[PATCH] KVM: Avoid debugfs warning caused by repeated vm fd number

[Ted Chen <znscnchen@xxxxxxxxx>]

Avoid debugfs warning like "KVM: debugfs: duplicate directory 59904-4"
caused by creating VMs with the same vm fd number in a single process.

As shown in the below test case, two test() are executed sequentially in a
single process, each creating a new VM.

Though the 2nd test() creates a new VM after the 1st test() closes the
vm_fd, KVM prints warnings like "KVM: debugfs: duplicate directory 59904-4"
on creating the 2nd VM.

This is due to the dup() of the vcpu_fd in test(). So, after closing the
1st vm_fd, kvm->users_count of the 1st VM is still > 0 when creating the
2nd VM. So, KVM has not yet invoked kvm_destroy_vm() and
kvm_destroy_vm_debugfs() for the 1st VM after closing the 1st vm_fd. The
2nd test() thus will be able to create a different VM with the same vm fd
number as the 1st VM.

Therefore, besides having "pid" and "fdname" in the dir_name of the
debugfs, add a random number to differentiate different VMs to avoid
printing warning, also allowing the 2nd VM to have a functional debugfs.

Use get_random_u32() to avoid dir_name() taking up too much memory while
greatly reducing the chance of printing warning.

void test(void)
{
        int kvm_fd, vm_fd, vcpu_fd;

        kvm_fd = open("/dev/kvm", O_RDWR);
        if (kvm_fd == -1)
                return;

        vm_fd = ioctl(kvm_fd, KVM_CREATE_VM, 0);
        if (vm_fd == -1)
                return;
        vcpu_fd = ioctl(vm_fd, KVM_CREATE_VCPU, 0);
        if (vcpu_fd == -1)
                return;

        dup(vcpu_fd);
        close(vcpu_fd);
        close(vm_fd);
        close(kvm_fd);
}

int main()
{
        test();
        test();

        return 0;
}

Signed-off-by: Ted Chen <znscnchen@xxxxxxxxx>
---
 virt/kvm/kvm_main.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6c07dd423458..f92a60ed5de8 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1017,7 +1017,7 @@ static int kvm_create_vm_debugfs(struct kvm *kvm, const char *fdname)
 {
 	static DEFINE_MUTEX(kvm_debugfs_lock);
 	struct dentry *dent;
-	char dir_name[ITOA_MAX_LEN * 2];
+	char dir_name[ITOA_MAX_LEN * 3];
 	struct kvm_stat_data *stat_data;
 	const struct _kvm_stats_desc *pdesc;
 	int i, ret = -ENOMEM;
@@ -1027,7 +1027,8 @@ static int kvm_create_vm_debugfs(struct kvm *kvm, const char *fdname)
 	if (!debugfs_initialized())
 		return 0;
 
-	snprintf(dir_name, sizeof(dir_name), "%d-%s", task_pid_nr(current), fdname);
+	snprintf(dir_name, sizeof(dir_name), "%d-%s-%u", task_pid_nr(current),
+		 fdname, get_random_u32());
 	mutex_lock(&kvm_debugfs_lock);
 	dent = debugfs_lookup(dir_name, kvm_debugfs_dir);
 	if (dent) {
-- 
2.39.2