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/*
* Test patching code, running in one thread, from another thread.
*
* Intel SDM calls this "cross-modifying code" and recommends a special
* sequence, which requires both threads to cooperate.
*
* Linux kernel uses a different sequence that does not require cooperation and
* involves patching the first byte with int3.
*
* Finally, there is user-mode software out there that simply uses atomics, and
* that seems to be good enough in practice. Test that QEMU has no problems
* with this as well.
*/
#include <assert.h>
#include <pthread.h>
#include <stdbool.h>
#include <stdlib.h>
void add1_or_nop(long *x);
asm(".pushsection .rwx,\"awx\",@progbits\n"
".globl add1_or_nop\n"
/* addq $0x1,(%rdi) */
"add1_or_nop: .byte 0x48, 0x83, 0x07, 0x01\n"
"ret\n"
".popsection\n");
#define THREAD_WAIT 0
#define THREAD_PATCH 1
#define THREAD_STOP 2
static void *thread_func(void *arg)
{
int val = 0x0026748d; /* nop */
while (true) {
switch (__atomic_load_n((int *)arg, __ATOMIC_SEQ_CST)) {
case THREAD_WAIT:
break;
case THREAD_PATCH:
val = __atomic_exchange_n((int *)&add1_or_nop, val,
__ATOMIC_SEQ_CST);
break;
case THREAD_STOP:
return NULL;
default:
assert(false);
__builtin_unreachable();
}
}
}
#define INITIAL 42
#define COUNT 1000000
int main(void)
{
int command = THREAD_WAIT;
pthread_t thread;
long x = 0;
int err;
int i;
err = pthread_create(&thread, NULL, &thread_func, &command);
assert(err == 0);
__atomic_store_n(&command, THREAD_PATCH, __ATOMIC_SEQ_CST);
for (i = 0; i < COUNT; i++) {
add1_or_nop(&x);
}
__atomic_store_n(&command, THREAD_STOP, __ATOMIC_SEQ_CST);
err = pthread_join(thread, NULL);
assert(err == 0);
assert(x >= INITIAL);
assert(x <= INITIAL + COUNT);
return EXIT_SUCCESS;
}
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