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/File.cpp

@@ -7,6 +7,7 @@
 #include <AK/StringView.h>
 #include <Kernel/FileSystem/File.h>
 #include <Kernel/FileSystem/FileDescription.h>
+#include <Kernel/Process.h>
 
 namespace Kernel {
 
@@ -53,5 +54,4 @@ void File::detach(FileDescription&)
 {
     m_attach_count--;
 }
-
 }

Kernel/FileSystem/FileDescription.cpp

@@ -74,6 +74,9 @@ FileDescription::~FileDescription()
     (void)m_file->close();
     if (m_inode)
         m_inode->detach(*this);
+
+    if (m_inode)
+        m_inode->remove_flocks_for_description(*this);
 }
 
 KResult FileDescription::attach()
@@ -446,4 +449,19 @@ FileBlockCondition& FileDescription::block_condition()
     return m_file->block_condition();
 }
 
+KResult FileDescription::apply_flock(Process const& process, Userspace<flock const*> lock)
+{
+    if (!m_inode)
+        return EBADF;
+
+    return m_inode->apply_flock(process, *this, lock);
+}
+
+KResult FileDescription::get_flock(Userspace<flock*> lock) const
+{
+    if (!m_inode)
+        return EBADF;
+
+    return m_inode->get_flock(*this, lock);
+}
 }

Kernel/FileSystem/FileDescription.h

@@ -127,6 +127,9 @@ public:
 
     FileBlockCondition& block_condition();
 
+    KResult apply_flock(Process const&, Userspace<flock const*>);
+    KResult get_flock(Userspace<flock*>) const;
+
 private:
     friend class VirtualFileSystem;
     explicit FileDescription(File&);

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--);
+    }
+}
 }

Kernel/FileSystem/Inode.h

@@ -97,6 +97,11 @@ public:
 
     NonnullRefPtr<FIFO> fifo();
 
+    KResult can_apply_flock(FileDescription const&, flock const&) const;
+    KResult apply_flock(Process const&, FileDescription const&, Userspace<flock const*>);
+    KResult get_flock(FileDescription const&, Userspace<flock*>) const;
+    void remove_flocks_for_description(FileDescription const&);
+
 protected:
     Inode(FileSystem&, InodeIndex);
     void set_metadata_dirty(bool);
@@ -119,6 +124,16 @@ private:
     RefPtr<FIFO> m_fifo;
     IntrusiveListNode<Inode> m_inode_list_node;
 
+    struct Flock {
+        short type;
+        off_t start;
+        off_t len;
+        FileDescription const* owner;
+        pid_t pid;
+    };
+
+    Vector<Flock> m_flocks;
+
 public:
     using List = IntrusiveList<Inode, RawPtr<Inode>, &Inode::m_inode_list_node>;
 };

Kernel/Syscalls/fcntl.cpp

@@ -43,6 +43,10 @@ KResultOr<FlatPtr> Process::sys$fcntl(int fd, int cmd, u32 arg)
         break;
     case F_ISTTY:
         return description->is_tty();
+    case F_GETLK:
+        return description->get_flock(Userspace<flock*>(arg));
+    case F_SETLK:
+        return description->apply_flock(*Process::current(), Userspace<const flock*>(arg));
     default:
         return EINVAL;
     }

Kernel/UnixTypes.h

@@ -108,6 +108,9 @@ enum {
 #define F_GETFL 3
 #define F_SETFL 4
 #define F_ISTTY 5
+#define F_GETLK 6
+#define F_SETLK 7
+#define F_SETLKW 8
 
 #define FD_CLOEXEC 1
 
@@ -746,3 +749,15 @@ struct statvfs {
     unsigned long f_flag;
     unsigned long f_namemax;
 };
+
+#define F_RDLCK ((short)0)
+#define F_WRLCK ((short)1)
+#define F_UNLCK ((short)2)
+
+struct flock {
+    short l_type;
+    short l_whence;
+    off_t l_start;
+    off_t l_len;
+    pid_t l_pid;
+};

Userland/Libraries/LibC/fcntl.h

@@ -18,6 +18,9 @@ __BEGIN_DECLS
 #define F_GETFL 3
 #define F_SETFL 4
 #define F_ISTTY 5
+#define F_GETLK 6
+#define F_SETLK 7
+#define F_SETLKW 8
 
 #define FD_CLOEXEC 1
 
@@ -48,12 +51,9 @@ int create_inode_watcher(unsigned flags);
 int inode_watcher_add_watch(int fd, const char* path, size_t path_length, unsigned event_mask);
 int inode_watcher_remove_watch(int fd, int wd);
 
-#define F_RDLCK 0
-#define F_WRLCK 1
-#define F_UNLCK 2
-#define F_GETLK 5
-#define F_SETLK 6
-#define F_SETLKW 7
+#define F_RDLCK ((short)0)
+#define F_WRLCK ((short)1)
+#define F_UNLCK ((short)2)
 
 struct flock {
     short l_type;