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userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c

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Ciro Santilli 六四事件 法轮功
2020-06-25 04:00:03 +00:00
parent 09cbc26819
commit c9d15228ab
3 changed files with 57 additions and 0 deletions
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1
README.adoc
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@@ -22968,6 +22968,7 @@ Examples:
* link:userland/arch/aarch64/inline_asm/wfe_ldxr_stxr.cpp[]
* link:userland/arch/aarch64/inline_asm/wfe_ldxr_str.cpp[]
* link:userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c[]: tests that ldxr and stxr do not interact with futexes. This was leading to problems in <<gem5-syscall-emulation-mode>> at one point: https://gem5.atlassian.net/browse/GEM5-537
SEV is not the only thing that can wake up a WFE, it is only an explicit software way to do it.

55
userland/arch/aarch64/inline_asm/futex_ldxr_stxr.c Normal file
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@@ -0,0 +1,55 @@
/* https://cirosantilli.com/linux-kernel-module-cheat#futex-system-call
* https://cirosantilli.com/linux-kernel-module-cheat#arm-wfe-global-monitor-events */
#define _GNU_SOURCE
#include <assert.h>
#include <pthread.h>
#include <stdatomic.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <inttypes.h>
#include <lkmc/futex.h>
static int futex1 = 1;
static int futex2 = 1;
atomic_int ldxr_done = 0;
atomic_int stdr_wake_done = 0;
static uint64_t ldxr_var = 0;
void __attribute__ ((noinline)) busy_loop(
unsigned long long max,
unsigned long long max2
) {
for (unsigned long long i = 0; i < max2; i++) {
for (unsigned long long j = 0; j < max; j++) {
__asm__ __volatile__ ("" : "+g" (i), "+g" (j) : :);
}
}
}
void* thread_main(void *arg) {
(void)arg;
__asm__ __volatile__ ("ldxr x0, [%0]" : : "r" (&ldxr_var) : "x0");
ldxr_done = 1;
lkmc_futex(&futex1, FUTEX_WAIT, futex1, NULL, NULL, 0);
lkmc_futex(&futex2, FUTEX_WAIT, futex2, NULL, NULL, 0);
return NULL;
}
int main(void) {
pthread_t thread;
pthread_create(&thread, NULL, thread_main, NULL);
while (!ldxr_done) {}
/* Wait for thread1 to sleep on futex1. */
busy_loop(1000, 1);
/* Wrongly wake up the thread with a SEV. */
__asm__ __volatile__ ("mov x0, 1;ldxr x0, [%0]; stxr w1, x0, [%0]" : : "r" (&ldxr_var) : "x0", "x1");
/* Wait for thread1 to sleep on futex2. */
busy_loop(1000, 1);
/* Wrongly wake thread from futex1 again. */
/* But it is now sleeping on futex2, so this is wrong. */
lkmc_futex(&futex1, FUTEX_WAKE, 1, NULL, NULL, 0);
assert(!pthread_join(thread, NULL));
}

1
userland/linux/futex.c
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@@ -1,5 +1,6 @@
/* https://cirosantilli.com/linux-kernel-module-cheat#futex-system-call */
#define _GNU_SOURCE
#include <assert.h>
#include <pthread.h>
#include <stdio.h>