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Kernel: Implement flock downgrading

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This commit makes it possible for a process to downgrade a file lock it
holds from a write (exclusive) lock to a read (shared) lock. For this,
the process must point to the exact range of the flock, and must be the
owner of the lock.
This commit is contained in:
sin-ack 2022-10-01 19:28:30 +00:00 committed by Andrew Kaster
parent 9b425b860c
commit 3275015786
Notes: sideshowbarker 2024-07-17 07:19:27 +09:00 
Author: https://github.com/sin-ack
Commit: https://github.com/SerenityOS/serenity/commit/3275015786
Pull-request: https://github.com/SerenityOS/serenity/pull/15428
Reviewed-by: https://github.com/ADKaster ✅
Reviewed-by: https://github.com/AtkinsSJ
Reviewed-by: https://github.com/BertalanD
Reviewed-by: https://github.com/linusg
Reviewed-by: https://github.com/timschumi
2 changed files with 39 additions and 19 deletions
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56
Kernel/FileSystem/Inode.cpp
View file

@ -298,7 +298,7 @@ static inline ErrorOr<void> normalize_flock(OpenFileDescription const& descripti
return {};
}
bool Inode::can_apply_flock(flock const& new_lock) const
bool Inode::can_apply_flock(flock const& new_lock, Optional<OpenFileDescription const&> description) const
{
VERIFY(new_lock.l_whence == SEEK_SET);
@ -310,8 +310,19 @@ bool Inode::can_apply_flock(flock const& new_lock) const
if (!range_overlap(lock.start, lock.len, new_lock.l_start, new_lock.l_len))
continue;
// There are two cases where we can attempt downgrade:
//
// 1) We're the owner of this lock. The downgrade will immediately
// succeed.
// 2) We're not the owner of this lock. Our downgrade attempt will
// fail, and the thread will start blocking on an FlockBlocker.
//
// For the first case, we get the description from try_apply_flock
// below. For the second case, the check below would always be
// false, so there is no need to store the description in the
// blocker in the first place.
if (new_lock.l_type == F_RDLCK && lock.type == F_WRLCK)
return false;
return description.has_value() && lock.owner == &description.value() && lock.start == new_lock.l_start && lock.len == new_lock.l_len;
if (new_lock.l_type == F_WRLCK)
return false;
@ -320,29 +331,38 @@ bool Inode::can_apply_flock(flock const& new_lock) const
});
}
ErrorOr<bool> Inode::try_apply_flock(Process const& process, OpenFileDescription const& description, flock const& lock)
ErrorOr<bool> Inode::try_apply_flock(Process const& process, OpenFileDescription const& description, flock const& new_lock)
{
return m_flocks.with([&](auto& flocks) -> ErrorOr<bool> {
if (!can_apply_flock(lock))
if (!can_apply_flock(new_lock, description))
return false;
if (lock.l_type == F_UNLCK) {
bool any_locks_unlocked = false;
for (size_t i = 0; i < flocks.size(); ++i) {
if (&description == flocks[i].owner && flocks[i].start == lock.l_start && flocks[i].len == lock.l_len) {
flocks.remove(i);
any_locks_unlocked |= true;
}
bool did_manipulate_lock = false;
for (size_t i = 0; i < flocks.size(); ++i) {
auto const& lock = flocks[i];
bool is_potential_downgrade = new_lock.l_type == F_RDLCK && lock.type == F_WRLCK;
bool is_potential_unlock = new_lock.l_type == F_UNLCK;
bool is_lock_owner = &description == lock.owner;
bool lock_range_exactly_matches = lock.start == new_lock.l_start && lock.len == new_lock.l_len;
bool can_manage_this_lock = is_lock_owner && lock_range_exactly_matches;
if ((is_potential_downgrade || is_potential_unlock) && can_manage_this_lock) {
flocks.remove(i);
did_manipulate_lock = true;
break;
}
if (any_locks_unlocked)
m_flock_blocker_set.unblock_all_blockers_whose_conditions_are_met();
// Judging by the Linux implementation, unlocking a non-existent lock also works.
return true;
}
TRY(flocks.try_append(Flock { lock.l_start, lock.l_len, &description, process.pid().value(), lock.l_type }));
if (new_lock.l_type != F_UNLCK)
TRY(flocks.try_append(Flock { new_lock.l_start, new_lock.l_len, &description, process.pid().value(), new_lock.l_type }));
if (did_manipulate_lock)
m_flock_blocker_set.unblock_all_blockers_whose_conditions_are_met();
// Judging by the Linux implementation, unlocking a non-existent lock
// also works.
return true;
});
}

2
Kernel/FileSystem/Inode.h
View file

@ -104,7 +104,7 @@ public:
ErrorOr<NonnullLockRefPtr<FIFO>> fifo();
bool can_apply_flock(flock const&) const;
bool can_apply_flock(flock const&, Optional<OpenFileDescription const&> = {}) const;
ErrorOr<void> apply_flock(Process const&, OpenFileDescription const&, Userspace<flock const*>, ShouldBlock);
ErrorOr<void> get_flock(OpenFileDescription const&, Userspace<flock*>) const;
void remove_flocks_for_description(OpenFileDescription const&);