                 printf("Unknown event '%s'\n", event);
125                 return -1;
126         }
127 
128         if (prog_cnt == MAX_PROGS)
129                 return -1;
130 
131         fd = bpf_load_program(prog_type, prog, insns_cnt, license, kern_version,
132                               bpf_log_buf, BPF_LOG_BUF_SIZE);
133         if (fd < 0) {
134                 printf("bpf_load_program() err=%d\n%s", errno, bpf_log_buf);
135                 return -1;
136         }
137 
138         prog_fd[prog_cnt++] = fd;
139 
140         if (is_xdp || is_perf_event || is_cgroup_skb || is_cgroup_sk)
141                 return 0;
142 
143         if (is_socket || is_sockops || is_sk_skb || is_sk_msg) {
144                 if (is_socket)
145                         event += 6;
146                 else
147                         event += 7;
148                 if (*event != '/')
149                         return 0;
150                 event++;
151                 if (!isdigit(*event)) {
152                         printf("invalid prog number\n");
153                         return -1;
154                 }
155                 return populate_prog_array(event, fd);
156         }
157 
158         if (is_raw_tracepoint) {
159                 efd = bpf_raw_tracepoint_open(event + 15, fd);
160                 if (efd < 0) {
161                         printf("tracepoint %s %s\n", event + 15, strerror(errno));
162                         return -1;
163                 }
164                 event_fd[prog_cnt - 1] = efd;
165                 return 0;
166         }
167 
168         if (is_kprobe || is_kretprobe) {
169                 bool need_normal_check = true;
170                 const char *event_prefix = "";
171 
172                 if (is_kprobe)
173                         event += 7;
174                 else
175                         event += 10;
176 
177                 if (*event == 0) {
178                         printf("event name cannot be empty\n");
179                         return -1;
180                 }
181 
182                 if (isdigit(*event))
183                         return populate_prog_array(event, fd);
184 
185 #ifdef __x86_64__
186                 if (strncmp(event, "sys_", 4) == 0) {
187                         snprintf(buf, sizeof(buf), "%c:__x64_%s __x64_%s",
188                                 is_kprobe ? 'p' : 'r', event, event);
189                         err = write_kprobe_events(buf);
190                         if (err >= 0) {
191                                 need_normal_check = false;
192                                 event_prefix = "__x64_";
193                         }
194                 }
195 #endif
196                 if (need_normal_check) {
197                         snprintf(buf, sizeof(buf), "%c:%s %s",
198                                 is_kprobe ? 'p' : 'r', event, event);
199                         err = write_kprobe_events(buf);
200                         if (err < 0) {
201                                 printf("failed to create kprobe '%s' error '%s'\n",
202                                        event, strerror(errno));
203                                 return -1;
204                         }
205                 }
206 
207                 strcpy(buf, DEBUGFS);
208                 strcat(buf, "events/kprobes/");
209                 strcat(buf, event_prefix);
210                 strcat(buf, event);
211                 strcat(buf, "/id");
212         } else if (is_tracepoint) {
213                 event += 11;
214 
215                 if (*event == 0) {
216                         printf("event name cannot be empty\n");
217                         return -1;
218                 }
219                 strcpy(buf, DEBUGFS);
220                 strcat(buf, "events/");
221                 strcat(buf, event);
222                 strcat(buf, "/id");
223         }
224 
225         efd = open(buf, O_RDONLY, 0);
226         if (efd < 0) {
227                 printf("failed to open event %s\n", event);
228                 return -1;
229         }
230 
231         err = read(efd, buf, sizeof(buf));
232         if (err < 0 || err >= sizeof(buf)) {
233                 printf("read from '%s' failed '%s'\n", event, strerror(errno));
234                 return -1;
235         }
236 
237         close(efd);
238 
239         buf[err] = 0;
240         id = atoi(buf);
241         attr.config = id;
242 
243         efd = sys_perf_event_open(&attr, -1/*pid*/, 0/*cpu*/, -1/*group_fd*/, 0);
244         if (efd < 0) {
245                 printf("event %d fd %d err %s\n", id, efd, strerror(errno));
246                 return -1;
247         }
248         event_fd[prog_cnt - 1] = efd;
249         err = ioctl(efd, PERF_EVENT_IOC_ENABLE, 0);
250         if (err < 0) {
251                 printf("ioctl PERF_EVENT_IOC_ENABLE failed err %s\n",
252                        strerror(errno));
253                 return -1;
254         }
255         err = ioctl(efd, PERF_EVENT_IOC_SET_BPF, fd);
256         if (err < 0) {
257                 printf("ioctl PERF_EVENT_IOC_SET_BPF failed err %s\n",
258                        strerror(errno));
259                 return -1;
260         }
261 
262         return 0;
263 }
264 
265 static int load_maps(struct bpf_map_data *maps, int nr_maps,
266                      fixup_map_cb fixup_map)
267 {
268         int i, numa_node;
269 
270         for (i = 0; i < nr_maps; i++) {
271                 if (fixup_map) {
272                         fixup_map(&maps[i], i);
273                         /* Allow userspace to assign map FD prior to creation */
274                         if (maps[i].fd != -1) {
275                                 map_fd[i] = maps[i].fd;
276                                 continue;
277                         }
278                 }
279 
280                 numa_node = maps[i].def.map_flags & BPF_F_NUMA_NODE ?
281                         maps[i].def.numa_node : -1;
282 
283                 if (maps[i].def.type == BPF_MAP_TYPE_ARRAY_OF_MAPS ||
284                     maps[i].def.type == BPF_MAP_TYPE_HASH_OF_MAPS) {
285                         int inner_map_fd = map_fd[maps[i].def.inner_map_idx];
286 
287                         map_fd[i] = bpf_create_map_in_map_node(maps[i].def.type,
288                                                         maps[i].name,
289                                                         maps[i].def.key_size,
290                                                         inner_map_fd,
291                                                         maps[i].def.max_entries,
292                                                         maps[i].def.map_flags,
293                                                         numa_node);
294                 } else {
295                         map_fd[i] = bpf_create_map_node(maps[i].def.type,
296                                                         maps[i].name,
297                                                         maps[i].def.key_size,
298                                                         maps[i].def.value_size,
299                                                         maps[i].def.max_entries,
300                                                         maps[i].def.map_flags,
301                                                         numa_node);
302                 }
303                 if (map_fd[i] < 0) {
304                         printf("failed to create map %d (%s): %d %s\n",
305                                i, maps[i].name, errno, strerror(errno));
306                         return 1;
307                 }
308                 maps[i].fd = map_fd[i];
309 
310                 if (maps[i].def.type == BPF_MAP_TYPE_PROG_ARRAY)
311                         prog_array_fd = map_fd[i];
312         }
313         return 0;
314 }
315 
316 static int get_sec(Elf *elf, int i, GElf_Ehdr *ehdr, char **shname,
317                    GElf_Shdr *shdr, Elf_Data **data)
318 {
319         Elf_Scn *scn;
320 
321         scn = elf_getscn(elf, i);
322         if (!scn)
323                 return 1;
324 
325         if (gelf_getshdr(scn, shdr) != shdr)
326                 return 2;
327 
328         *shname = elf_strptr(elf, ehdr->e_shstrndx, shdr->sh_name);
329         if (!*shname || !shdr->sh_size)
330                 return 3;
331 
332         *data = elf_getdata(scn, 0);
333         if (!*data || elf_getdata(scn, *data) != NULL)
334                 return 4;
335 
336         return 0;
337 }
338 
339 static int parse_relo_and_apply(Elf_Data *data, Elf_Data *symbols,
340                                 GElf_Shdr *shdr, struct bpf_insn *insn,
341                                 struct bpf_map_data *maps, int nr_maps)
342 {
343         int i, nrels;
344 
345         nrels = shdr->sh_size / shdr->sh_entsize;
346 
347         for (i = 0; i < nrels; i++) {
348                 GElf_Sym sym;
349                 GElf_Rel rel;
350                 unsigned int insn_idx;
351                 bool match = false;
352                 int j, map_idx;
353 
354                 gelf_getrel(data, i, &rel);
355 
356                 insn_idx = rel.r_offset / sizeof(struct bpf_insn);
357 
358                 gelf_getsym(symbols, GELF_R_SYM(rel.r_info), &sym);
359 
360                 if (insn[insn_idx].code != (BPF_LD | BPF_IMM | BPF_DW)) {
361                         printf("invalid relo for insn[%d].code 0x%x\n",
362                                insn_idx, insn[insn_idx].code);
363                         return 1;
364                 }
365                 insn[insn_idx].src_reg = BPF_PSEUDO_MAP_FD;
366 
367                 /* Match FD relocation against recorded map_data[] offset */
368                 for (map_idx = 0; map_idx < nr_maps; map_idx++) {
369                         if (maps[map_idx].elf_offset == sym.st_value) {
370                                 match = true;
371                                 break;
372                         }
373                 }
374                 if (match) {
375                         insn[insn_idx].imm = maps[map_idx].fd;
376                 } else {
377                         printf("invalid relo for insn[%d] no map_data match\n",
378                                insn_idx);
379                         return 1;
380                 }
381         }
382 
383         return 0;
384 }
385 
386 static int cmp_symbols(const void *l, const void *r)
387 {
388         const GElf_Sym *lsym = (const GElf_Sym *)l;
389         const GElf_Sym *rsym = (const GElf_Sym *)r;
390 
391         if (lsym->st_value < rsym->st_value)
392                 return -1;
393         else if (lsym->st_value > rsym->st_value)
394                 return 1;
395         else
396                 return 0;
397 }
398 
399 static int load_elf_maps_section(struct bpf_map_data *maps, int maps_shndx,
400                                  Elf *elf, Elf_Data *symbols, int strtabidx)
401 {
402         int map_sz_elf, map_sz_copy;
403         bool validate_zero = false;
404         Elf_Data *data_maps;
405         int i, nr_maps;
406         GElf_Sym *sym;
407         Elf_Scn *scn;
408         int copy_sz;
409 
410         if (maps_shndx < 0)
411                 return -EINVAL;
412         if (!symbols)
413                 return -EINVAL;
414 
415         /* Get data for maps section via elf index */
416         scn = elf_getscn(elf, maps_shndx);
417         if (scn)
418                 data_maps = elf_getdata(scn, NULL);
419         if (!scn || !data_maps) {
420                 printf("Failed to get Elf_Data from maps section %d\n",
421                        maps_shndx);
422                 return -EINVAL;
423         }
424 
425         /* For each map get corrosponding symbol table entry */
426         sym = calloc(MAX_MAPS+1, sizeof(GElf_Sym));
427         for (i = 0, nr_maps = 0; i < symbols->d_size / sizeof(GElf_Sym); i++) {
428                 assert(nr_maps < MAX_MAPS+1);
429                 if (!gelf_getsym(symbols, i, &sym[nr_maps]))
430                         continue;
431                 if (sym[nr_maps].st_shndx != maps_shndx)
432                         continue;
433                 /* Only increment iif maps section */
434                 nr_maps++;
435         }
436 
437         /* Align to map_fd[] order, via sort on offset in sym.st_value */
438         qsort(sym, nr_maps, sizeof(GElf_Sym), cmp_symbols);
439 
440         /* Keeping compatible with ELF maps section changes
441          * ------------------------------------------------
442          * The program size of struct bpf_load_map_def is known by loader
443          * code, but struct stored in ELF file can be different.
444          *
445          * Unfortunately sym[i].st_size is zero.  To calculate the
446          * struct size stored in the ELF file, assume all struct have
447          * the same size, and simply divide with number of map
448          * symbols.
449          */
450         map_sz_elf = data_maps->d_size / nr_maps;
451         map_sz_copy = sizeof(struct bpf_load_map_def);
452         if (map_sz_elf < map_sz_copy) {
453                 /*
454                  * Backward compat, loading older ELF file with
455                  * smaller struct, keeping remaining bytes zero.
456                  */
457                 map_sz_copy = map_sz_elf;
458         } else if (map_sz_elf > map_sz_copy) {
459                 /*
460                  * Forward compat, loading newer ELF file with larger
461                  * struct with unknown features. Assume zero means
462                  * feature not used.  Thus, validate rest of struct
463                  * data is zero.
464                  */
465                 validate_zero = true;
466         }
467 
468         /* Memcpy relevant part of ELF maps data to loader maps */
469         for (i = 0; i < nr_maps; i++) {
470                 struct bpf_load_map_def *def;
471                 unsigned char *addr, *end;
472                 const char *map_name;
473                 size_t offset;
474 
475                 map_name = elf_strptr(elf, strtabidx, sym[i].st_name);
476                 maps[i].name = strdup(map_name);
477                 if (!maps[i].name) {
478                         printf("strdup(%s): %s(%d)\n", map_name,
479                                strerror(errno), errno);
480                         free(sym);
481                         return -errno;
482                 }
483 
484                 /* Symbol value is offset into ELF maps section data area */
485                 offset = sym[i].st_value;
486                 def = (struct bpf_load_map_def *)(data_maps->d_buf + offset);
487                 maps[i].elf_offset = offset;
488                 memset(&maps[i].def, 0, sizeof(struct bpf_load_map_def));
489                 memcpy(&maps[i].def, def, map_sz_copy);
490 
491                 /* Verify no newer features were requested */
492                 if (validate_zero) {
493                         addr = (unsigned char*) def + map_sz_copy;
494                         end  = (unsigned char*) def + map_sz_elf;
495                         for (; addr < end; addr++) {
496                                 if (*addr != 0) {
497                                         free(sym);
498                                         return -EFBIG;
499                                 }
500                         }
501                 }
502         }
503 
504         free(sym);
505         return nr_maps;
506 }
507 
508 static int do_load_bpf_file(const char *path, fixup_map_cb fixup_map)
509 {
510         int fd, i, ret, maps_shndx = -1, strtabidx = -1;
511         Elf *elf;
512         GElf_Ehdr ehdr;
513         GElf_Shdr shdr, shdr_prog;
514         Elf_Data *data, *data_prog, *data_maps = NULL, *symbols = NULL;
515         char *shname, *shname_prog;
516         int nr_maps = 0;
517 
518         /* reset global variables */
519         kern_version = 0;
520         memset(license, 0, sizeof(license));
521         memset(processed_sec, 0, sizeof(processed_sec));
522 
523         if (elf_version(EV_CURRENT) == EV_NONE)
524                 return 1;
525 
526         fd = open(path, O_RDONLY, 0);
527         if (fd < 0)
528                 return 1;
529 
530         elf = elf_begin(fd, ELF_C_READ, NULL);
531 
532         if (!elf)
533                 return 1;
534 
535         if (gelf_getehdr(elf, &ehdr) != &ehdr)
536                 return 1;
537 
538         /* clear all kprobes */
539         i = write_kprobe_events("");
540 
541         /* scan over all elf sections to get license and map info */
542         for (i = 1; i < ehdr.e_shnum; i++) {
543 
544                 if (get_sec(elf, i, &ehdr, &shname, &shdr, &data))
545                         continue;
546 
547                 if (0) /* helpful for llvm debugging */
548                         printf("section %d:%s data %p size %zd link %d flags %d\n",
549                                i, shname, data->d_buf, data->d_size,
550                                shdr.sh_link, (int) shdr.sh_flags);
551 
552                 if (strcmp(shname, "license") == 0) {
553                         processed_sec[i] = true;
554                         memcpy(license, data->d_buf, data->d_size);
555                 } else if (strcmp(shname, "version") == 0) {
556                         processed_sec[i] = true;
557                         if (data->d_size != sizeof(int)) {
558                                 printf("invalid size of version section %zd\n",
559                                        data->d_size);
560                                 return 1;
561                         }
562                         memcpy(&kern_version, data->d_buf, sizeof(int));
563                 } else if (strcmp(shname, "maps") == 0) {
564                         int j;
565 
566                         maps_shndx = i;
567                         data_maps = data;
568                         for (j = 0; j < MAX_MAPS; j++)
569                                 map_data[j].fd = -1;
570                 } else if (shdr.sh_type == SHT_SYMTAB) {
571                         strtabidx = shdr.sh_link;
572                         symbols = data;
573                 }
574         }
575 
576         ret = 1;
577 
578         if (!symbols) {
579                 printf("missing SHT_SYMTAB section\n");
580                 goto done;
581         }
582 
583         if (data_maps) {
584                 nr_maps = load_elf_maps_section(map_data, maps_shndx,
585                                                 elf, symbols, strtabidx);
586                 if (nr_maps < 0) {
587                         printf("Error: Failed loading ELF maps (errno:%d):%s\n",
588                                nr_maps, strerror(-nr_maps));
589                         goto done;
590                 }
591                 if (load_maps(map_data, nr_maps, fixup_map))
592                         goto done;
593                 map_data_count = nr_maps;
594 
595                 processed_sec[maps_shndx] = true;
596         }
597 
598         /* process all relo sections, and rewrite bpf insns for maps */
599         for (i = 1; i < ehdr.e_shnum; i++) {
600                 if (processed_sec[i])
601                         continue;
602 
603                 if (get_sec(elf, i, &ehdr, &shname, &shdr, &data))
604                         continue;
605 
606                 if (shdr.sh_type == SHT_REL) {
607                         struct bpf_insn *insns;
608 
609                         /* locate prog sec that need map fixup (relocations) */
610                         if (get_sec(elf, shdr.sh_info, &ehdr, &shname_prog,
611                                     &shdr_prog, &data_prog))
612                                 continue;
613 
614                         if (shdr_prog.sh_type != SHT_PROGBITS ||
615                             !(shdr_prog.sh_flags & SHF_EXECINSTR))
616                                 continue;
617 
618                         insns = (struct bpf_insn *) data_prog->d_buf;
619                         processed_sec[i] = true; /* relo section */
620 
621                         if (parse_relo_and_apply(data, symbols, &shdr, insns,
622                                                  map_data, nr_maps))
623                                 continue;
624                 }
625         }
626 
627         /* load programs */
628         for (i = 1; i < ehdr.e_shnum; i++) {
629 
630                 if (processed_sec[i])
631                         continue;
632 
633                 if (get_sec(elf, i, &ehdr, &shname, &shdr, &data))
634                         continue;
635 
636                 if (memcmp(shname, "kprobe/", 7) == 0 ||
637                     memcmp(shname, "kretprobe/", 10) == 0 ||
638                     memcmp(shname, "tracepoint/", 11) == 0 ||
639                     memcmp(shname, "raw_tracepoint/", 15) == 0 ||
640                     memcmp(shname, "xdp", 3) == 0 ||
641                     memcmp(shname, "perf_event", 10) == 0 ||
642                     memcmp(shname, "socket", 6) == 0 ||
643                     memcmp(shname, "cgroup/", 7) == 0 ||
644                     memcmp(shname, "sockops", 7) == 0 ||
645                     memcmp(shname, "sk_skb", 6) == 0 ||
646                     memcmp(shname, "sk_msg", 6) == 0) {
647                         ret = load_and_attach(shname, data->d_buf,
648                                               data->d_size);
649                         if (ret != 0)
650                                 goto done;
651                 }
652         }
653 
654 done:
655         close(fd);
656         return ret;
657 }
658 
659 int load_bpf_file(char *path)
660 {
661         return do_load_bpf_file(path, NULL);
662 }
663 
664 int load_bpf_file_fixup_map(const char *path, fixup_map_cb fixup_map)
665 {
666         return do_load_bpf_file(path, fixup_map);
667 }
668 
669 void read_trace_pipe(void)
670 {
671         int trace_fd;
672 
673         trace_fd = open(DEBUGFS "trace_pipe", O_RDONLY, 0);
674         if (trace_fd < 0)
675                 return;
676 
677         while (1) {
678                 static char buf[4096];
679                 ssize_t sz;
680 
681                 sz = read(trace_fd, buf, sizeof(buf));
682                 if (sz > 0) {
683                         buf[sz] = 0;
684                         puts(buf);
685                 }
686         }
687 }
688 

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