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cleaner.c
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#include <stdio.h>
#include <string.h>
#include <stdint.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#define VERSION "1.0"
#define DEBUG 1
#define DEBUG_VERBOSE 0
#define MAX_TYPES 256
#define MAX_FUNCS 256 /* this includes imports! */
uint64_t leb(
uint8_t** buf,
uint8_t* bufend,
int is_signed)
{
uint64_t val = 0, shift = 0, i = 0;
while (*buf + i < bufend)
{
uint64_t b = (uint64_t)((*buf)[i]);
uint64_t last = val;
val += (b & 0x7FU) << shift;
if (val < last)
{
fprintf(stderr, "LEB128 overflow in input wasm at offset %ld.\n",
i);
exit(100);
}
++i;
if (b & 0x80U)
{
shift += 7;
continue;
}
*buf += i;
if (is_signed && shift < 64 && (b & 0x40U))
val |= (~0 << shift);
return val;
}
return 0;
}
void leb_out(
uint64_t i,
uint8_t** o)
{
do
{
uint8_t b = i & 0x7FU;
i >>= 7U;
if (i != 0)
b |= 0x80;
**o = b;
(*o)++;
} while (i != 0);
}
void leb_out_pad(
uint64_t i,
uint8_t** o,
int padto)
{
printf("Leb_out_pad(i=%ld, pad=%d): [", i, padto);
padto--;
do
{
uint8_t b = i & 0x7FU;
i >>= 7U;
if (i != 0 || padto > 0)
b |= 0x80;
**o = b;
(*o)++;
printf(" 0x%02X", b);
padto--;
} while (i > 0 || padto >= 0);
printf(" ]\n");
}
int cleaner (
uint8_t* w, // web assembly input buffer
uint8_t* o, // web assembly output buffer
ssize_t* len) // length of input buffer when called, and len of output buffer when returned
{
// require at least `need` bytes
#define REQUIRE(need)\
{\
if (DEBUG && DEBUG_VERBOSE)\
printf("Require %ld b\tfrom 0x%lX to 0x%lX\n",\
((uint64_t)(need)),\
((uint64_t)(w-wstart)),\
((uint64_t)(w+need-wstart)));\
if (wlen - (w - wstart) < need)\
return \
fprintf(stderr,\
"Truncated web assembly input. SrcLine: %d. Illegally short at position %ld [0x%lx].\n"\
"wlen: %ld w-wstart: %ld need:%ld\n"\
"%08lX : %02X %02X %02X %02X\n",\
__LINE__,\
((uint64_t)(w - wstart)),\
((uint64_t)(w - wstart)),\
((uint64_t)(wlen)),\
((uint64_t)(w-wstart)),\
((uint64_t)(need)),\
((uint64_t)((w - wstart) - 4)),\
*(w-4), *(w-3), *(w-2), *(w-1));\
}
// advance `adv` bytes
#define ADVANCE(adv)\
{\
if (DEBUG && DEBUG_VERBOSE)\
printf("Advance %ld b\tfrom 0x%lX to 0x%lX\n",\
((uint64_t)(adv)),\
((uint64_t)(w-wstart)),\
((uint64_t)(w+adv-wstart)));\
w += adv;\
REQUIRE(0);\
}
#define LEB()\
(tmp2=w-wstart,tmp=leb(&w, wend, 0),\
(DEBUG && DEBUG_VERBOSE &&\
printf("Leb read at 0x%lX: %ld\n", tmp2, tmp)),tmp)
#define SIGNED_LEB()\
(tmp2=w-wstart,tmp=leb(&w, wend, 1),\
(DEBUG && DEBUG_VERBOSE &&\
printf("Signed Leb read at 0x%lX: %ld\n", tmp2, tmp)),tmp)
uint8_t* wstart = w; // remember start of buffer
ssize_t wlen = *len;
uint8_t* wend = w + wlen;
uint64_t tmp, tmp2;
// read magic number
REQUIRE(4);
//00 61 73 6D
if (w[0] != 0x00U || w[1] != 0x61U || w[2] != 0x73U || w[3] != 0x6DU)
return fprintf(stderr, "Magic number missing or invalid %02X=%d %02X=%d %02X=%d %02X=%d\n",
w[0], w[0] == 0x00U, w[1], w[1] == 0x61U, w[2], w[2] == 0x73U, w[3], w[3] == 0x6DU);
ADVANCE(4);
// read version
REQUIRE(4)
if (w[0] != 0x01U || w[1] || w[2] || w[3])
return fprintf(stderr, "Only version 1.00 of WASM standard is supported\n");
ADVANCE(4);
// first section loop
if (DEBUG)
printf("First pass start\n");
int func_hook = -1;
int func_cbak = -1;
int mem_export = -1; // RH UPTO: find out what memory is exported and carry it over (do we need this??)
int out_import_count = -1; // the number of imports there will be in the output file
ssize_t out_import_size = 0; // the size ofthe import section in the output
ssize_t out_code_size = 0;
int func_count = -1;
int hook_cbak_type = -1;
int func_type[MAX_FUNCS]; // each function we discover import/func has its type id recorded here
struct {
uint8_t set;
uint8_t rc;
uint8_t r[30];
uint8_t pc;
uint8_t p[31];
} types[MAX_TYPES];
for (int x = 0; x < MAX_TYPES; ++x)
{
func_type[x] = -1;
types[x].set = 0;
}
int guard_func_idx = -1;
uint8_t* next_section_start = 0;
while (w < wend)
{
if (next_section_start && w != next_section_start)
{
return fprintf(stderr, "Internal sanity check failed. w = %ld, next_section_start = %ld\n",
w - wstart, next_section_start - wstart);
}
REQUIRE(1);
uint8_t section_type = w[0];
ADVANCE(1);
uint64_t section_len = LEB();
if (DEBUG)
printf("Section type: %d, Section len: %ld, Section offset: 0x%lX\n",
section_type, section_len, w - wstart);
REQUIRE(section_len);
next_section_start = w + section_len;
switch (section_type)
{
case 0x01U: // types
{
int type_count = LEB();
for (int i = 0; i < type_count; ++i)
{
REQUIRE(1);
if (w[0] != 0x60U)
return fprintf(stderr, "Illegal func type didn't start with 0x60U at %lX\n",
(w - wstart));
ADVANCE(1);
int param_count = LEB();
types[i].pc = param_count;
int is_P32 = 0;
for (int j = 0; j < param_count; ++j)
{
int param_type = LEB();
types[i].p[j] = param_type;
if (param_type == 0x7FU && param_count == 1)
is_P32 = 1;
}
int result_count = LEB();
types[i].set = 1;
types[i].rc = result_count;
if (result_count == 1)
for (int j = 0; j < result_count; ++j)
{
int result_type = LEB();
types[i].r[j] = result_type;
if (result_type = 0x7EU && result_count == 1 && is_P32)
{
if (DEBUG)
printf("Hook/Cbak type: %d\n", i);
if (hook_cbak_type != -1)
return fprintf(stderr, "int64_t func(int32_t) appears in type section twice!\n");
hook_cbak_type = i;
}
}
}
continue;
}
case 0x02U: // imports
{
// just get an import count
int count = LEB();
if (DEBUG)
printf("Import count: %d\n", count);
int func_upto = 0;
for (int i = 0; i < count; ++i)
{
uint8_t* import_start = w;
// module name
int mod_length = LEB();
REQUIRE(mod_length);
if (mod_length != 3 || w[0] != 'e' || w[1] != 'n' || w[2] != 'v')
return fprintf(stderr, "Did not import only from module 'env'\n");
ADVANCE(mod_length);
// import name
int name_length = LEB();
REQUIRE(name_length);
if (name_length == 2 && w[0] == '_' && w[1] == 'g')
{
guard_func_idx = func_upto;
printf("Guard function found at index: %d\n", guard_func_idx);
}
ADVANCE(name_length);
REQUIRE(1);
uint8_t import_type = w[0];
ADVANCE(1);
// only function imports
if (import_type != 0x00U)
{
if (guard_func_idx == i)
return fprintf(stderr, "Guard import _g was not imported as a function!\n");
if (import_type == 0x01U)
{
// table type
REQUIRE(1);
ADVANCE(1);
int dualLimit = (*w == 0x00U);
ADVANCE(1);
LEB();
if (dualLimit)
LEB();
}
else if (import_type == 0x02U)
{
// mem type
int dualLimit = (*w == 0x00U);
LEB();
if (dualLimit)
LEB();
}
else if (import_type == 0x03U)
{
REQUIRE(2);
ADVANCE(2);
}
}
else
{
uint64_t import_idx = LEB();
func_type[func_upto++] = import_idx;
out_import_size += (w - import_start);
if (DEBUG)
printf("Import %d type %ld out_import_size = %ld\n",
func_upto, import_idx, out_import_size);
}
}
out_import_count = func_upto;
if (out_import_count > 127*127)
return fprintf(stderr, "Unsupported number of imports: %d\n", out_import_count);
out_import_size += (out_import_count <= 127 ? 1U : 2U);
continue;
}
case 0x03U: // funcs
{
func_count = LEB();
if (DEBUG)
printf("Function count: %d\n", func_count);
for (int i = 0; i < func_count; ++i)
{
func_type[out_import_count + i] = LEB();
if (DEBUG)
printf("Func %d is type %d\n",
out_import_count + i, func_type[out_import_count + i]);
}
continue;
}
case 0x07U: // exports
{
uint8_t* export_end = w + section_len;
uint64_t export_count = LEB();
for (uint64_t i = 0; i < export_count; ++i)
{
// we only care about two exports: hook and cbak
// since we have to parse name first we'll read it in passing
// and store info about it here
int status = 0; // 1 = hook() 2 = cbak(), 0 = irrelevant
// read export name
uint64_t export_name_len = LEB();
REQUIRE(export_name_len);
if (export_name_len == 4)
{
if (w[0] == 'h' && w[1] == 'o' && w[2] == 'o' && w[3] == 'k')
status = 1;
else
if (w[0] == 'c' && w[1] == 'b' && w[2] == 'a' && w[3] == 'k')
status = 2;
}
ADVANCE(export_name_len);
// export type
REQUIRE(1);
uint8_t export_type = w[0];
ADVANCE(1);
// export idx
uint64_t export_idx = LEB();
if (status == 1)
func_hook = export_idx;
else if (status == 2)
func_cbak = export_idx;
if (func_hook > -1 && func_cbak > -1)
break;
}
// hook() is required at minimum
if (func_hook < 0)
return fprintf(stderr, "Could not find hook() export in wasm input\n");
w = export_end;
continue;
}
case 0x0AU:
{
uint64_t code_count = LEB();
for (uint64_t i = 0; i < code_count; ++i)
{
uint8_t* code_start = w;
uint64_t code_size = LEB();
ADVANCE(code_size);
if (i == (func_hook - out_import_count) || i == (func_cbak - out_import_count))
out_code_size += (w - code_start);
}
continue;
}
default:
{
ADVANCE(section_len);
continue;
}
}
}
if (hook_cbak_type == -1)
return fprintf(stderr, "Hook/cbak has the wrong function signature. Must be int64_t (*) (uint32_t).\n");
printf("hook idx: %d, cbak idx: %d\n", func_hook, func_cbak);
if (guard_func_idx == -1)
return fprintf(stderr, "Guard function _g was not imported / missing.\n");
// reset to top
w = wstart;
// pass two: write out
if (DEBUG)
printf("Second pass start\n");
uint8_t* ostart = o;
// magic number and version: 8 bytes
for (int i = 0; i < 8; ++i)
*o++ = *w++;
int type_new[MAX_TYPES];
memset(type_new, 0, sizeof(type_new));
next_section_start = 0;
while (w < wend)
{
if (next_section_start && w != next_section_start)
{
return fprintf(stderr, "Internal sanity check failed. w = %ld, next_section_start = %ld\n",
w - wstart, next_section_start - wstart);
}
REQUIRE(1);
uint8_t section_type = w[0];
ADVANCE(1);
uint64_t section_len = LEB();
if (DEBUG)
printf("Source section type: %d, Section len: %ld, Section offset: 0x%lX\n",
section_type, section_len, w - wstart);
REQUIRE(section_len);
next_section_start = w + section_len;
switch (section_type)
{
case 0x04U: // tables
case 0x08U: // start
case 0x09U: // elements
case 0x00U: // custom section
{
// these sections are dropped
ADVANCE(section_len);
continue;
}
case 0x05U: // memory
case 0x0BU: // data section
case 0x0CU: // data count section
{
// copied as is
*o++ = section_type;
leb_out(section_len, &o);
memcpy(o, w, section_len);
o += section_len;
ADVANCE(section_len);
continue;
}
case 0x01U: // type section
{
ADVANCE(section_len);
*o++ = 0x01U; // write section type
uint8_t used[MAX_TYPES];
memset(used, 0, MAX_TYPES);
// count types
int type_count = 0;
int imports_use_hook_cbak_type = 0;
int section_size = 0;
for (int i = 0; i < out_import_count; ++i)
{
if (!used[func_type[i]])
{
type_count++;
used[func_type[i]] = 1;
section_size += 3U + types[func_type[i]].rc + types[func_type[i]].pc;
if (func_type[i] == hook_cbak_type && !imports_use_hook_cbak_type)
{
imports_use_hook_cbak_type = 1;
hook_cbak_type = type_count-1;
if (DEBUG)
printf("Imports DO use hook_cbak_type = %d\n", hook_cbak_type);
}
}
}
if (!imports_use_hook_cbak_type)
{
hook_cbak_type = type_count++;
section_size += 5U;
if (DEBUG)
printf("Imports do not use hook_cbak_type = %d\n", hook_cbak_type);
}
if (type_count > 127*127)
return fprintf(stderr, "Too many types in wasm!\n");
// account for the type vector size bytes
section_size += (type_count > 127 ? 2U : 1U);
if (DEBUG)
printf("Writing type section, proposed size: %d\n", section_size);
// write out section size
leb_out(section_size, &o);
uint8_t* out_start = o;
// write type vector len
leb_out(type_count, &o);
// write out types
memset(used, 0, MAX_TYPES);
int upto = 0;
for (int i = 0; i < out_import_count; ++i)
{
int t = func_type[i];
if (!types[t].set)
return fprintf(stderr, "Tried to write unset type %d from func %d\n", func_type[i], i);
if (used[t])
continue;
used[t] = 1;
*o++ = 0x60U; // functype lead in byte
// write parameter count
leb_out(types[t].pc, &o);
// write each parameter type
for (int j = 0; j < types[t].pc; ++j)
leb_out(types[t].p[j], &o);
leb_out(types[t].rc, &o);
for (int j = 0; j < types[t].rc; ++j)
leb_out(types[t].r[j], &o);
type_new[t] = upto++;
// done for this record
}
// write out cbak/hook type if needed
if (!imports_use_hook_cbak_type)
{
*o++ = 0x60U;
*o++ = 0x01U;
*o++ = 0x7FU;
*o++ = 0x01U;
*o++ = 0x7EU;
}
if (DEBUG)
printf("Actually written type section size: %ld\n", o - out_start);
continue;
}
case 0x02U: // imports
{
*o++ = 0x02U;
if (DEBUG)
{
printf("Writing import section, proposed size, count: %ld, %d\n",
out_import_size, out_import_count);
}
leb_out(out_import_size, &o);
uint8_t* import_start = o;
leb_out(out_import_count, &o);
int type_count = 0;
int count = LEB();
for (int i = 0; i < count; ++i)
{
// module name
int mod_length = LEB();
REQUIRE(mod_length);
uint8_t* mod = w;
ADVANCE(mod_length);
// import name
int name_length = LEB();
REQUIRE(name_length);
uint8_t* name = w;
ADVANCE(name_length);
// only function imports
if (*w != 0x00U)
{
int it = *w;
ADVANCE(1);
if (it == 0x01U)
{
// table type
REQUIRE(1);
ADVANCE(1);
int dualLimit = (*w == 0x00U);
ADVANCE(1);
LEB();
if (dualLimit)
LEB();
}
else if (it == 0x02U)
{
// mem type
int dualLimit = (*w == 0x00U);
LEB();
if (dualLimit)
LEB();
}
else if (it == 0x03U)
{
REQUIRE(2);
ADVANCE(2);
}
continue;
}
ADVANCE(1);
// write mod
leb_out(mod_length, &o);
memcpy(o, mod, mod_length);
o += mod_length;
// write name
leb_out(name_length, &o);
memcpy(o, name, name_length);
o += name_length;
// write import type (always 0)
*o++ = 0x00U;
if (DEBUG)
printf("New import: %d old type: %d new type: %d\n", i, func_type[i], type_new[func_type[i]]);
// write new type idx
leb_out(type_new[func_type[i]], &o);
LEB(); // discard old type
// advance to next entry
}
if (DEBUG)
printf("Actually written import size: %ld\n", o - import_start);
continue;
}
case 0x03U: // functions
{
*o++ = 0x03U;
ssize_t s = (func_cbak == -1 ? 0x01U : 0x02U);
if (hook_cbak_type > 127U*127U)
return fprintf(stderr, "Illegally large hook_cbak type index\n");
if (hook_cbak_type > 127U)
s <<= 1U; // double size if > 127
s++; // one byte for the vector size
if (DEBUG)
printf("Writing function section, proposed size: %ld\n", s);
leb_out(s, &o); // sections size
uint8_t* function_start = o;
*o++ = (func_cbak == -1 ? 0x01U : 0x02U); // vector size
leb_out(hook_cbak_type, &o); // vector entries
if (func_cbak != -1)
{
leb_out(hook_cbak_type, &o);
if (DEBUG)
printf("Writing cbak [idx=%d, type=%d]\n", func_cbak, hook_cbak_type);
}
ADVANCE(section_len);
if (DEBUG)
printf("Actually written function size: %ld\n", o - function_start);
continue;
}
case 0x06U: // globals
{
// globals are copied byte for byte
*o++ = 0x06U;
leb_out(section_len, &o);
memcpy(o, w, section_len);
o += section_len;
ADVANCE(section_len);
continue;
}
case 0x07U: // exports
{
*o++ = 0x07U;
// size
// V M NNNN 0 1 [ M NNNN 0 2 ]
*o++ = (func_cbak == -1 ? 0x08U : 0x0FU);
// vec len
*o++ = (func_cbak == -1 ? 0x01U : 0x02U);
int cbak_first = (func_cbak < func_hook);
if (cbak_first && func_cbak != -1)
{
*o++ = 0x04U;
*o++ = 'c'; *o++ = 'b'; *o++ = 'a'; *o++ = 'k';
*o++ = 0x00U;
leb_out(out_import_count + 0, &o);
*o++ = 0x04U;
*o++ = 'h'; *o++ = 'o'; *o++ = 'o'; *o++ = 'k';
*o++ = 0x00U;
leb_out(out_import_count + 1, &o);
}
else
{
*o++ = 0x04U;
*o++ = 'h'; *o++ = 'o'; *o++ = 'o'; *o++ = 'k';
*o++ = 0x00U;
leb_out(out_import_count + 0, &o);
if (func_cbak != -1)
{
*o++ = 0x04U;
*o++ = 'c'; *o++ = 'b'; *o++ = 'a'; *o++ = 'k';
*o++ = 0x00U;
leb_out(out_import_count + 1, &o);
}
}
ADVANCE(section_len);
continue;
}
case 0x0AU: // code section (aka function body)
{
*o++ = 0x0AU;
if (DEBUG)
printf("Output code size: %ld\n", out_code_size + 1);
// RH NOTE:
// In addition to moving a properly formed clean guard of the form { i32.const, i32.const, _g, drop }
// we can also reinterpret a badly formed guard like { i32.con, i32.store, ..., i32.con, _g, drop }.
// This becomes what's known as a guard rewrite. In this case additional instructions beyond the
// original size of the hook will be inserted at the start of the relevant loop. This counter tracks
// these, and the reserved space allows us to output the right LEB128 at the end.
int total_guard_rewrite_bytes = 0;
uint8_t* codesec_out_size_ptr = o;
// we need to correct this at the end
leb_out_pad(out_code_size + 1 /* allow for vec len */, &o, 3);
*o++ = (func_cbak == -1 ? 0x01U : 0x02U); // vec len
uint64_t count = LEB();
for (uint64_t i = 0; i < count; ++i)
{
uint8_t* code_start = w;
uint64_t code_size = LEB();
if (i == (func_hook - out_import_count) || i == (func_cbak - out_import_count))
{
//leb_out(code_size, &o);
int guard_rewrite_bytes = 0;
uint8_t* code_size_ptr = o;
// we need to correct this at the end
leb_out_pad(code_size, &o, 3);
int pad_len = 3 - (w-code_start);
if (pad_len < 0)
return fprintf(stderr,
"Codesec %ld was too large! Size must fit in 3 leb128 bytes!\n", i);
total_guard_rewrite_bytes += pad_len;
//memcpy(o, code_start, w-code_start);
// parse locals
uint8_t* locals_start = w;
uint64_t locals_count = LEB();
printf("Locals count: %ld\n", locals_count);
for (int i = 0; i < locals_count; ++i)
{
LEB(); // inner len
REQUIRE(1); // local type
ADVANCE(1);
}
memcpy(o, locals_start, w-locals_start);
o += (w-locals_start);
uint8_t* expr_start = w;
uint64_t expr_size = code_size - (w-locals_start);
printf("Expr start: %ld [0x%lx]\n", expr_size, expr_size);
// parse code
uint8_t* last_loop = 0; // where the start of the last loop instruction is in the input
uint8_t* last_loop_out = 0; // where the start of the last loop instruction is in the output
int i32_found = 0;
uint8_t* call_guard_found = 0;
uint8_t* last_i32 = 0;
uint64_t last_i32_actual = 0; // the actual leb value
uint8_t* second_last_i32 = 0;
uint64_t second_last_i32_actual = 0; // the actual leb value
int between_const_and_guard = 0;
#define RESET_GUARD_FINDER()\
{\
i32_found = 0;\
call_guard_found = 0;\
last_i32 = 0;\
last_i32_actual = 0;\
second_last_i32 = 0;\
second_last_i32_actual = 0;\
between_const_and_guard = 0;\
}
while (w - expr_start < expr_size)
{
uint8_t* instr_start = w;
REQUIRE(1);
uint8_t ins = *w;
ADVANCE(1);
REQUIRE(1);
if (ins == 0x02U || ins == 0x03U || ins == 0x04U) // block, loop, if
{
REQUIRE(1);
uint8_t block_type = *w;
if ((block_type >= 0x7CU && block_type <= 0x7FU) ||
block_type == 0x7BU || block_type == 0x70U ||
block_type == 0x7BU || block_type == 0x40U)
{
ADVANCE(1);
}
else
SIGNED_LEB();
memcpy(o, instr_start, w-instr_start);
o += (w - instr_start);
if (ins == 0x03U) // loop
{
last_loop = w;
last_loop_out = o;
}
RESET_GUARD_FINDER();
continue;
}
if (ins == 0x1AU) // drop
{
*o++ = ins;
if (i32_found >= 2 && call_guard_found && last_loop)
{
if (between_const_and_guard > 0)
{
if (second_last_i32_actual < last_i32_actual)
{
uint64_t swap = last_i32_actual;
last_i32_actual = second_last_i32_actual;
second_last_i32_actual = swap;
}
uint8_t guard_code[128];
uint8_t* g = guard_code;
*g++ = 0x41U;
leb_out(second_last_i32_actual, &g);
*g++ = 0x41U;
leb_out(last_i32_actual, &g);
*g++ = 0x10U;
leb_out(guard_func_idx, &g);
*g++ = 0x1AU;
ssize_t guard_len = g - guard_code;
ssize_t rest_len = w - last_loop;
char guard_print[128]; guard_print[0] = '\0';
snprintf(guard_print, 128, "_g(0x%08lx,%ld)", second_last_i32_actual,
last_i32_actual);
int guard_pad_len = 20 - strlen(guard_print);
if (guard_pad_len < 0) guard_pad_len = 0;
snprintf(guard_print, 128, "_g(0x%08lx,%.*s%ld)",
second_last_i32_actual,
guard_pad_len,
" ",
last_i32_actual);
printf("Found dirty guard %s\tat: %ld [0x%lx] - %ld [0x%lx],\t"
"rewriting to %ld [0x%lx] - %ld [0x%lx]\n",
guard_print,
second_last_i32 - wstart,
second_last_i32 - wstart,
w - wstart,
w - wstart,
last_loop - wstart,
last_loop - wstart,
last_loop - wstart + guard_len,
last_loop - wstart + guard_len
);
// erase guard call with nops and an additional drop
// to preserve the stack at this location during runtime
int bytes_to_fill = w - call_guard_found - 2;
*call_guard_found = 0x1AU; // drop
while (bytes_to_fill-- > 0)
*(++call_guard_found) = 0x01U; // nop
// first move the instructions down
memcpy(last_loop_out + guard_len, last_loop, rest_len);
// then copy the guard into position