bpf-prologue.c

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// SPDX-License-Identifier: GPL-2.0
/*
 * bpf-prologue.c
 *
 * Copyright (C) 2015 He Kuang <hekuang@huawei.com>
 * Copyright (C) 2015 Wang Nan <wangnan0@huawei.com>
 * Copyright (C) 2015 Huawei Inc.
 */

#include <bpf/libbpf.h>
#include "perf.h"
#include "debug.h"
#include "bpf-loader.h"
#include "bpf-prologue.h"
#include "probe-finder.h"
#include <errno.h>
#include <dwarf-regs.h>
#include <linux/filter.h>

#define BPF_REG_SIZE        8

#define JMP_TO_ERROR_CODE   -1
#define JMP_TO_SUCCESS_CODE -2
#define JMP_TO_USER_CODE    -3

struct bpf_insn_pos {
    struct bpf_insn *begin;
    struct bpf_insn *end;
    struct bpf_insn *pos;
};

static inline int
pos_get_cnt(struct bpf_insn_pos *pos)
{
    return pos->pos - pos->begin;
}

static int
append_insn(struct bpf_insn new_insn, struct bpf_insn_pos *pos)
{
    if (!pos->pos)
        return -BPF_LOADER_ERRNO__PROLOGUE2BIG;

    if (pos->pos + 1 >= pos->end) {
        pr_err("bpf prologue: prologue too long\n");
        pos->pos = NULL;
        return -BPF_LOADER_ERRNO__PROLOGUE2BIG;
    }

    *(pos->pos)++ = new_insn;
    return 0;
}

static int
check_pos(struct bpf_insn_pos *pos)
{
    if (!pos->pos || pos->pos >= pos->end)
        return -BPF_LOADER_ERRNO__PROLOGUE2BIG;
    return 0;
}

/*
 * Convert type string (u8/u16/u32/u64/s8/s16/s32/s64 ..., see
 * Documentation/trace/kprobetrace.txt) to size field of BPF_LDX_MEM
 * instruction (BPF_{B,H,W,DW}).
 */
static int
argtype_to_ldx_size(const char *type)
{
    int arg_size = type ? atoi(&type[1]) : 64;

    switch (arg_size) {
    case 8:
        return BPF_B;
    case 16:
        return BPF_H;
    case 32:
        return BPF_W;
    case 64:
    default:
        return BPF_DW;
    }
}

static const char *
insn_sz_to_str(int insn_sz)
{
    switch (insn_sz) {
    case BPF_B:
        return "BPF_B";
    case BPF_H:
        return "BPF_H";
    case BPF_W:
        return "BPF_W";
    case BPF_DW:
        return "BPF_DW";
    default:
        return "UNKNOWN";
    }
}

/* Give it a shorter name */
#define ins(i, p) append_insn((i), (p))

/*
 * Give a register name (in 'reg'), generate instruction to
 * load register into an eBPF register rd:
 *   'ldd target_reg, offset(ctx_reg)', where:
 * ctx_reg is pre initialized to pointer of 'struct pt_regs'.
 */
static int
gen_ldx_reg_from_ctx(struct bpf_insn_pos *pos, int ctx_reg,
             const char *reg, int target_reg)
{
    int offset = regs_query_register_offset(reg);

    if (offset < 0) {
        pr_err("bpf: prologue: failed to get register %s\n",
               reg);
        return offset;
    }
    ins(BPF_LDX_MEM(BPF_DW, target_reg, ctx_reg, offset), pos);

    return check_pos(pos);
}

/*
 * Generate a BPF_FUNC_probe_read function call.
 *
 * src_base_addr_reg is a register holding base address,
 * dst_addr_reg is a register holding dest address (on stack),
 * result is:
 *
 *  *[dst_addr_reg] = *([src_base_addr_reg] + offset)
 *
 * Arguments of BPF_FUNC_probe_read:
 *     ARG1: ptr to stack (dest)
 *     ARG2: size (8)
 *     ARG3: unsafe ptr (src)
 */
static int
gen_read_mem(struct bpf_insn_pos *pos,
         int src_base_addr_reg,
         int dst_addr_reg,
         long offset)
{
    /* mov arg3, src_base_addr_reg */
    if (src_base_addr_reg != BPF_REG_ARG3)
        ins(BPF_MOV64_REG(BPF_REG_ARG3, src_base_addr_reg), pos);
    /* add arg3, #offset */
    if (offset)
        ins(BPF_ALU64_IMM(BPF_ADD, BPF_REG_ARG3, offset), pos);

    /* mov arg2, #reg_size */
    ins(BPF_ALU64_IMM(BPF_MOV, BPF_REG_ARG2, BPF_REG_SIZE), pos);

    /* mov arg1, dst_addr_reg */
    if (dst_addr_reg != BPF_REG_ARG1)
        ins(BPF_MOV64_REG(BPF_REG_ARG1, dst_addr_reg), pos);

    /* Call probe_read  */
    ins(BPF_EMIT_CALL(BPF_FUNC_probe_read), pos);
    /*
     * Error processing: if read fail, goto error code,
     * will be relocated. Target should be the start of
     * error processing code.
     */
    ins(BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, JMP_TO_ERROR_CODE),
        pos);

    return check_pos(pos);
}

/*
 * Each arg should be bare register. Fetch and save them into argument
 * registers (r3 - r5).
 *
 * BPF_REG_1 should have been initialized with pointer to
 * 'struct pt_regs'.
 */
static int
gen_prologue_fastpath(struct bpf_insn_pos *pos,
              struct probe_trace_arg *args, int nargs)
{
    int i, err = 0;

    for (i = 0; i < nargs; i++) {
        err = gen_ldx_reg_from_ctx(pos, BPF_REG_1, args[i].value,
                       BPF_PROLOGUE_START_ARG_REG + i);
        if (err)
            goto errout;
    }

    return check_pos(pos);
errout:
    return err;
}

/*
 * Slow path:
 *   At least one argument has the form of 'offset($rx)'.
 *
 * Following code first stores them into stack, then loads all of then
 * to r2 - r5.
 * Before final loading, the final result should be:
 *
 * low address
 * BPF_REG_FP - 24  ARG3
 * BPF_REG_FP - 16  ARG2
 * BPF_REG_FP - 8   ARG1
 * BPF_REG_FP
 * high address
 *
 * For each argument (described as: offn(...off2(off1(reg)))),
 * generates following code:
 *
 *  r7 <- fp
 *  r7 <- r7 - stack_offset  // Ideal code should initialize r7 using
 *                           // fp before generating args. However,
 *                           // eBPF won't regard r7 as stack pointer
 *                           // if it is generated by minus 8 from
 *                           // another stack pointer except fp.
 *                           // This is why we have to set r7
 *                           // to fp for each variable.
 *  r3 <- value of 'reg'-> generated using gen_ldx_reg_from_ctx()
 *  (r7) <- r3       // skip following instructions for bare reg
 *  r3 <- r3 + off1  . // skip if off1 == 0
 *  r2 <- 8           \
 *  r1 <- r7           |-> generated by gen_read_mem()
 *  call probe_read    /
 *  jnei r0, 0, err  ./
 *  r3 <- (r7)
 *  r3 <- r3 + off2  . // skip if off2 == 0
 *  r2 <- 8           \  // r2 may be broken by probe_read, so set again
 *  r1 <- r7           |-> generated by gen_read_mem()
 *  call probe_read    /
 *  jnei r0, 0, err  ./
 *  ...
 */
static int
gen_prologue_slowpath(struct bpf_insn_pos *pos,
              struct probe_trace_arg *args, int nargs)
{
    int err, i;

    for (i = 0; i < nargs; i++) {
        struct probe_trace_arg *arg = &args[i];
        const char *reg = arg->value;
        struct probe_trace_arg_ref *ref = NULL;
        int stack_offset = (i + 1) * -8;

        pr_debug("prologue: fetch arg %d, base reg is %s\n",
             i, reg);

        /* value of base register is stored into ARG3 */
        err = gen_ldx_reg_from_ctx(pos, BPF_REG_CTX, reg,
                       BPF_REG_ARG3);
        if (err) {
            pr_err("prologue: failed to get offset of register %s\n",
                   reg);
            goto errout;
        }

        /* Make r7 the stack pointer. */
        ins(BPF_MOV64_REG(BPF_REG_7, BPF_REG_FP), pos);
        /* r7 += -8 */
        ins(BPF_ALU64_IMM(BPF_ADD, BPF_REG_7, stack_offset), pos);
        /*
         * Store r3 (base register) onto stack
         * Ensure fp[offset] is set.
         * fp is the only valid base register when storing
         * into stack. We are not allowed to use r7 as base
         * register here.
         */
        ins(BPF_STX_MEM(BPF_DW, BPF_REG_FP, BPF_REG_ARG3,
                stack_offset), pos);

        ref = arg->ref;
        while (ref) {
            pr_debug("prologue: arg %d: offset %ld\n",
                 i, ref->offset);
            err = gen_read_mem(pos, BPF_REG_3, BPF_REG_7,
                       ref->offset);
            if (err) {
                pr_err("prologue: failed to generate probe_read function call\n");
                goto errout;
            }

            ref = ref->next;
            /*
             * Load previous result into ARG3. Use
             * BPF_REG_FP instead of r7 because verifier
             * allows FP based addressing only.
             */
            if (ref)
                ins(BPF_LDX_MEM(BPF_DW, BPF_REG_ARG3,
                        BPF_REG_FP, stack_offset), pos);
        }
    }

    /* Final pass: read to registers */
    for (i = 0; i < nargs; i++) {
        int insn_sz = (args[i].ref) ? argtype_to_ldx_size(args[i].type) : BPF_DW;

        pr_debug("prologue: load arg %d, insn_sz is %s\n",
             i, insn_sz_to_str(insn_sz));
        ins(BPF_LDX_MEM(insn_sz, BPF_PROLOGUE_START_ARG_REG + i,
                BPF_REG_FP, -BPF_REG_SIZE * (i + 1)), pos);
    }

    ins(BPF_JMP_IMM(BPF_JA, BPF_REG_0, 0, JMP_TO_SUCCESS_CODE), pos);

    return check_pos(pos);
errout:
    return err;
}

static int
prologue_relocate(struct bpf_insn_pos *pos, struct bpf_insn *error_code,
          struct bpf_insn *success_code, struct bpf_insn *user_code)
{
    struct bpf_insn *insn;

    if (check_pos(pos))
        return -BPF_LOADER_ERRNO__PROLOGUE2BIG;

    for (insn = pos->begin; insn < pos->pos; insn++) {
        struct bpf_insn *target;
        u8 class = BPF_CLASS(insn->code);
        u8 opcode;

        if (class != BPF_JMP)
            continue;
        opcode = BPF_OP(insn->code);
        if (opcode == BPF_CALL)
            continue;

        switch (insn->off) {
        case JMP_TO_ERROR_CODE:
            target = error_code;
            break;
        case JMP_TO_SUCCESS_CODE:
            target = success_code;
            break;
        case JMP_TO_USER_CODE:
            target = user_code;
            break;
        default:
            pr_err("bpf prologue: internal error: relocation failed\n");
            return -BPF_LOADER_ERRNO__PROLOGUE;
        }

        insn->off = target - (insn + 1);
    }
    return 0;
}

int bpf__gen_prologue(struct probe_trace_arg *args, int nargs,
              struct bpf_insn *new_prog, size_t *new_cnt,
              size_t cnt_space)
{
    struct bpf_insn *success_code = NULL;
    struct bpf_insn *error_code = NULL;
    struct bpf_insn *user_code = NULL;
    struct bpf_insn_pos pos;
    bool fastpath = true;
    int err = 0, i;

    if (!new_prog || !new_cnt)
        return -EINVAL;

    if (cnt_space > BPF_MAXINSNS)
        cnt_space = BPF_MAXINSNS;

    pos.begin = new_prog;
    pos.end = new_prog + cnt_space;
    pos.pos = new_prog;

    if (!nargs) {
        ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 0),
            &pos);

        if (check_pos(&pos))
            goto errout;

        *new_cnt = pos_get_cnt(&pos);
        return 0;
    }

    if (nargs > BPF_PROLOGUE_MAX_ARGS) {
        pr_warning("bpf: prologue: %d arguments are dropped\n",
               nargs - BPF_PROLOGUE_MAX_ARGS);
        nargs = BPF_PROLOGUE_MAX_ARGS;
    }

    /* First pass: validation */
    for (i = 0; i < nargs; i++) {
        struct probe_trace_arg_ref *ref = args[i].ref;

        if (args[i].value[0] == '@') {
            /* TODO: fetch global variable */
            pr_err("bpf: prologue: global %s%+ld not support\n",
                args[i].value, ref ? ref->offset : 0);
            return -ENOTSUP;
        }

        while (ref) {
            /* fastpath is true if all args has ref == NULL */
            fastpath = false;

            /*
             * Instruction encodes immediate value using
             * s32, ref->offset is long. On systems which
             * can't fill long in s32, refuse to process if
             * ref->offset too large (or small).
             */
#ifdef __LP64__
#define OFFSET_MAX  ((1LL << 31) - 1)
#define OFFSET_MIN  ((1LL << 31) * -1)
            if (ref->offset > OFFSET_MAX ||
                    ref->offset < OFFSET_MIN) {
                pr_err("bpf: prologue: offset out of bound: %ld\n",
                       ref->offset);
                return -BPF_LOADER_ERRNO__PROLOGUEOOB;
            }
#endif
            ref = ref->next;
        }
    }
    pr_debug("prologue: pass validation\n");

    if (fastpath) {
        /* If all variables are registers... */
        pr_debug("prologue: fast path\n");
        err = gen_prologue_fastpath(&pos, args, nargs);
        if (err)
            goto errout;
    } else {
        pr_debug("prologue: slow path\n");

        /* Initialization: move ctx to a callee saved register. */
        ins(BPF_MOV64_REG(BPF_REG_CTX, BPF_REG_ARG1), &pos);

        err = gen_prologue_slowpath(&pos, args, nargs);
        if (err)
            goto errout;
        /*
         * start of ERROR_CODE (only slow pass needs error code)
         *   mov r2 <- 1  // r2 is error number
         *   mov r3 <- 0  // r3, r4... should be touched or
         *                // verifier would complain
         *   mov r4 <- 0
         *   ...
         *   goto usercode
         */
        error_code = pos.pos;
        ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 1),
            &pos);

        for (i = 0; i < nargs; i++)
            ins(BPF_ALU64_IMM(BPF_MOV,
                      BPF_PROLOGUE_START_ARG_REG + i,
                      0),
                &pos);
        ins(BPF_JMP_IMM(BPF_JA, BPF_REG_0, 0, JMP_TO_USER_CODE),
                &pos);
    }

    /*
     * start of SUCCESS_CODE:
     *   mov r2 <- 0
     *   goto usercode  // skip
     */
    success_code = pos.pos;
    ins(BPF_ALU64_IMM(BPF_MOV, BPF_PROLOGUE_FETCH_RESULT_REG, 0), &pos);

    /*
     * start of USER_CODE:
     *   Restore ctx to r1
     */
    user_code = pos.pos;
    if (!fastpath) {
        /*
         * Only slow path needs restoring of ctx. In fast path,
         * register are loaded directly from r1.
         */
        ins(BPF_MOV64_REG(BPF_REG_ARG1, BPF_REG_CTX), &pos);
        err = prologue_relocate(&pos, error_code, success_code,
                    user_code);
        if (err)
            goto errout;
    }

    err = check_pos(&pos);
    if (err)
        goto errout;

    *new_cnt = pos_get_cnt(&pos);
    return 0;
errout:
    return err;
}