linux-user: calling SYS_get_thread_area and SYS_get_thread_area has incorrent result on multithread environment

Host environment

• Operating system: Ubuntu 22.10
• OS/kernel version: Linux DESKTOP-388IRR7 5.15.79.1-microsoft-standard-WSL2
• Architecture: x86
• QEMU flavor: qemu-i386 (user-mode)
• QEMU version: qemu-i386 version 7.2.50 (v7.2.0-333-g222059a0)
• QEMU command line:

  ./qemu-i386 test.out

Description of problem

Steps to reproduce

1. Compile test.out by Command and source code:

gcc -m32 -g test.c -lpthread -o test.out

#include <sys/syscall.h>
#include <unistd.h>
#include <stdio.h>
#include <pthread.h>
#include <asm/ldt.h>

static inline int set_thread_area( struct user_desc *ptr )
{
    return syscall( SYS_set_thread_area, ptr );
}

static inline int get_thread_area( struct user_desc *ptr )
{
    return syscall( SYS_get_thread_area, ptr );
}

static unsigned int entry_number;

static void* start_routine(void* ptr) 
{
    struct user_desc user_desc0 = { entry_number };
    struct user_desc user_desc1 = { entry_number };
    struct user_desc user_desc2 = { entry_number };
    get_thread_area(&user_desc0);
    printf("child thread: %u\n", user_desc0.base_addr);

    user_desc1.base_addr = 2;
    user_desc1.limit     = 0xFFF;
    user_desc1.seg_32bit = 1;
    set_thread_area( &user_desc1 );

    get_thread_area(&user_desc2);
    printf("child thread: %u\n", user_desc2.base_addr);
    return NULL;
}

int main(void) {
    struct user_desc user_desc0 = { -1 }, user_desc1 = { 0 }, user_desc2 = { 0 };
    user_desc0.seg_32bit = 1;
    user_desc0.useable = 1;
    set_thread_area( &user_desc0 );

    entry_number = user_desc0.entry_number;

    user_desc1.entry_number = entry_number;
    user_desc1.base_addr = 1;
    user_desc1.limit     = 0xFFF;
    user_desc1.seg_32bit = 1;
    set_thread_area( &user_desc1 );

    pthread_t thread_id;
    pthread_create(&thread_id, NULL, &start_routine, NULL);
    pthread_join(thread_id, NULL);

    user_desc2.entry_number = entry_number;
    get_thread_area(&user_desc2);
    printf("main  thread: %u\n", user_desc2.base_addr); // main  thread: 1
    return 0;
}

2. Correct Result:

child thread: 1
child thread: 2
main  thread: 1

qemu-i386 Print Result:

child thread: 1
child thread: 2
main  thread: 2

Additional information

patch for fix the bug:

https://lists.nongnu.org/archive/html/qemu-devel/2023-02/msg02203.html

CPUX86State::gdt::base on differect threads must have different vaules, but it points to same memory. value of CPUX86State::gdt::base must be copied when clone thread.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kernel/tls.c

SYS_set_thread_area call do_set_thread_area in kernel, it set user_desc to different memroy area on differernt threads. tls_array is in thread local memory.

static void set_tls_desc(struct task_struct *p, int idx,
			 const struct user_desc *info, int n)
{
	struct thread_struct *t = &p->thread;
	struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];
	int cpu;

	/*
	 * We must not get preempted while modifying the TLS.
	 */
	cpu = get_cpu();

	while (n-- > 0) {
		if (LDT_empty(info) || LDT_zero(info))
			memset(desc, 0, sizeof(*desc));
		else
			fill_ldt(desc, info);
		++info;
		++desc;
	}

	if (t == &current->thread)
		load_TLS(t, cpu);

	put_cpu();
}