5.10-stable review patch. If anyone has any objections, please let me know.
------------------
From: Eric W. Biederman ebiederm@xmission.com
[ Upstream commit e9a53aeb5e0a838f10fcea74235664e7ad5e6e1a ]
As a companion to fget_task and task_lookup_fd_rcu implement task_lookup_next_fd_rcu that will return the struct file for the first file descriptor number that is equal or greater than the fd argument value, or NULL if there is no such struct file.
This allows file descriptors of foreign processes to be iterated through safely, without needed to increment the count on files_struct.
Some concern[1] has been expressed that this function takes the task_lock for each iteration and thus for each file descriptor. This place where this function will be called in a commonly used code path is for listing /proc/<pid>/fd. I did some small benchmarks and did not see any measurable performance differences. For ordinary users ls is likely to stat each of the directory entries and tid_fd_mode called from tid_fd_revalidae has always taken the task lock for each file descriptor. So this does not look like it will be a big change in practice.
At some point is will probably be worth changing put_files_struct to free files_struct after an rcu grace period so that task_lock won't be needed at all.
[1] https://lkml.kernel.org/r/20200817220425.9389-10-ebiederm@xmission.com v1: https://lkml.kernel.org/r/20200817220425.9389-9-ebiederm@xmission.com Link: https://lkml.kernel.org/r/20201120231441.29911-14-ebiederm@xmission.com Signed-off-by: Eric W. Biederman ebiederm@xmission.com Signed-off-by: Chuck Lever chuck.lever@oracle.com Signed-off-by: Sasha Levin sashal@kernel.org --- fs/file.c | 21 +++++++++++++++++++++ include/linux/fdtable.h | 1 + 2 files changed, 22 insertions(+)
diff --git a/fs/file.c b/fs/file.c index 60a3ccba728cd..9fa49e6298fba 100644 --- a/fs/file.c +++ b/fs/file.c @@ -926,6 +926,27 @@ struct file *task_lookup_fd_rcu(struct task_struct *task, unsigned int fd) return file; }
+struct file *task_lookup_next_fd_rcu(struct task_struct *task, unsigned int *ret_fd) +{ + /* Must be called with rcu_read_lock held */ + struct files_struct *files; + unsigned int fd = *ret_fd; + struct file *file = NULL; + + task_lock(task); + files = task->files; + if (files) { + for (; fd < files_fdtable(files)->max_fds; fd++) { + file = files_lookup_fd_rcu(files, fd); + if (file) + break; + } + } + task_unlock(task); + *ret_fd = fd; + return file; +} + /* * Lightweight file lookup - no refcnt increment if fd table isn't shared. * diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h index a88f68f740677..b0c6a959c6a00 100644 --- a/include/linux/fdtable.h +++ b/include/linux/fdtable.h @@ -111,6 +111,7 @@ static inline struct file *lookup_fd_rcu(unsigned int fd) }
struct file *task_lookup_fd_rcu(struct task_struct *task, unsigned int fd); +struct file *task_lookup_next_fd_rcu(struct task_struct *task, unsigned int *fd);
struct task_struct;