Messages in this thread |  | | Date | Fri, 17 Jan 2020 15:06:02 -0800 | From | Matthew Wilcox <> | Subject | Re: [PATCH v8 2/3] pid: Introduce pidfd_getfd syscall |
| |
On Fri, Jan 03, 2020 at 08:29:27AM -0800, Sargun Dhillon wrote: > +++ b/kernel/pid.c > @@ -578,3 +578,93 @@ void __init pid_idr_init(void) > init_pid_ns.pid_cachep = KMEM_CACHE(pid, > SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT); > } > + > +static struct file *__pidfd_fget(struct task_struct *task, int fd) > +{ > + struct file *file; > + int ret; > + > + ret = mutex_lock_killable(&task->signal->cred_guard_mutex); > + if (ret) > + return ERR_PTR(ret); > + > + if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS)) > + file = fget_task(task, fd); > + else > + file = ERR_PTR(-EPERM); > + > + mutex_unlock(&task->signal->cred_guard_mutex); > + > + return file ?: ERR_PTR(-EBADF); > +} > + > +static int pidfd_getfd(struct pid *pid, int fd) > +{ > + struct task_struct *task; > + struct file *file; > + int ret; > + > + task = get_pid_task(pid, PIDTYPE_PID); > + if (!task) > + return -ESRCH; > + > + file = __pidfd_fget(task, fd); > + put_task_struct(task); > + if (IS_ERR(file)) > + return PTR_ERR(file); > + > + ret = security_file_receive(file); > + if (ret) { > + fput(file); > + return ret; > + } > + > + ret = get_unused_fd_flags(O_CLOEXEC); > + if (ret < 0) > + fput(file); > + else > + fd_install(ret, file); > + > + return ret; > +} > + > +/** > + * sys_pidfd_getfd() - Get a file descriptor from another process > + * > + * @pidfd: the pidfd file descriptor of the process > + * @fd: the file descriptor number to get > + * @flags: flags on how to get the fd (reserved) > + * > + * This syscall gets a copy of a file descriptor from another process > + * based on the pidfd, and file descriptor number. It requires that > + * the calling process has the ability to ptrace the process represented > + * by the pidfd. The process which is having its file descriptor copied > + * is otherwise unaffected. > + * > + * Return: On success, a cloexec file descriptor is returned. > + * On error, a negative errno number will be returned. > + */
We don't usually kernel-doc syscalls. They should have manpages instead.
> +SYSCALL_DEFINE3(pidfd_getfd, int, pidfd, int, fd, > + unsigned int, flags) > +{ > + struct pid *pid; > + struct fd f; > + int ret; > + > + /* flags is currently unused - make sure it's unset */ > + if (flags) > + return -EINVAL;
Is EINVAL the right errno here? Often we use ENOSYS for bad flags to syscalls.
> + f = fdget(pidfd); > + if (!f.file) > + return -EBADF; > + > + pid = pidfd_pid(f.file); > + if (IS_ERR(pid)) > + ret = PTR_ERR(pid); > + else > + ret = pidfd_getfd(pid, fd);
You can simplify this by having pidfd_pid() return ERR_PTR(-EBADF) if !f.file, and having pidfd_getfd() return PTR_ERR() if IS_ERR(pid). Then this function looks like:
if (flags) return -EINVAL;
f = fdget(pidfd); pid = pidfd_pid(f.file); ret = pidfd_getfd(pid, fd); fdput(f); return ret;
You could even eliminate the 'pid' variable and just do:
ret = pidfd_getfd(pidfd_pid(f.file), fd);
but that's a step too far for me.
It's unfortunate that -EBADF might mean that either the first or second argument is a bad fd number. I'm not sure I have a good alternative though.
|  |