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introduce __fcheck_files() to fix rcu_dereference_check_fdtable(), kill rcu_my_thread_group_empty()
rcu_dereference_check_fdtable() looks very wrong, 1. rcu_my_thread_group_empty() was added by 844b9a8707f1 "vfs: fix RCU-lockdep false positive due to /proc" but it doesn't really fix the problem. A CLONE_THREAD (without CLONE_FILES) task can hit the same race with get_files_struct(). And otoh rcu_my_thread_group_empty() can suppress the correct warning if the caller is the CLONE_FILES (without CLONE_THREAD) task. 2. files->count == 1 check is not really right too. Even if this files_struct is not shared it is not safe to access it lockless unless the caller is the owner. Otoh, this check is sub-optimal. files->count == 0 always means it is safe to use it lockless even if files != current->files, but put_files_struct() has to take rcu_read_lock(). See the next patch. This patch removes the buggy checks and turns fcheck_files() into __fcheck_files() which uses rcu_dereference_raw(), the "unshared" callers, fget_light() and fget_raw_light(), can use it to avoid the warning from RCU-lockdep. fcheck_files() is trivially reimplemented as rcu_lockdep_assert() plus __fcheck_files(). Signed-off-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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@ -707,7 +707,7 @@ struct file *fget_light(unsigned int fd, int *fput_needed)
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*fput_needed = 0;
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if (atomic_read(&files->count) == 1) {
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file = fcheck_files(files, fd);
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file = __fcheck_files(files, fd);
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if (file && (file->f_mode & FMODE_PATH))
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file = NULL;
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} else {
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@ -735,7 +735,7 @@ struct file *fget_raw_light(unsigned int fd, int *fput_needed)
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*fput_needed = 0;
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if (atomic_read(&files->count) == 1) {
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file = fcheck_files(files, fd);
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file = __fcheck_files(files, fd);
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} else {
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rcu_read_lock();
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file = fcheck_files(files, fd);
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@ -59,29 +59,36 @@ struct files_struct {
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struct file __rcu * fd_array[NR_OPEN_DEFAULT];
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};
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#define rcu_dereference_check_fdtable(files, fdtfd) \
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(rcu_dereference_check((fdtfd), \
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lockdep_is_held(&(files)->file_lock) || \
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atomic_read(&(files)->count) == 1 || \
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rcu_my_thread_group_empty()))
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#define files_fdtable(files) \
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(rcu_dereference_check_fdtable((files), (files)->fdt))
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struct file_operations;
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struct vfsmount;
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struct dentry;
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extern void __init files_defer_init(void);
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static inline struct file * fcheck_files(struct files_struct *files, unsigned int fd)
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#define rcu_dereference_check_fdtable(files, fdtfd) \
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rcu_dereference_check((fdtfd), lockdep_is_held(&(files)->file_lock))
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#define files_fdtable(files) \
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rcu_dereference_check_fdtable((files), (files)->fdt)
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/*
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* The caller must ensure that fd table isn't shared or hold rcu or file lock
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*/
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static inline struct file *__fcheck_files(struct files_struct *files, unsigned int fd)
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{
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struct file * file = NULL;
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struct fdtable *fdt = files_fdtable(files);
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struct fdtable *fdt = rcu_dereference_raw(files->fdt);
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if (fd < fdt->max_fds)
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file = rcu_dereference_check_fdtable(files, fdt->fd[fd]);
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return file;
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return rcu_dereference_raw(fdt->fd[fd]);
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return NULL;
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}
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static inline struct file *fcheck_files(struct files_struct *files, unsigned int fd)
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{
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rcu_lockdep_assert(rcu_read_lock_held() ||
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lockdep_is_held(&files->file_lock),
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"suspicious rcu_dereference_check() usage");
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return __fcheck_files(files, fd);
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}
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/*
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@ -448,8 +448,6 @@ static inline int rcu_read_lock_sched_held(void)
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#ifdef CONFIG_PROVE_RCU
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extern int rcu_my_thread_group_empty(void);
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/**
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* rcu_lockdep_assert - emit lockdep splat if specified condition not met
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* @c: condition to check
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@ -195,17 +195,6 @@ void wait_rcu_gp(call_rcu_func_t crf)
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}
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EXPORT_SYMBOL_GPL(wait_rcu_gp);
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#ifdef CONFIG_PROVE_RCU
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/*
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* wrapper function to avoid #include problems.
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*/
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int rcu_my_thread_group_empty(void)
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{
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return thread_group_empty(current);
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}
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EXPORT_SYMBOL_GPL(rcu_my_thread_group_empty);
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#endif /* #ifdef CONFIG_PROVE_RCU */
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#ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
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static inline void debug_init_rcu_head(struct rcu_head *head)
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{
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