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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
24struct bpf_insn_pos {
25 struct bpf_insn *begin;
26 struct bpf_insn *end;
27 struct bpf_insn *pos;
28};
29
30static inline int
31pos_get_cnt(struct bpf_insn_pos *pos)
32{
33 return pos->pos - pos->begin;
34}
35
36static int
37append_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
52static int
53check_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 */
69static int
70gen_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 */
99static int
100gen_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 */
139static int
140gen_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);
153errout:
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 */
198static int
199gen_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);
267errout:
268 return err;
269}
270
271static int
272prologue_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
311int 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;
453errout:
454 return err;
455}