sysret_ss_attrs: Commit 64-bit set_thread_area for posterity

Signed-off-by: Andy Lutomirski <luto@kernel.org>

1 files changed, 40 insertions, 1 deletions

diff --git a/sysret_ss_attrs.c b/sysret_ss_attrs.c
index fa81ce2..9d80edd 100644
--- a/sysret_ss_attrs.c
+++ b/sysret_ss_attrs.c
@@ -18,6 +18,7 @@
 #include <stdbool.h>
 #include <sys/ptrace.h>
 #include <sys/user.h>
+#include <errno.h>
 #include <pthread.h>
 
 static unsigned short GDT3(int idx)
@@ -25,6 +26,32 @@ static unsigned short GDT3(int idx)
 	return (idx << 3) | 3;
 }
 
+static int force_set_thread_area(struct user_desc *u_info)
+{
+#ifdef __x86_64__
+	int ret;
+	struct user_desc *low = mmap(NULL, sizeof(struct user_desc),
+				     PROT_READ | PROT_WRITE,
+				     MAP_ANONYMOUS | MAP_PRIVATE | MAP_32BIT,
+				     -1, 0);
+	memcpy(low, u_info, sizeof(struct user_desc));
+	/* Call the 32-bit set_thread_area (nr 243) using int80 */
+	asm volatile ("int $0x80"
+		      : "=a" (ret) : "a" (243), "b" (low)
+		      : "memory");
+	if (ret < 0) {
+		munmap(low, sizeof(struct user_desc));
+		errno = -ret;
+		return -1;
+	}
+	memcpy(u_info, low, sizeof(struct user_desc));
+	munmap(low, sizeof(struct user_desc));
+	return ret;
+#else
+	return syscall(SYS_set_thread_area, u_info);
+#endif
+}
+
 static void *threadproc(void *ctx)
 {
 	struct user_desc desc = {
@@ -39,26 +66,38 @@ static void *threadproc(void *ctx)
 		.useable         = 0,
 	};
 
-	if (syscall(SYS_set_thread_area, &desc) != 0)
+	if (force_set_thread_area(&desc) != 0)
 		err(1, "set_thread_area");
 
 	asm volatile (
 		"mov %%ax, %%ss\n\t"
 		"1:\n\t"
+		/*
 		"movl $0xbfffffff, %%eax\n\t"
 		"int $0x80\n\t"
+		*/
 		"jmp 1b"
 		: : "a" (GDT3(desc.entry_number))
 		);
+
 	return NULL;
 }
 
+
+
 int main()
 {
+	cpu_set_t cpuset;
+	CPU_ZERO(&cpuset);
+	CPU_SET(0, &cpuset);
+	sched_setaffinity(0, sizeof(cpuset), &cpuset);
+
 	pthread_t thread;
 	if (pthread_create(&thread, 0, threadproc, 0) != 0)
 		err(1, "pthread_create");
 
+	pause();
+
 	while (1) {
 		usleep(1);
 	}