Kernel+LibC: Implement fcntl(2) advisory locks

Advisory locks don't actually prevent other processes from writing to
the file, but they do prevent other processes looking to acquire and
advisory lock on the file.

This implementation currently only adds non-blocking locks, which are
all I need for now.

Kernel/FileSystem/Inode.cpp

@@ -11,11 +11,13 @@
 #include <AK/StringView.h>
 #include <Kernel/API/InodeWatcherEvent.h>
 #include <Kernel/FileSystem/Custody.h>
+#include <Kernel/FileSystem/FileDescription.h>
 #include <Kernel/FileSystem/Inode.h>
 #include <Kernel/FileSystem/InodeWatcher.h>
 #include <Kernel/FileSystem/VirtualFileSystem.h>
 #include <Kernel/KBufferBuilder.h>
 #include <Kernel/Net/LocalSocket.h>
+#include <Kernel/Process.h>
 #include <Kernel/VM/SharedInodeVMObject.h>
 
 namespace Kernel {
@@ -272,4 +274,126 @@ RefPtr<SharedInodeVMObject> Inode::shared_vmobject() const
     return m_shared_vmobject.strong_ref();
 }
 
+template<typename T>
+static inline bool range_overlap(T start1, T len1, T start2, T len2)
+{
+    return ((start1 < start2 + len2) || len2 == 0) && ((start2 < start1 + len1) || len1 == 0);
+}
+
+static inline KResult normalize_flock(FileDescription const& description, flock& lock)
+{
+    off_t start;
+    switch (lock.l_whence) {
+    case SEEK_SET:
+        start = lock.l_start;
+        break;
+    case SEEK_CUR:
+        start = description.offset() + lock.l_start;
+        break;
+    case SEEK_END:
+        // FIXME: Implement SEEK_END and negative lengths.
+        return ENOTSUP;
+    default:
+        return EINVAL;
+    }
+    lock = { lock.l_type, SEEK_SET, start, lock.l_len, 0 };
+    return KSuccess;
+}
+
+KResult Inode::can_apply_flock(FileDescription const& description, flock const& new_lock) const
+{
+    VERIFY(new_lock.l_whence == SEEK_SET);
+
+    MutexLocker locker(m_inode_lock, Mutex::Mode::Shared);
+
+    if (new_lock.l_type == F_UNLCK) {
+        for (auto& lock : m_flocks) {
+            if (&description == lock.owner && lock.start == new_lock.l_start && lock.len == new_lock.l_len)
+                return KSuccess;
+        }
+        return EINVAL;
+    }
+
+    for (auto& lock : m_flocks) {
+        if (!range_overlap(lock.start, lock.len, new_lock.l_start, new_lock.l_len))
+            continue;
+
+        if (new_lock.l_type == F_RDLCK && lock.type == F_WRLCK)
+            return EAGAIN;
+
+        if (new_lock.l_type == F_WRLCK)
+            return EAGAIN;
+    }
+    return KSuccess;
+}
+
+KResult Inode::apply_flock(Process const& process, FileDescription const& description, Userspace<flock const*> input_lock)
+{
+    flock new_lock;
+    if (!copy_from_user(&new_lock, input_lock))
+        return EFAULT;
+
+    auto rc = normalize_flock(description, new_lock);
+    if (rc.is_error())
+        return rc;
+
+    MutexLocker locker(m_inode_lock);
+
+    rc = can_apply_flock(description, new_lock);
+    if (rc.is_error())
+        return rc;
+
+    if (new_lock.l_type == F_UNLCK) {
+        for (size_t i = 0; i < m_flocks.size(); ++i) {
+            if (&description == m_flocks[i].owner && m_flocks[i].start == new_lock.l_start && m_flocks[i].len == new_lock.l_len) {
+                m_flocks.remove(i);
+                return KSuccess;
+            }
+        }
+        return EINVAL;
+    }
+
+    m_flocks.append(Flock { new_lock.l_type, new_lock.l_start, new_lock.l_len, &description, process.pid().value() });
+    return KSuccess;
+}
+
+KResult Inode::get_flock(FileDescription const& description, Userspace<flock*> reference_lock) const
+{
+    flock lookup;
+    if (!copy_from_user(&lookup, reference_lock))
+        return EFAULT;
+
+    auto rc = normalize_flock(description, lookup);
+    if (rc.is_error())
+        return rc;
+
+    MutexLocker locker(m_inode_lock, Mutex::Mode::Shared);
+
+    for (auto& lock : m_flocks) {
+        if (!range_overlap(lock.start, lock.len, lookup.l_start, lookup.l_len))
+            continue;
+
+        if ((lookup.l_type == F_RDLCK && lock.type == F_WRLCK) || lookup.l_type == F_WRLCK) {
+            lookup = { lock.type, SEEK_SET, lock.start, lock.len, lock.pid };
+            if (!copy_to_user(reference_lock, &lookup))
+                return EFAULT;
+            return KSuccess;
+        }
+    }
+
+    lookup.l_type = F_UNLCK;
+    if (!copy_to_user(reference_lock, &lookup))
+        return EFAULT;
+    return KSuccess;
+}
+
+void Inode::remove_flocks_for_description(FileDescription const& description)
+{
+    MutexLocker locker(m_inode_lock);
+
+    for (size_t i = 0; i < m_flocks.size(); ++i) {
+        if (&description == m_flocks[i].owner)
+            m_flocks.remove(i--);
+    }
+}
 }