kernel/linux.git/include/linux/fdtable.h, branch v6.6.132

close_range(): fix the logics in descriptor table trimming

2024-10-10T09:58:00+00:00

commit 678379e1d4f7443b170939525d3312cfc37bf86b upstream. Cloning a descriptor table picks the size that would cover all currently opened files. That's fine for clone() and unshare(), but for close_range() there's an additional twist - we clone before we close, and it would be a shame to have close_range(3, ~0U, CLOSE_RANGE_UNSHARE) leave us with a huge descriptor table when we are not going to keep anything past stderr, just because some large file descriptor used to be open before our call has taken it out. Unfortunately, it had been dealt with in an inherently racy way - sane_fdtable_size() gets a "don't copy anything past that" argument (passed via unshare_fd() and dup_fd()), close_range() decides how much should be trimmed and passes that to unshare_fd(). The problem is, a range that used to extend to the end of descriptor table back when close_range() had looked at it might very well have stuff grown after it by the time dup_fd() has allocated a new files_struct and started to figure out the capacity of fdtable to be attached to that. That leads to interesting pathological cases; at the very least it's a QoI issue, since unshare(CLONE_FILES) is atomic in a sense that it takes a snapshot of descriptor table one might have observed at some point. Since CLOSE_RANGE_UNSHARE close_range() is supposed to be a combination of unshare(CLONE_FILES) with plain close_range(), ending up with a weird state that would never occur with unshare(2) is confusing, to put it mildly. It's not hard to get rid of - all it takes is passing both ends of the range down to sane_fdtable_size(). There we are under ->files_lock, so the race is trivially avoided. So we do the following: * switch close_files() from calling unshare_fd() to calling dup_fd(). * undo the calling convention change done to unshare_fd() in 60997c3d45d9 "close_range: add CLOSE_RANGE_UNSHARE" * introduce struct fd_range, pass a pointer to that to dup_fd() and sane_fdtable_size() instead of "trim everything past that point" they are currently getting. NULL means "we are not going to be punching any holes"; NR_OPEN_MAX is gone. * make sane_fdtable_size() use find_last_bit() instead of open-coding it; it's easier to follow that way. * while we are at it, have dup_fd() report errors by returning ERR_PTR(), no need to use a separate int *errorp argument. Fixes: 60997c3d45d9 "close_range: add CLOSE_RANGE_UNSHARE" Cc: stable@vger.kernel.org Signed-off-by: Al Viro Signed-off-by: Greg Kroah-Hartman

Unify the primitives for file descriptor closing

2022-05-14T22:49:01+00:00

Currently we have 3 primitives for removing an opened file from descriptor table - pick_file(), __close_fd_get_file() and close_fd_get_file(). Their calling conventions are rather odd and there's a code duplication for no good reason. They can be unified - 1) have __range_close() cap max_fd in the very beginning; that way we don't need separate way for pick_file() to report being past the end of descriptor table. 2) make {__,}close_fd_get_file() return file (or NULL) directly, rather than returning it via struct file ** argument. Don't bother with (bogus) return value - nobody wants that -ENOENT. 3) make pick_file() return NULL on unopened descriptor - the only caller that used to care about the distinction between descriptor past the end of descriptor table and finding NULL in descriptor table doesn't give a damn after (1). 4) lift ->files_lock out of pick_file() That actually simplifies the callers, as well as the primitives themselves. Code duplication is also gone... Reviewed-by: Christian Brauner (Microsoft) Signed-off-by: Al Viro

file: Remove get_files_struct

2020-12-10T18:42:59+00:00

When discussing[1] exec and posix file locks it was realized that none of the callers of get_files_struct fundamentally needed to call get_files_struct, and that by switching them to helper functions instead it will both simplify their code and remove unnecessary increments of files_struct.count. Those unnecessary increments can result in exec unnecessarily unsharing files_struct which breaking posix locks, and it can result in fget_light having to fallback to fget reducing system performance. Now that get_files_struct has no more users and can not cause the problems for posix file locking and fget_light remove get_files_struct so that it does not gain any new users. [1] https://lkml.kernel.org/r/20180915160423.GA31461@redhat.com Suggested-by: Oleg Nesterov Acked-by: Christian Brauner v1: https://lkml.kernel.org/r/20200817220425.9389-13-ebiederm@xmission.com Link: https://lkml.kernel.org/r/20201120231441.29911-24-ebiederm@xmission.com Signed-off-by: Eric W. Biederman

file: Rename __close_fd_get_file close_fd_get_file

2020-12-10T18:42:59+00:00

The function close_fd_get_file is explicitly a variant of __close_fd[1]. Now that __close_fd has been renamed close_fd, rename close_fd_get_file to be consistent with close_fd. When __alloc_fd, __close_fd and __fd_install were introduced the double underscore indicated that the function took a struct files_struct parameter. The function __close_fd_get_file never has so the naming has always been inconsistent. This just cleans things up so there are not any lingering mentions or references __close_fd left in the code. [1] 80cd795630d6 ("binder: fix use-after-free due to ksys_close() during fdget()") Link: https://lkml.kernel.org/r/20201120231441.29911-23-ebiederm@xmission.com Signed-off-by: Eric W. Biederman

file: Rename __close_fd to close_fd and remove the files parameter

2020-12-10T18:42:59+00:00

The function __close_fd was added to support binder[1]. Now that binder has been fixed to no longer need __close_fd[2] all calls to __close_fd pass current->files. Therefore transform the files parameter into a local variable initialized to current->files, and rename __close_fd to close_fd to reflect this change, and keep it in sync with the similar changes to __alloc_fd, and __fd_install. This removes the need for callers to care about the extra care that needs to be take if anything except current->files is passed, by limiting the callers to only operation on current->files. [1] 483ce1d4b8c3 ("take descriptor-related part of close() to file.c") [2] 44d8047f1d87 ("binder: use standard functions to allocate fds") Acked-by: Christian Brauner v1: https://lkml.kernel.org/r/20200817220425.9389-17-ebiederm@xmission.com Link: https://lkml.kernel.org/r/20201120231441.29911-21-ebiederm@xmission.com Signed-off-by: Eric W. Biederman

file: Merge __alloc_fd into alloc_fd

2020-12-10T18:42:59+00:00

The function __alloc_fd was added to support binder[1]. With binder fixed[2] there are no more users. As alloc_fd just calls __alloc_fd with "files=current->files", merge them together by transforming the files parameter into a local variable initialized to current->files. [1] dcfadfa4ec5a ("new helper: __alloc_fd()") [2] 44d8047f1d87 ("binder: use standard functions to allocate fds") Acked-by: Christian Brauner v1: https://lkml.kernel.org/r/20200817220425.9389-16-ebiederm@xmission.com Link: https://lkml.kernel.org/r/20201120231441.29911-20-ebiederm@xmission.com Signed-off-by: Eric W. Biederman

file: Merge __fd_install into fd_install

2020-12-10T18:42:58+00:00

The function __fd_install was added to support binder[1]. With binder fixed[2] there are no more users. As fd_install just calls __fd_install with "files=current->files", merge them together by transforming the files parameter into a local variable initialized to current->files. [1] f869e8a7f753 ("expose a low-level variant of fd_install() for binder") [2] 44d8047f1d87 ("binder: use standard functions to allocate fds") Acked-by: Christian Brauner v1:https://lkml.kernel.org/r/20200817220425.9389-14-ebiederm@xmission.com Link: https://lkml.kernel.org/r/20201120231441.29911-18-ebiederm@xmission.com Signed-off-by: Eric W. Biederman

file: Implement task_lookup_next_fd_rcu

2020-12-10T18:42:58+00:00

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//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

file: Implement task_lookup_fd_rcu

2020-12-10T18:40:10+00:00

As a companion to lookup_fd_rcu implement task_lookup_fd_rcu for querying an arbitrary process about a specific file. Acked-by: Christian Brauner v1: https://lkml.kernel.org/r/20200818103713.aw46m7vprsy4vlve@wittgenstein Link: https://lkml.kernel.org/r/20201120231441.29911-11-ebiederm@xmission.com Signed-off-by: Eric W. Biederman

file: Rename fcheck lookup_fd_rcu

2020-12-10T18:40:07+00:00

Also remove the confusing comment about checking if a fd exists. I could not find one instance in the entire kernel that still matches the description or the reason for the name fcheck. The need for better names became apparent in the last round of discussion of this set of changes[1]. [1] https://lkml.kernel.org/r/CAHk-=wj8BQbgJFLa+J0e=iT-1qpmCRTbPAJ8gd6MJQ=kbRPqyQ@mail.gmail.com Link: https://lkml.kernel.org/r/20201120231441.29911-10-ebiederm@xmission.com Signed-off-by: Eric W. Biederman