linux-kernel - Re: [RFC PATCH kernel] KVM: Stop leaking memory in debugfs

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Message-ID: <CABgObfb+M9Qeow1EZy+eQwM1jwoZY3zdPJfZW+Q+MoWmkaqcFw@mail.gmail.com>
Date:   Tue, 3 Aug 2021 14:52:17 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Alexey Kardashevskiy <aik@...abs.ru>,
        "Kernel Mailing List, Linux" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH kernel] KVM: Stop leaking memory in debugfs

On Tue, Aug 3, 2021 at 1:16 PM Greg KH <gregkh@...uxfoundation.org> wrote:
> On Fri, Jul 30, 2021 at 02:32:17PM +1000, Alexey Kardashevskiy wrote:
> >       snprintf(dir_name, sizeof(dir_name), "%d-%d", task_pid_nr(current), fd);
> >       kvm->debugfs_dentry = debugfs_create_dir(dir_name, kvm_debugfs_dir);
> > +     if (IS_ERR_OR_NULL(kvm->debugfs_dentry)) {
> > +             pr_err("Failed to create %s\n", dir_name);
> > +             return 0;
> > +     }
>
> It should not matter if you fail a debugfs call at all.
>
> If there is a larger race at work here, please fix that root cause, do
> not paper over it by attempting to have debugfs catch the issue for you.

I don't think it's a race, it's really just a bug that is intrinsic in how
the debugfs files are named.  You can just do something like this:

#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <sys/ioctl.h>
#include <linux/kvm.h>
#include <stdlib.h>
int main() {
        int kvmfd = open("/dev/kvm", O_RDONLY);
        int fd = ioctl(kvmfd, KVM_CREATE_VM, 0);
        if (fork() == 0) {
                printf("before: %d\n", fd);
                sleep(2);
        } else {
                close(fd);
                sleep(1);
                int fd = ioctl(kvmfd, KVM_CREATE_VM, 0);
                printf("after: %d\n", fd);
                wait(NULL);
        }
}

So Alexey's patch is okay and I've queued it, though with pr_warn_ratelimited
instead of pr_err.

Paolo