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Reimplement memcpy, memmove, memcmp, and memset in assembly to work around potentially buggy compilers generating references to those function recursively within the function bodies

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mintsuki 2020-08-28 01:25:17 +02:00
parent e2a87ca33e
commit 155362570e
2 changed files with 71 additions and 36 deletions
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qloader2.bin
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src/lib/libc.c
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@ -1,6 +1,7 @@
#include <stddef.h>
#include <stdint.h>
#include <lib/libc.h>
#include <lib/asm.h>
int toupper(int c) {
if (c >= 'a' && c <= 'z') {
@ -16,54 +17,88 @@ int tolower(int c) {
return c;
}
__attribute__((naked))
void *memcpy(void *dest, const void *src, size_t n) {
uint8_t *pdest = dest;
const uint8_t *psrc = src;
for (size_t i = 0; i < n; i++) {
pdest[i] = psrc[i];
}
return dest;
ASM_BASIC(
"push esi\n\t"
"push edi\n\t"
"mov eax, dword ptr [esp+12]\n\t"
"mov edi, eax\n\t"
"mov esi, dword ptr [esp+16]\n\t"
"mov ecx, dword ptr [esp+20]\n\t"
"rep movsb\n\t"
"pop edi\n\t"
"pop esi\n\t"
"ret\n\t"
);
}
__attribute__((naked))
void *memset(void *s, int c, size_t n) {
uint8_t *p = s;
for (size_t i = 0; i < n; i++) {
p[i] = (uint8_t)c;
}
return s;
ASM_BASIC(
"push edi\n\t"
"mov edx, dword ptr [esp+8]\n\t"
"mov edi, edx\n\t"
"mov eax, dword ptr [esp+12]\n\t"
"mov ecx, dword ptr [esp+16]\n\t"
"rep stosb\n\t"
"mov eax, edx\n\t"
"pop edi\n\t"
"ret\n\t"
);
}
__attribute__((naked))
void *memmove(void *dest, const void *src, size_t n) {
uint8_t *pdest = dest;
const uint8_t *psrc = src;
ASM_BASIC(
"push esi\n\t"
"push edi\n\t"
"mov eax, dword ptr [esp+12]\n\t"
"mov edi, eax\n\t"
"mov esi, dword ptr [esp+16]\n\t"
"mov ecx, dword ptr [esp+20]\n\t"
if (src > dest) {
for (size_t i = 0; i < n; i++) {
pdest[i] = psrc[i];
}
} else if (src < dest) {
for (size_t i = n; i > 0; i--) {
pdest[i-1] = psrc[i-1];
}
}
return dest;
"cmp edi, esi\n\t"
"ja 1f\n\t"
"rep movsb\n\t"
"jmp 2f\n\t"
"1:\n\t"
"lea edi, [edi+ecx-1]\n\t"
"lea esi, [esi+ecx-1]\n\t"
"std\n\t"
"rep movsb\n\t"
"cld\n\t"
"2:\n\t"
"pop edi\n\t"
"pop esi\n\t"
"ret\n\t"
);
}
__attribute__((naked))
int memcmp(const void *s1, const void *s2, size_t n) {
const uint8_t *p1 = s1;
const uint8_t *p2 = s2;
for (size_t i = 0; i < n; i++) {
if (p1[i] != p2[i])
return p1[i] < p2[i] ? -1 : 1;
}
return 0;
ASM_BASIC(
"push esi\n\t"
"push edi\n\t"
"mov edi, dword ptr [esp+12]\n\t"
"mov esi, dword ptr [esp+16]\n\t"
"mov ecx, dword ptr [esp+20]\n\t"
"repe cmpsb\n\t"
"jecxz 1f\n\t"
"mov al, byte ptr [edi-1]\n\t"
"sub al, byte ptr [esi-1]\n\t"
"movsx eax, al\n\t"
"jmp 2f\n\t"
"1:\n\t"
"mov eax, ecx\n\t"
"2:\n\t"
"pop edi\n\t"
"pop esi\n\t"
"ret\n\t"
);
}
char *strcpy(char *dest, const char *src) {