tools/perf/util/bpf-prologue.c

   1 /*
   2  * bpf-prologue.c
   3  *
   4  * Copyright (C) 2015 He Kuang <hekuang@huawei.com>
   5  * Copyright (C) 2015 Wang Nan <wangnan0@huawei.com>
   6  * Copyright (C) 2015 Huawei Inc.
   7  */
   8
   9 #include <bpf/libbpf.h>
  10 #include "perf.h"
  11 #include "debug.h"
  12 #include "bpf-loader.h"
  13 #include "bpf-prologue.h"
  14 #include "probe-finder.h"
  15 #include <dwarf-regs.h>
  16 #include <linux/filter.h>
  17
  18 #define BPF_REG_SIZE            8
  19
  20 #define JMP_TO_ERROR_CODE       -1
  21 #define JMP_TO_SUCCESS_CODE     -2
  22 #define JMP_TO_USER_CODE        -3
  23
  24 struct bpf_insn_pos {
  25         struct bpf_insn *begin;
  26         struct bpf_insn *end;
  27         struct bpf_insn *pos;
  28 };
  29
  30 static inline int
  31 pos_get_cnt(struct bpf_insn_pos *pos)
  32 {
  33         return pos->pos - pos->begin;
  34 }
  35
  36 static int
  37 append_insn(struct bpf_insn new_insn, struct bpf_insn_pos *pos)
  38 {
  39         if (!pos->pos)
  40                 return -BPF_LOADER_ERRNO__PROLOGUE2BIG;
  41
  42         if (pos->pos + 1 >= pos->end) {
  43                 pr_err("bpf prologue: prologue too long\n");
  44                 pos->pos = NULL;
  45                 return -BPF_LOADER_ERRNO__PROLOGUE2BIG;
  46         }
  47
  48         *(pos->pos)++ = new_insn;
  49         return 0;
  50 }
  51
  52 static int
  53 check_pos(struct bpf_insn_pos *pos)
  54 {
  55         if (!pos->pos || pos->pos >= pos->end)
  56                 return -BPF_LOADER_ERRNO__PROLOGUE2BIG;
  57         return 0;
  58 }
  59
  60 /* Give it a shorter name */
  61 #define ins(i, p) append_insn((i), (p))
  62
  63 /*
  64  * Give a register name (in 'reg'), generate instruction to
  65  * load register into an eBPF register rd:
  66  *   'ldd target_reg, offset(ctx_reg)', where:
  67  * ctx_reg is pre initialized to pointer of 'struct pt_regs'.
  68  */
  69 static int
  70 gen_ldx_reg_from_ctx(struct bpf_insn_pos *pos, int ctx_reg,
  71                      const char *reg, int target_reg)
  72 {
  73         int offset = regs_query_register_offset(reg);
  74
  75         if (offset < 0) {
  76                 pr_err("bpf: prologue: failed to get register %s\n",
  77                        reg);
  78                 return offset;
  79         }
  80         ins(BPF_LDX_MEM(BPF_DW, target_reg, ctx_reg, offset), pos);
  81
  82         return check_pos(pos);
  83 }
  84
  85 /*
  86  * Generate a BPF_FUNC_probe_read function call.
  87  *
  88  * src_base_addr_reg is a register holding base address,
  89  * dst_addr_reg is a register holding dest address (on stack),
  90  * result is:
  91  *
  92  *  *[dst_addr_reg] = *([src_base_addr_reg] + offset)
  93  *
  94  * Arguments of BPF_FUNC_probe_read:
  95  *     ARG1: ptr to stack (dest)
  96  *     ARG2: size (8)
  97  *     ARG3: unsafe ptr (src)
  98  */
  99 static int
 100 gen_read_mem(struct bpf_insn_pos *pos,
 101              int src_base_addr_reg,
 102              int dst_addr_reg,
 103              long offset)
 104 {
 105         /* mov arg3, src_base_addr_reg */
 106         if (src_base_addr_reg != BPF_REG_ARG3)
 107                 ins(BPF_MOV64_REG(BPF_REG_ARG3, src_base_addr_reg), pos);
 108         /* add arg3, #offset */
 109         if (offset)
 110                 ins(BPF_ALU64_IMM(BPF_ADD, BPF_REG_ARG3, offset), pos);
 111
 112         /* mov arg2, #reg_size */
 113         ins(BPF_ALU64_IMM(BPF_MOV, BPF_REG_ARG2, BPF_REG_SIZE), pos);
 114
 115         /* mov arg1, dst_addr_reg */
 116         if (dst_addr_reg != BPF_REG_ARG1)
 117                 ins(BPF_MOV64_REG(BPF_REG_ARG1, dst_addr_reg), pos);
 118
 119         /* Call probe_read  */
 120         ins(BPF_EMIT_CALL(BPF_FUNC_probe_read), pos);
 121         /*
 122          * Error processing: if read fail, goto error code,
 123          * will be relocated. Target should be the start of
 124          * error processing code.
 125          */
 126         ins(BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, JMP_TO_ERROR_CODE),
 127             pos);
 128
 129         return check_pos(pos);
 130 }
 131
 132 /*
 133  * Each arg should be bare register. Fetch and save them into argument
 134  * registers (r3 - r5).
 135  *
 136  * BPF_REG_1 should have been initialized with pointer to
 137  * 'struct pt_regs'.
 138  */
 139 static int
 140 gen_prologue_fastpath(struct bpf_insn_pos *pos,
 141                       struct probe_trace_arg *args, int nargs)
 142 {
 143         int i, err = 0;
 144
 145         for (i = 0; i < nargs; i++) {
 146                 err = gen_ldx_reg_from_ctx(pos, BPF_REG_1, args[i].value,
 147                                            BPF_PROLOGUE_START_ARG_REG + i);
 148                 if (err)
 149                         goto errout;
 150         }
 151
 152         return check_pos(pos);
 153 errout:
 154         return err;
 155 }
 156
 157 /*
 158  * Slow path:
 159  *   At least one argument has the form of 'offset($rx)'.
 160  *
 161  * Following code first stores them into stack, then loads all of then
 162  * to r2 - r5.
 163  * Before final loading, the final result should be:
 164  *
 165  * low address
 166  * BPF_REG_FP - 24  ARG3
 167  * BPF_REG_FP - 16  ARG2
 168  * BPF_REG_FP - 8   ARG1
 169  * BPF_REG_FP
 170  * high address
 171  *
 172  * For each argument (described as: offn(...off2(off1(reg)))),
 173  * generates following code:
 174  *
 175  *  r7 <- fp
 176  *  r7 <- r7 - stack_offset  // Ideal code should initialize r7 using
 177  *                           // fp before generating args. However,
 178  *                           // eBPF won't regard r7 as stack pointer
 179  *                           // if it is generated by minus 8 from
 180  *                           // another stack pointer except fp.
 181  *                           // This is why we have to set r7
 182  *                           // to fp for each variable.
 183  *  r3 <- value of 'reg'-> generated using gen_ldx_reg_from_ctx()
 184  *  (r7) <- r3       // skip following instructions for bare reg
 185  *  r3 <- r3 + off1  . // skip if off1 == 0
 186  *  r2 <- 8           \
 187  *  r1 <- r7           |-> generated by gen_read_mem()
 188  *  call probe_read    /
 189  *  jnei r0, 0, err  ./
 190  *  r3 <- (r7)
 191  *  r3 <- r3 + off2  . // skip if off2 == 0
 192  *  r2 <- 8           \  // r2 may be broken by probe_read, so set again
 193  *  r1 <- r7           |-> generated by gen_read_mem()
 194  *  call probe_read    /
 195  *  jnei r0, 0, err  ./
 196  *  ...
 197  */
 198 static int
 199 gen_prologue_slowpath(struct bpf_insn_pos *pos,
 200                       struct probe_trace_arg *args, int nargs)
 201 {
 202         int err, i;
 203
 204         for (i = 0; i < nargs; i++) {
 205                 struct probe_trace_arg *arg = &args[i];
 206                 const char *reg = arg->value;
 207                 struct probe_trace_arg_ref *ref = NULL;
 208                 int stack_offset = (i + 1) * -8;
 209
 210                 pr_debug("prologue: fetch arg %d, base reg is %s\n",
 211                          i, reg);
 212
 213                 /* value of base register is stored into ARG3 */
 214                 err = gen_ldx_reg_from_ctx(pos, BPF_REG_CTX, reg,
 215                                            BPF_REG_ARG3);
 216                 if (err) {
 217                         pr_err("prologue: failed to get offset of register %s\n",
 218                                reg);
 219                         goto errout;
 220                 }
 221
 222                 /* Make r7 the stack pointer. */
 223                 ins(BPF_MOV64_REG(BPF_REG_7, BPF_REG_FP), pos);
 224                 /* r7 += -8 */
 225                 ins(BPF_ALU64_IMM(BPF_ADD, BPF_REG_7, stack_offset), pos);
 226                 /*
 227                  * Store r3 (base register) onto stack
 228                  * Ensure fp[offset] is set.
 229                  * fp is the only valid base register when storing
 230                  * into stack. We are not allowed to use r7 as base
 231                  * register here.
 232                  */
 233                 ins(BPF_STX_MEM(BPF_DW, BPF_REG_FP, BPF_REG_ARG3,
 234                                 stack_offset), pos);
 235
 236                 ref = arg->ref;
 237                 while (ref) {
 238                         pr_debug("prologue: arg %d: offset %ld\n",
 239                                  i, ref->offset);
 240                         err = gen_read_mem(pos, BPF_REG_3, BPF_REG_7,
 241                                            ref->offset);
 242                         if (err) {
 243                                 pr_err("prologue: failed to generate probe_read function call\n");
 244                                 goto errout;
 245                         }
 246
 247                         ref = ref->next;
 248                         /*
 249                          * Load previous result into ARG3. Use
 250                          * BPF_REG_FP instead of r7 because verifier
 251                          * allows FP based addressing only.
 252                          */
 253                         if (ref)
 254                                 ins(BPF_LDX_MEM(BPF_DW, BPF_REG_ARG3,
 255                                                 BPF_REG_FP, stack_offset), pos);
 256                 }
 257         }
 258
 259         /* Final pass: read to registers */
 260         for (i = 0; i < nargs; i++)
 261                 ins(BPF_LDX_MEM(BPF_DW, BPF_PROLOGUE_START_ARG_REG + i,
 262                                 BPF_REG_FP, -BPF_REG_SIZE * (i + 1)), pos);
 263
 264         ins(BPF_JMP_IMM(BPF_JA, BPF_REG_0, 0, JMP_TO_SUCCESS_CODE), pos);
 265
 266         return check_pos(pos);
 267 errout:
 268         return err;
 269 }
 270
 271 static int
 272 prologue_relocate(struct bpf_insn_pos *pos, struct bpf_insn *error_code,
 273                   struct bpf_insn *success_code, struct bpf_insn *user_code)
 274 {
 275         struct bpf_insn *insn;
 276
 277         if (check_pos(pos))
 278                 return -BPF_LOADER_ERRNO__PROLOGUE2BIG;
 279
 280         for (insn = pos->begin; insn < pos->pos; insn++) {
 281                 struct bpf_insn *target;
 282                 u8 class = BPF_CLASS(insn->code);
 283                 u8 opcode;
 284
 285                 if (class != BPF_JMP)
 286                         continue;
 287                 opcode = BPF_OP(insn->code);
 288                 if (opcode == BPF_CALL)
 289                         continue;
 290
 291                 switch (insn->off) {
 292                 case JMP_TO_ERROR_CODE:
 293                         target = error_code;
 294                         break;
 295                 case JMP_TO_SUCCESS_CODE:
 296                         target = success_code;
 297                         break;
 298                 case JMP_TO_USER_CODE:
 299                         target = user_code;
 300                         break;
 301                 default:
 302                         pr_err("bpf prologue: internal error: relocation failed\n");
 303                         return -BPF_LOADER_ERRNO__PROLOGUE;
 304                 }
 305
 306                 insn->off = target - (insn + 1);
 307         }
 308         return 0;
 309 }
 310
 311 int bpf__gen_prologue(struct probe_trace_arg *args, int nargs,
 312                       struct bpf_insn *new_prog, size_t *new_cnt,
 313                       size_t cnt_space)
 314 {
 315         struct bpf_insn *success_code = NULL;
 316         struct bpf_insn *error_code = NULL;
 317         struct bpf_insn *user_code = NULL;
 318         struct bpf_insn_pos pos;
 319         bool fastpath = true;
 320         int err = 0, i;
 321
 322         if (!new_prog || !new_cnt)
 323                 return -EINVAL;
 324
 325         if (cnt_space > BPF_MAXINSNS)
 326                 cnt_space = BPF_MAXINSNS;
 327
 328         pos.begin = new_prog;
 329         pos.end = new_prog + cnt_space;
 330         pos.pos = new_prog;
 331
 332         if (!nargs) {
 333                 ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 0),
 334                     &pos);
 335
 336                 if (check_pos(&pos))
 337                         goto errout;
 338
 339                 *new_cnt = pos_get_cnt(&pos);
 340                 return 0;
 341         }
 342
 343         if (nargs > BPF_PROLOGUE_MAX_ARGS) {
 344                 pr_warning("bpf: prologue: %d arguments are dropped\n",
 345                            nargs - BPF_PROLOGUE_MAX_ARGS);
 346                 nargs = BPF_PROLOGUE_MAX_ARGS;
 347         }
 348
 349         /* First pass: validation */
 350         for (i = 0; i < nargs; i++) {
 351                 struct probe_trace_arg_ref *ref = args[i].ref;
 352
 353                 if (args[i].value[0] == '@') {
 354                         /* TODO: fetch global variable */
 355                         pr_err("bpf: prologue: global %s%+ld not support\n",
 356                                 args[i].value, ref ? ref->offset : 0);
 357                         return -ENOTSUP;
 358                 }
 359
 360                 while (ref) {
 361                         /* fastpath is true if all args has ref == NULL */
 362                         fastpath = false;
 363
 364                         /*
 365                          * Instruction encodes immediate value using
 366                          * s32, ref->offset is long. On systems which
 367                          * can't fill long in s32, refuse to process if
 368                          * ref->offset too large (or small).
 369                          */
 370 #ifdef __LP64__
 371 #define OFFSET_MAX      ((1LL << 31) - 1)
 372 #define OFFSET_MIN      ((1LL << 31) * -1)
 373                         if (ref->offset > OFFSET_MAX ||
 374                                         ref->offset < OFFSET_MIN) {
 375                                 pr_err("bpf: prologue: offset out of bound: %ld\n",
 376                                        ref->offset);
 377                                 return -BPF_LOADER_ERRNO__PROLOGUEOOB;
 378                         }
 379 #endif
 380                         ref = ref->next;
 381                 }
 382         }
 383         pr_debug("prologue: pass validation\n");
 384
 385         if (fastpath) {
 386                 /* If all variables are registers... */
 387                 pr_debug("prologue: fast path\n");
 388                 err = gen_prologue_fastpath(&pos, args, nargs);
 389                 if (err)
 390                         goto errout;
 391         } else {
 392                 pr_debug("prologue: slow path\n");
 393
 394                 /* Initialization: move ctx to a callee saved register. */
 395                 ins(BPF_MOV64_REG(BPF_REG_CTX, BPF_REG_ARG1), &pos);
 396
 397                 err = gen_prologue_slowpath(&pos, args, nargs);
 398                 if (err)
 399                         goto errout;
 400                 /*
 401                  * start of ERROR_CODE (only slow pass needs error code)
 402                  *   mov r2 <- 1  // r2 is error number
 403                  *   mov r3 <- 0  // r3, r4... should be touched or
 404                  *                // verifier would complain
 405                  *   mov r4 <- 0
 406                  *   ...
 407                  *   goto usercode
 408                  */
 409                 error_code = pos.pos;
 410                 ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 1),
 411                     &pos);
 412
 413                 for (i = 0; i < nargs; i++)
 414                         ins(BPF_ALU64_IMM(BPF_MOV,
 415                                           BPF_PROLOGUE_START_ARG_REG + i,
 416                                           0),
 417                             &pos);
 418                 ins(BPF_JMP_IMM(BPF_JA, BPF_REG_0, 0, JMP_TO_USER_CODE),
 419                                 &pos);
 420         }
 421
 422         /*
 423          * start of SUCCESS_CODE:
 424          *   mov r2 <- 0
 425          *   goto usercode  // skip
 426          */
 427         success_code = pos.pos;
 428         ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 0), &pos);
 429
 430         /*
 431          * start of USER_CODE:
 432          *   Restore ctx to r1
 433          */
 434         user_code = pos.pos;
 435         if (!fastpath) {
 436                 /*
 437                  * Only slow path needs restoring of ctx. In fast path,
 438                  * register are loaded directly from r1.
 439                  */
 440                 ins(BPF_MOV64_REG(BPF_REG_ARG1, BPF_REG_CTX), &pos);
 441                 err = prologue_relocate(&pos, error_code, success_code,
 442                                         user_code);
 443                 if (err)
 444                         goto errout;
 445         }
 446
 447         err = check_pos(&pos);
 448         if (err)
 449                 goto errout;
 450
 451         *new_cnt = pos_get_cnt(&pos);
 452         return 0;
 453 errout:
 454         return err;
 455 }