/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8                                :vi│
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2020 Justine Alexandra Roberts Tunney                              │
│                                                                              │
│ Permission to use, copy, modify, and/or distribute this software for         │
│ any purpose with or without fee is hereby granted, provided that the         │
│ above copyright notice and this permission notice appear in all copies.      │
│                                                                              │
│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL                │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED                │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE             │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL         │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR        │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER               │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR             │
│ PERFORMANCE OF THIS SOFTWARE.                                                │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/calls/calls.h"
#include "libc/calls/struct/sigset.h"
#include "libc/dce.h"
#include "libc/errno.h"
#include "libc/intrin/bits.h"
#include "libc/log/libfatal.internal.h"
#include "libc/runtime/runtime.h"
#include "libc/runtime/symbols.internal.h"
#include "libc/str/str.h"
#include "libc/sysv/consts/prot.h"

/**
 * Rewrites code in memory to hook function calls.
 *
 * We do this by searching each function for the nop instruction
 * inserted by GCC when we use the -pg -mnop-mcount flags. There's no
 * risk of corrupting data since the linker scripts won't mix code and
 * data.
 *
 * Modules built with -O3 and without the profiling flags might have
 * these same nop instructions, but that shouldn't be problematic since
 * they're only there for the puposes of aligning jumps, and therefore
 * aren't actually executed. However codebases that use huge function
 * alignments with wide-nop slides could pose minor issues. Further note
 * that Cosmopolitan sources are almost never intentionally written to
 * use code alignment, since we've only seen a few cases where it helps.
 *
 * @see ape/ape.lds
 */
privileged noinstrument noasan int __hook(void *ifunc,
                                          struct SymbolTable *symbols) {
  int rc;
  size_t i;
  char *p, *pe;
  intptr_t addr;
  uint64_t code, mcode;
  sigset_t mask, oldmask;
  intptr_t kMcount = (intptr_t)&mcount;
  intptr_t kProgramCodeStart = (intptr_t)_ereal;
  intptr_t kPrivilegedStart = (intptr_t)__privileged_addr;
  if (!symbols) return -1;
  __morph_begin();
  for (i = 0; i < symbols->count; ++i) {
    if (symbols->addr_base + symbols->symbols[i].x < kProgramCodeStart) {
      continue;
    }
    if (symbols->addr_base + symbols->symbols[i].y >= kPrivilegedStart) {
      break;
    }
    for (p = (char *)symbols->addr_base + symbols->symbols[i].x,
        pe = (char *)symbols->addr_base + symbols->symbols[i].y;
         p + 8 - 1 <= pe; ++p) {
      code = ((uint64_t)(255 & p[7]) << 070 | (uint64_t)(255 & p[6]) << 060 |
              (uint64_t)(255 & p[5]) << 050 | (uint64_t)(255 & p[4]) << 040 |
              (uint64_t)(255 & p[3]) << 030 | (uint64_t)(255 & p[2]) << 020 |
              (uint64_t)(255 & p[1]) << 010 | (uint64_t)(255 & p[0]) << 000);

      /*
       * Test for -mrecord-mcount (w/ -fpie or -fpic)
       *
       *   nopw 0x00(%rax,%rax,1)  ← morphed by package.com
       *   call *mcount(%rip)      ← linked w/o -static
       *   addr32 call mcount      ← relaxed w/ -static
       *   addr32 call mcount      ← relaxed w/ -static
       *
       * Note that gcc refuses to insert the six byte nop.
       */
      if ((code & 0x0000FFFFFFFFFFFF) == 0x0000441F0F66 ||
          (code & 0x0000FFFFFFFFFFFF) ==
              ((((kMcount - ((intptr_t)&p[2] + 4)) << 16) | 0xE867) &
               0x0000FFFFFFFFFFFF) ||
          (code & 0x0000FFFFFFFFFFFF) ==
              ((((kMcount - ((intptr_t)&p[2] + 4)) << 16) | 0xFF15) &
               0x0000FFFFFFFFFFFF)) {
        p[0] = 0x67;
        p[1] = 0xE8;
        addr = (intptr_t)ifunc - ((intptr_t)&p[2] + 4);
        p[2] = (addr & 0x000000ff) >> 000;
        p[3] = (addr & 0x0000ff00) >> 010;
        p[4] = (addr & 0x00ff0000) >> 020;
        p[5] = (addr & 0xff000000) >> 030;
        break;
      }

      /*
       * Test for -mnop-mcount (w/ -fno-pie)
       */
      mcode = code & 0x000000FFFFFFFFFF;
      if ((mcode == 0x00441F0F /*   nopl 0x00(%eax,%eax,1) [canonical] */) ||
          (mcode == 0x00041F0F67 /* nopl (%eax,%eax,1)     [older gcc] */)) {
        if (p[-1] != 0x66 /*        nopw 0x0(%rax,%rax,1)  [donotwant] */) {
          p[0] = 0xE8 /* call Jvds */;
          addr = (intptr_t)ifunc - ((intptr_t)&p[1] + 4);
          p[1] = (addr & 0x000000ff) >> 000;
          p[2] = (addr & 0x0000ff00) >> 010;
          p[3] = (addr & 0x00ff0000) >> 020;
          p[4] = (addr & 0xff000000) >> 030;
        }
        break;
      }
    }
  }
  __morph_end();
  return 0;
}