Re: [PATCH v2 1/3] pidfd: extend pidfd_get_pid() and de-duplicate pid lookup
From: Suren Baghdasaryan
Date: Tue Oct 15 2024 - 15:41:04 EST
On Fri, Oct 11, 2024 at 4:06 AM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> The means by which a pid is determined from a pidfd is duplicated, with
> some callers holding a reference to the (pid)fd, and others explicitly
> pinning the pid.
>
> Introduce __pidfd_get_pid() which abstracts both approaches and provide
> optional output parameters for file->f_flags and the fd (the latter of
> which, if provided, prevents the function from decrementing the fd's
> refernce count).
>
> Additionally, allow the ability to open a pidfd by opening a /proc/<pid>
> directory, utilised by the pidfd_send_signal() system call, providing a
> pidfd_get_pid_proc() helper function to do so.
>
> Doing this allows us to eliminate open-coded pidfd pid lookup and to
> consistently handle this in one place.
>
> This lays the groundwork for a subsequent patch which adds a new sentinel
> pidfd to explicitly reference the current process (i.e. thread group
> leader) without the need for a pidfd.
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> ---
> include/linux/pid.h | 42 +++++++++++++++++++++++++++++++-
> kernel/pid.c | 58 ++++++++++++++++++++++++++++++---------------
> kernel/signal.c | 22 ++++-------------
> 3 files changed, 84 insertions(+), 38 deletions(-)
>
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index a3aad9b4074c..68b02eab7509 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -2,6 +2,7 @@
> #ifndef _LINUX_PID_H
> #define _LINUX_PID_H
>
> +#include <linux/file.h>
> #include <linux/pid_types.h>
> #include <linux/rculist.h>
> #include <linux/rcupdate.h>
> @@ -72,8 +73,47 @@ extern struct pid init_struct_pid;
>
> 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.
> + * @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.
> + * @fd: Output variable, if non-NULL, then the pidfd reference will
> + * remain elevated and the caller will need to decrement it
> + * themselves.
> + *
> + * Returns: If successful, the pid associated with the pidfd, otherwise an
> + * error.
> + */
> +struct pid *__pidfd_get_pid(unsigned int pidfd, bool pin_pid,
> + bool allow_proc, unsigned int *flags,
> + struct fd *fd);
> +
> +static inline struct pid *pidfd_get_pid(unsigned int pidfd, unsigned int *flags)
> +{
> + return __pidfd_get_pid(pidfd, /* pin_pid = */ true,
> + /* allow_proc = */ false,
> + flags, /* fd = */ NULL);
> +}
> +
> +static inline struct pid *pidfd_to_pid_proc(unsigned int pidfd,
> + unsigned int *flags,
> + struct fd *fd)
> +{
> + return __pidfd_get_pid(pidfd, /* pin_pid = */ false,
> + /* allow_proc = */ true,
> + flags, fd);
> +}
> +
> struct pid *pidfd_pid(const struct file *file);
> -struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags);
> struct task_struct *pidfd_get_task(int pidfd, unsigned int *flags);
> int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret);
> void do_notify_pidfd(struct task_struct *task);
> diff --git a/kernel/pid.c b/kernel/pid.c
> index 2715afb77eab..25cc1c36a1b1 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -36,6 +36,7 @@
> #include <linux/pid_namespace.h>
> #include <linux/init_task.h>
> #include <linux/syscalls.h>
> +#include <linux/proc_fs.h>
> #include <linux/proc_ns.h>
> #include <linux/refcount.h>
> #include <linux/anon_inodes.h>
> @@ -534,22 +535,46 @@ struct pid *find_ge_pid(int nr, struct pid_namespace *ns)
> }
> EXPORT_SYMBOL_GPL(find_ge_pid);
>
> -struct pid *pidfd_get_pid(unsigned int fd, unsigned int *flags)
> +struct pid *__pidfd_get_pid(unsigned int pidfd, bool pin_pid,
> + bool allow_proc, unsigned int *flags,
> + struct fd *fd)
> {
> - struct fd f;
> + struct file *file;
> struct pid *pid;
> + struct fd f = fdget(pidfd);
>
> - f = fdget(fd);
> - if (!fd_file(f))
> + file = fd_file(f);
> + if (!file)
> return ERR_PTR(-EBADF);
>
> - pid = pidfd_pid(fd_file(f));
> - if (!IS_ERR(pid)) {
> - get_pid(pid);
> - *flags = fd_file(f)->f_flags;
> + 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;
> }
>
> - fdput(f);
> + if (pin_pid)
> + get_pid(pid);
> + else
> + WARN_ON_ONCE(!fd); /* Nothing to keep pid/pidfd around? */
> +
> + if (flags)
> + *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);
> +
> return pid;
> }
There is an EXPORT_SYMBOL_GPL(pidfd_get_pid) right after this line. It
should also be changed to EXPORT_SYMBOL_GPL(__pidfd_get_pid),
otherwise __pidfd_get_pid() will not be exported. A module calling
pidfd_get_pid() now inlined in the header file will try to call
__pidfd_get_pid() and will have trouble resolving this symbol.
>
> @@ -747,23 +772,18 @@ 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;
>
> - f = fdget(pidfd);
> - if (!fd_file(f))
> - return -EBADF;
> -
> - pid = pidfd_pid(fd_file(f));
> + pid = pidfd_get_pid(pidfd, NULL);
> if (IS_ERR(pid))
> - ret = PTR_ERR(pid);
> - else
> - ret = pidfd_getfd(pid, fd);
> + return PTR_ERR(pid);
>
> - fdput(f);
> + ret = pidfd_getfd(pid, fd);
> +
> + put_pid(pid);
> return ret;
> }
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 4344860ffcac..868bfa674c62 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -3875,17 +3875,6 @@ static int copy_siginfo_from_user_any(kernel_siginfo_t *kinfo,
> return copy_siginfo_from_user(kinfo, info);
> }
>
> -static struct pid *pidfd_to_pid(const struct file *file)
> -{
> - struct pid *pid;
> -
> - pid = pidfd_pid(file);
> - if (!IS_ERR(pid))
> - return pid;
> -
> - return tgid_pidfd_to_pid(file);
> -}
> -
> #define PIDFD_SEND_SIGNAL_FLAGS \
> (PIDFD_SIGNAL_THREAD | PIDFD_SIGNAL_THREAD_GROUP | \
> PIDFD_SIGNAL_PROCESS_GROUP)
> @@ -3908,10 +3897,11 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> siginfo_t __user *, info, unsigned int, flags)
> {
> int ret;
> - struct fd f;
> struct pid *pid;
> kernel_siginfo_t kinfo;
> enum pid_type type;
> + unsigned int f_flags;
> + struct fd f;
>
> /* Enforce flags be set to 0 until we add an extension. */
> if (flags & ~PIDFD_SEND_SIGNAL_FLAGS)
> @@ -3921,12 +3911,8 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> if (hweight32(flags & PIDFD_SEND_SIGNAL_FLAGS) > 1)
> return -EINVAL;
>
> - f = fdget(pidfd);
> - if (!fd_file(f))
> - return -EBADF;
> -
> /* Is this a pidfd? */
> - pid = pidfd_to_pid(fd_file(f));
> + pid = pidfd_to_pid_proc(pidfd, &f_flags, &f);
> if (IS_ERR(pid)) {
> ret = PTR_ERR(pid);
> goto err;
> @@ -3939,7 +3925,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> switch (flags) {
> case 0:
> /* Infer scope from the type of pidfd. */
> - if (fd_file(f)->f_flags & PIDFD_THREAD)
> + if (f_flags & PIDFD_THREAD)
> type = PIDTYPE_PID;
> else
> type = PIDTYPE_TGID;
> --
> 2.46.2
>