@@ -77,18 +77,19 @@ struct file;
/**
* __pidfd_get_pid() - Retrieve a pid associated with the specified pidfd.
*
- * @pidfd: The pidfd whose pid we want, or the fd of a /proc/<pid> file if
- * @alloc_proc is also set.
+ * @pidfd: The pidfd whose pid we want, the fd of a /proc/<pid> file if
+ * @alloc_proc is also set, or PIDFD_SELF_* to refer to the current
+ * thread or thread group leader.
* @pin_pid: If set, then the reference counter of the returned pid is
* incremented. If not set, then @fd should be provided to pin the
* pidfd.
* @allow_proc: If set, then an fd of a /proc/<pid> file can be passed instead
* of a pidfd, and this will be used to determine the pid.
* @flags: Output variable, if non-NULL, then the file->f_flags of the
- * pidfd will be set here.
+ * pidfd will be set here. If PIDFD_SELF_* set, this is zero.
* @fd: Output variable, if non-NULL, then the pidfd reference will
* remain elevated and the caller will need to decrement it
- * themselves.
+ * themselves. If PIDFD_SELF_* set, this is empty.
*
* Returns: If successful, the pid associated with the pidfd, otherwise an
* error.
@@ -29,4 +29,19 @@
#define PIDFD_GET_USER_NAMESPACE _IO(PIDFS_IOCTL_MAGIC, 9)
#define PIDFD_GET_UTS_NAMESPACE _IO(PIDFS_IOCTL_MAGIC, 10)
+/*
+ * Special sentinel values which can be used to refer to the current thread or
+ * thread group leader (which from a userland perspective is the process).
+ */
+#define PIDFD_SELF PIDFD_SELF_THREAD
+#define PIDFD_SELF_PROCESS PIDFD_SELF_THREAD_GROUP
+
+#define PIDFD_SELF_THREAD -100 /* Current thread. */
+#define PIDFD_SELF_THREAD_GROUP -200 /* Current thread group leader. */
+
+static inline bool pidfd_is_self_sentinel(pid_t pid)
+{
+ return pid == PIDFD_SELF_THREAD || pid == PIDFD_SELF_THREAD_GROUP;
+}
+
#endif /* _UAPI_LINUX_PIDFD_H */
@@ -71,6 +71,7 @@
#include <linux/user_events.h>
#include <linux/uaccess.h>
+#include <uapi/linux/pidfd.h>
#include <uapi/linux/wait.h>
#include <asm/unistd.h>
@@ -1739,7 +1740,7 @@ int kernel_waitid_prepare(struct wait_opts *wo, int which, pid_t upid,
break;
case P_PIDFD:
type = PIDTYPE_PID;
- if (upid < 0)
+ if (upid < 0 && !pidfd_is_self_sentinel(upid))
return -EINVAL;
pid = pidfd_get_pid(upid, &f_flags);
@@ -550,6 +550,7 @@ SYSCALL_DEFINE2(setns, int, fd, int, flags)
struct nsset nsset = {};
int err = 0;
+ /* If fd is PIDFD_SELF_*, implicitly fail here, as invalid. */
if (!fd_file(f))
return -EBADF;
@@ -539,22 +539,31 @@ struct pid *__pidfd_get_pid(unsigned int pidfd, bool pin_pid,
bool allow_proc, unsigned int *flags,
struct fd *fd)
{
- struct file *file;
+ struct file *file = NULL;
struct pid *pid;
- struct fd f = fdget(pidfd);
-
- file = fd_file(f);
- if (!file)
- return ERR_PTR(-EBADF);
-
- pid = pidfd_pid(file);
- /* If we allow opening a pidfd via /proc/<pid>, do so. */
- if (IS_ERR(pid) && allow_proc)
- pid = tgid_pidfd_to_pid(file);
-
- if (IS_ERR(pid)) {
- fdput(f);
- return pid;
+ unsigned int f_flags = 0;
+ struct fd f = {};
+
+ if (pidfd == PIDFD_SELF_THREAD) {
+ pid = *task_pid_ptr(current, PIDTYPE_PID);
+ f_flags = PIDFD_THREAD;
+ } else if (pidfd == PIDFD_SELF_THREAD_GROUP) {
+ pid = *task_pid_ptr(current, PIDTYPE_TGID);
+ } else {
+ f = fdget(pidfd);
+ file = fd_file(f);
+ if (!file)
+ return ERR_PTR(-EBADF);
+
+ pid = pidfd_pid(file);
+ /* If we allow opening a pidfd via /proc/<pid>, do so. */
+ if (IS_ERR(pid) && allow_proc)
+ pid = tgid_pidfd_to_pid(file);
+
+ if (IS_ERR(pid)) {
+ fdput(f);
+ return pid;
+ }
}
if (pin_pid)
@@ -562,18 +571,22 @@ struct pid *__pidfd_get_pid(unsigned int pidfd, bool pin_pid,
else
WARN_ON_ONCE(!fd); /* Nothing to keep pid/pidfd around? */
- if (flags)
- *flags = file->f_flags;
+ if (file) {
+ f_flags = file->f_flags;
- /*
- * If the user provides an fd output then it will handle decrementing
- * its reference counter.
- */
- if (fd)
- *fd = f;
- else
- /* Otherwise we release it. */
- fdput(f);
+ /*
+ * If the user provides an fd output then it will handle decrementing
+ * its reference counter.
+ */
+ if (fd)
+ *fd = f;
+ else
+ /* Otherwise we release it. */
+ fdput(f);
+ }
+
+ if (flags)
+ *flags = f_flags;
return pid;
}
It is useful to be able to utilise pidfd mechanisms to reference the current thread or process (from a userland point of view - thread group leader from the kernel's point of view). Therefore introduce PIDFD_SELF_THREAD to refer to the current thread, and PIDFD_SELF_THREAD_GROUP to refer to the current thread group leader. For convenience and to avoid confusion from userland's perspective we alias these: * PIDFD_SELF is an alias for PIDFD_SELF_THREAD - This is nearly always what the user will want to use, as they would find it surprising if for instance fd's were unshared()'d and they wanted to invoke pidfd_getfd() and that failed. * PIDFD_SELF_PROCESS is an alias for PIDFD_SELF_THREAD_GROUP - Most users have no concept of thread groups or what a thread group leader is, and from userland's perspective and nomenclature this is what userland considers to be a process. Due to the refactoring of the central __pidfd_get_pid() function we can implement this functionality centrally, providing the use of this sentinel in most functionality which utilises pidfd's. We need to explicitly adjust kernel_waitid_prepare() to permit this (though it wouldn't really make sense to use this there, we provide the ability for consistency). We explicitly disallow use of this in setns(), which would otherwise have required explicit custom handling, as it doesn't make sense to set the current calling thread to join the namespace of itself. As the callers of pidfd_get_pid() expect an increased reference count on the pid we do so in the self case, reducing churn and avoiding any breakage from existing logic which decrements this reference count. In the pidfd_send_signal() system call, we can continue to fdput() the struct fd output by pidfs_to_pid_proc() even if PIDFD_SELF_* is specified, as this will be empty and the invocation will be a no-op. This change implicitly provides PIDFD_SELF_* support in the waitid(P_PIDFS, ...), process_madvise(), process_mrelease(), pidfd_send_signal(), and pidfd_getfd() system calls. Things such as polling a pidfs and general fd operations are not supported, this strictly provides the sentinel for APIs which explicitly accept a pidfd. Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> --- include/linux/pid.h | 9 +++--- include/uapi/linux/pidfd.h | 15 +++++++++ kernel/exit.c | 3 +- kernel/nsproxy.c | 1 + kernel/pid.c | 65 +++++++++++++++++++++++--------------- 5 files changed, 62 insertions(+), 31 deletions(-)