exec: Move unshare_files and guarantee files_struct.count is correct

A while ago it was reported that posix file locking goes wrong when a
multi-threaded process calls exec.  I looked into the history and this
is definitely a regression, that should be fixed if we can.

This set of changes cleanups of the code in exec so hopefully this code
will not regress again.  Then it adds helpers and fixes the users of
files_struct so the reference count is only incremented if COPY_FILES is
passed to clone (or if io_uring takes a reference).  Then it removes
helpers (get_files_struct, __install_fd, __alloc_fd, __close_fd) that
are no longer needed and if used would encourage code that increments
the count of files_struct somewhere besides in clone when COPY_FILES is
passed.

In addition to fixing the bug in exec and simplifing the code this set
of changes by virtue of getting files_struct.count correct it optimizes
fdget.  With proc and other places not temporarily increasing the count
on files_struct __fget_light should succeed more often in being able to
return a struct file without touching it's reference count.

Fixing the count in files_struct was suggested by Oleg[1].

For those that are interested in the history of this issue I have
included as much of it as I could find in the first change.

Since v1:

- Renamed the functions
  __fcheck_files      -> files_lookup_fd_raw
  fcheck_files        -> files_lookup_fd_locked
  fcheck_files        -> files_lookup_fd_rcu
  fcheck_files        -> lookup_fd_rcu
  fcheck_task         -> task_lookup_fd_rcu
  fnext_task          -> task_lookup_next_fd_rcu
  __close_fd_get_file -> close_fd_get_file

- Simplified get_file_raw_ptr

- Removed ksys_close

- Examined the penalty for taking task_lock.  The helper
  task_lookup_next_fd_rcu takes task_lock each iteration.  Concern was
  expressed that this might be a problem.  The function tid_fd_mode
  isn called from tid_fd_revalidate which is called when ever a file
  descriptor file is stat'ed, opened, or otherwise accessed.  The
  function tid_fd_mode histrocally called get_files_struct which took
  and dropped task_lock.  So the volume of task_lock calls is already
  proportional to the number of file descriptors.  A micro benchmark
  did not see the move to task_lookup_next_fd_rcu making a difference
  in performance.  Which suggests that the change to taking the task
  lock for every file descriptor found in task_lookup_next_fd will not
  be a problem.

- Reviewed the code for conflicts with io_uring (especially the
  removal of get_files_struct).  To my surprise no conflicts were
  found as io_uring does not use standard helpers but instead rolls
  it's own version of get_files_struct by hand.

 Documentation/filesystems/files.rst          |   8 +-
 arch/powerpc/platforms/cell/spufs/coredump.c |   2 +-
 drivers/android/binder.c                     |   2 +-
 fs/autofs/dev-ioctl.c                        |   5 +-
 fs/coredump.c                                |   5 +-
 fs/exec.c                                    |  29 +++----
 fs/file.c                                    | 124 +++++++++++++--------------
 fs/io_uring.c                                |   2 +-
 fs/locks.c                                   |  14 +--
 fs/notify/dnotify/dnotify.c                  |   2 +-
 fs/open.c                                    |   2 +-
 fs/proc/fd.c                                 |  48 ++++-------
 include/linux/fdtable.h                      |  40 +++++----
 include/linux/syscalls.h                     |  12 ---
 kernel/bpf/syscall.c                         |  20 +----
 kernel/bpf/task_iter.c                       |  44 +++-------
 kernel/fork.c                                |  12 +--
 kernel/kcmp.c                                |  29 ++-----
 18 files changed, 153 insertions(+), 247 deletions(-)

Eric W. Biederman (25):
      exec: Don't open code get_close_on_exec
      exec: Move unshare_files to fix posix file locking during exec
      exec: Simplify unshare_files
      exec: Remove reset_files_struct
      kcmp: In kcmp_epoll_target use fget_task
      bpf: In bpf_task_fd_query use fget_task
      proc/fd: In proc_fd_link use fget_task
      file: Rename __fcheck_files to files_lookup_fd_raw
      file: Factor files_lookup_fd_locked out of fcheck_files
      file: Replace fcheck_files with files_lookup_fd_rcu
      file: Rename fcheck lookup_fd_rcu
      file: Implement task_lookup_fd_rcu
      proc/fd: In tid_fd_mode use task_lookup_fd_rcu
      kcmp: In get_file_raw_ptr use task_lookup_fd_rcu
      file: Implement task_lookup_next_fd_rcu
      proc/fd: In proc_readfd_common use task_lookup_next_fd_rcu
      bpf/task_iter: In task_file_seq_get_next use task_lookup_next_fd_rcu
      proc/fd: In fdinfo seq_show don't use get_files_struct
      file: Merge __fd_install into fd_install
      file: In f_dupfd read RLIMIT_NOFILE once.
      file: Merge __alloc_fd into alloc_fd
      file: Rename __close_fd to close_fd and remove the files parameter
      file: Replace ksys_close with close_fd
      file: Rename __close_fd_get_file close_fd_get_file
      file: Remove get_files_struct

[1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com
v1: https://lkml.kernel.org/r/87ft8l6ic3.fsf@x220.int.ebiederm.org
Reported-by: Jeff Layton <jlayton@redhat.com>
Reported-by: Daniel P. Berrangé <berrange@redhat.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lkml.kernel.org/r/87r1on1v62.fsf@x220.int.ebiederm.org
Link: https://lists.openvz.org/pipermail/criu/2020-November/045123.html
Link: https://marc.info/?l=openvz-criu&m=160591423214257
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

2 files changed, 21 insertions, 31 deletions

diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index a32bf47c593e..d0e78174874a 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -80,7 +80,7 @@ struct dentry;
 /*
  * The caller must ensure that fd table isn't shared or hold rcu or file lock
  */
-static inline struct file *__fcheck_files(struct files_struct *files, unsigned int fd)
+static inline struct file *files_lookup_fd_raw(struct files_struct *files, unsigned int fd)
 {
 	struct fdtable *fdt = rcu_dereference_raw(files->fdt);
 
@@ -91,39 +91,41 @@ static inline struct file *__fcheck_files(struct files_struct *files, unsigned i
 	return NULL;
 }
 
-static inline struct file *fcheck_files(struct files_struct *files, unsigned int fd)
+static inline struct file *files_lookup_fd_locked(struct files_struct *files, unsigned int fd)
 {
-	RCU_LOCKDEP_WARN(!rcu_read_lock_held() &&
-			   !lockdep_is_held(&files->file_lock),
+	RCU_LOCKDEP_WARN(!lockdep_is_held(&files->file_lock),
 			   "suspicious rcu_dereference_check() usage");
-	return __fcheck_files(files, fd);
+	return files_lookup_fd_raw(files, fd);
 }
 
-/*
- * Check whether the specified fd has an open file.
- */
-#define fcheck(fd)	fcheck_files(current->files, fd)
+static inline struct file *files_lookup_fd_rcu(struct files_struct *files, unsigned int fd)
+{
+	RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
+			   "suspicious rcu_dereference_check() usage");
+	return files_lookup_fd_raw(files, fd);
+}
+
+static inline struct file *lookup_fd_rcu(unsigned int fd)
+{
+	return files_lookup_fd_rcu(current->files, 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;
 
-struct files_struct *get_files_struct(struct task_struct *);
 void put_files_struct(struct files_struct *fs);
-void reset_files_struct(struct files_struct *);
-int unshare_files(struct files_struct **);
+int unshare_files(void);
 struct files_struct *dup_fd(struct files_struct *, unsigned, int *) __latent_entropy;
 void do_close_on_exec(struct files_struct *);
 int iterate_fd(struct files_struct *, unsigned,
 		int (*)(const void *, struct file *, unsigned),
 		const void *);
 
-extern int __alloc_fd(struct files_struct *files,
-		      unsigned start, unsigned end, unsigned flags);
-extern void __fd_install(struct files_struct *files,
-		      unsigned int fd, struct file *file);
-extern int __close_fd(struct files_struct *files,
-		      unsigned int fd);
+extern int close_fd(unsigned int fd);
 extern int __close_range(unsigned int fd, unsigned int max_fd, unsigned int flags);
-extern int __close_fd_get_file(unsigned int fd, struct file **res);
+extern int close_fd_get_file(unsigned int fd, struct file **res);
 extern int unshare_fd(unsigned long unshare_flags, unsigned int max_fds,
 		      struct files_struct **new_fdp);
 
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index 37bea07c12f2..0f72f380db72 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1295,18 +1295,6 @@ static inline long ksys_ftruncate(unsigned int fd, loff_t length)
 	return do_sys_ftruncate(fd, length, 1);
 }
 
-extern int __close_fd(struct files_struct *files, unsigned int fd);
-
-/*
- * In contrast to sys_close(), this stub does not check whether the syscall
- * should or should not be restarted, but returns the raw error codes from
- * __close_fd().
- */
-static inline int ksys_close(unsigned int fd)
-{
-	return __close_fd(current->files, fd);
-}
-
 extern long do_sys_truncate(const char __user *pathname, loff_t length);
 
 static inline long ksys_truncate(const char __user *pathname, loff_t length)