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pid: add pidfd_prepare()

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commit 6ae930d9dbf2d093157be33428538c91966d8a9f upstream.

Add a new helper that allows to reserve a pidfd and allocates a new
pidfd file that stashes the provided struct pid. This will allow us to
remove places that either open code this function or that call
pidfd_create() but then have to call close_fd() because there are still
failure points after pidfd_create() has been called.

Reviewed-by: Jan Kara <jack@suse.cz>
Message-Id: <20230327-pidfd-file-api-v1-1-5c0e9a3158e4@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Christian Brauner 2023-03-27 20:22:51 +02:00 • committed by Greg Kroah-Hartman
commit 0a841d0a48
2 changed files with 86 additions and 0 deletions
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1
include/linux/pid.h
View file

@ -75,6 +75,7 @@ extern const struct file_operations pidfd_fops;
struct file;
extern struct pid *pidfd_pid(const struct file *file);
int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret);
static inline struct pid *get_pid(struct pid *pid)
{

85
kernel/fork.c
View file

@ -1750,6 +1750,91 @@ const struct file_operations pidfd_fops = {
#endif
};
/**
* __pidfd_prepare - allocate a new pidfd_file and reserve a pidfd
* @pid: the struct pid for which to create a pidfd
* @flags: flags of the new @pidfd
* @pidfd: the pidfd to return
*
* Allocate a new file that stashes @pid and reserve a new pidfd number in the
* caller's file descriptor table. The pidfd is reserved but not installed yet.
* The helper doesn't perform checks on @pid which makes it useful for pidfds
* created via CLONE_PIDFD where @pid has no task attached when the pidfd and
* pidfd file are prepared.
*
* If this function returns successfully the caller is responsible to either
* call fd_install() passing the returned pidfd and pidfd file as arguments in
* order to install the pidfd into its file descriptor table or they must use
* put_unused_fd() and fput() on the returned pidfd and pidfd file
* respectively.
*
* This function is useful when a pidfd must already be reserved but there
* might still be points of failure afterwards and the caller wants to ensure
* that no pidfd is leaked into its file descriptor table.
*
* Return: On success, a reserved pidfd is returned from the function and a new
* pidfd file is returned in the last argument to the function. On
* error, a negative error code is returned from the function and the
* last argument remains unchanged.
*/
static int __pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
{
int pidfd;
struct file *pidfd_file;
if (flags & ~(O_NONBLOCK | O_RDWR | O_CLOEXEC))
return -EINVAL;
pidfd = get_unused_fd_flags(O_RDWR | O_CLOEXEC);
if (pidfd < 0)
return pidfd;
pidfd_file = anon_inode_getfile("[pidfd]", &pidfd_fops, pid,
flags | O_RDWR | O_CLOEXEC);
if (IS_ERR(pidfd_file)) {
put_unused_fd(pidfd);
return PTR_ERR(pidfd_file);
}
get_pid(pid); /* held by pidfd_file now */
*ret = pidfd_file;
return pidfd;
}
/**
* pidfd_prepare - allocate a new pidfd_file and reserve a pidfd
* @pid: the struct pid for which to create a pidfd
* @flags: flags of the new @pidfd
* @pidfd: the pidfd to return
*
* Allocate a new file that stashes @pid and reserve a new pidfd number in the
* caller's file descriptor table. The pidfd is reserved but not installed yet.
*
* The helper verifies that @pid is used as a thread group leader.
*
* If this function returns successfully the caller is responsible to either
* call fd_install() passing the returned pidfd and pidfd file as arguments in
* order to install the pidfd into its file descriptor table or they must use
* put_unused_fd() and fput() on the returned pidfd and pidfd file
* respectively.
*
* This function is useful when a pidfd must already be reserved but there
* might still be points of failure afterwards and the caller wants to ensure
* that no pidfd is leaked into its file descriptor table.
*
* Return: On success, a reserved pidfd is returned from the function and a new
* pidfd file is returned in the last argument to the function. On
* error, a negative error code is returned from the function and the
* last argument remains unchanged.
*/
int pidfd_prepare(struct pid *pid, unsigned int flags, struct file **ret)
{
if (!pid || !pid_has_task(pid, PIDTYPE_TGID))
return -EINVAL;
return __pidfd_prepare(pid, flags, ret);
}
static void __delayed_free_task(struct rcu_head *rhp)
{
struct task_struct *tsk = container_of(rhp, struct task_struct, rcu);