Kernel/FATFS: Avoid creating LFN entries when an SFN will suffice

SerenityOS · Nov 2, 2024 · a9083f9 · a9083f9
1 parent 0ba7a03
commit a9083f9
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Showing 2 changed files with 83 additions and 10 deletions.
diff --git a/Kernel/FileSystem/FATFS/Inode.cpp b/Kernel/FileSystem/FATFS/Inode.cpp
@@ -93,6 +93,41 @@ static bool is_valid_sfn_char(char c)
     return valid_misc_sfn_chars.contains_slow(c);
 }
 
+static bool is_valid_sfn(StringView sfn)
+{
+    StringView name = {};
+    StringView extension = {};
+
+    auto dot = sfn.find('.');
+    if (!dot.has_value()) {
+        name = sfn;
+    } else {
+        if (*dot + 1 >= sfn.length())
+            return false;
+
+        name = sfn.substring_view(0, *dot);
+        extension = sfn.substring_view(*dot + 1, sfn.length() - *dot - 1);
+    }
+
+    if (name.length() > 8 || extension.length() > 3)
+        return false;
+
+    for (char c : name) {
+        if (!is_valid_sfn_char(c))
+            return false;
+    }
+
+    for (char c : extension) {
+        if (!is_valid_sfn_char(c))
+            return false;
+    }
+
+    if (name.length() == 0 || name[0] == ' ')
+        return false;
+
+    return true;
+}
+
 // http://www.osdever.net/documents/LongFileName.pdf
 static ErrorOr<NonnullRefPtr<SFN>> create_sfn_from_lfn(StringView lfn)
 {
@@ -177,6 +212,27 @@ ErrorOr<void> FATInode::create_unique_sfn_for(FATEntry& entry, NonnullRefPtr<SFN
     return {};
 }
 
+ErrorOr<void> FATInode::encode_known_good_sfn_for(FATEntry& entry, StringView name)
+{
+    memset(entry.filename, ' ', 8);
+    memset(entry.extension, ' ', 3);
+    auto dot = name.find('.');
+    if (dot.has_value()) {
+        auto extension_length = name.length() - dot.value() - 1;
+
+        VERIFY(dot.value() <= 8);
+        VERIFY(extension_length <= 3);
+
+        memcpy(entry.filename, name.bytes().data(), dot.value());
+        memcpy(entry.extension, name.bytes().data() + dot.value() + 1, extension_length);
+    } else {
+        VERIFY(name.bytes().size() <= 8);
+        memcpy(entry.filename, name.bytes().data(), name.bytes().size());
+    }
+
+    return {};
+}
+
 ErrorOr<Vector<FATLongFileNameEntry>> FATInode::create_lfn_entries(StringView name, u8 checksum)
 {
     u32 lfn_entry_count = ceil_div(name.length(), characters_per_lfn_entry);
@@ -642,18 +698,26 @@ ErrorOr<NonnullRefPtr<Inode>> FATInode::create_child(StringView name, mode_t mod
     dbgln_if(FAT_DEBUG, "FATInode[{}]::create_child(): creating inode \"{}\"", identifier(), name);
 
     FATEntry entry {};
-    auto sfn = TRY(create_sfn_from_lfn(name));
-    auto existing_sfns = TRY(collect_sfns());
-    TRY(create_unique_sfn_for(entry, move(sfn), move(existing_sfns)));
+
+    bool valid_sfn = is_valid_sfn(name);
+
+    if (valid_sfn) {
+        TRY(encode_known_good_sfn_for(entry, name));
+    } else {
+        auto sfn = TRY(create_sfn_from_lfn(name));
+        auto existing_sfns = TRY(collect_sfns());
+        TRY(create_unique_sfn_for(entry, move(sfn), move(existing_sfns)));
+    }
 
     // TODO: We should set the hidden attribute if the file starts with a dot or read only (the same way Linux does this).
     if (mode & S_IFDIR)
         entry.attributes |= FATAttributes::Directory;
 
     // FIXME: Set the dates
 
-    // FIXME: For some filenames lfn entries are not necessary
-    auto lfn_entries = TRY(create_lfn_entries(name, lfn_entry_checksum(entry)));
+    Vector<FATLongFileNameEntry> lfn_entries = {};
+    if (!valid_sfn)
+        lfn_entries = TRY(create_lfn_entries(name, lfn_entry_checksum(entry)));
 
     MutexLocker locker(m_inode_lock);
 
@@ -710,18 +774,26 @@ ErrorOr<void> FATInode::add_child(Inode& inode, StringView name, mode_t mode)
     dbgln_if(FAT_DEBUG, "FATInode[{}]::add_child(): appending inode {} as \"{}\"", identifier(), inode.identifier(), name);
 
     auto entry = bit_cast<FATInode*>(&inode)->m_entry;
-    auto sfn = TRY(create_sfn_from_lfn(name));
-    auto existing_sfns = TRY(collect_sfns());
-    TRY(create_unique_sfn_for(entry, move(sfn), move(existing_sfns)));
+
+    bool valid_sfn = is_valid_sfn(name);
+
+    if (valid_sfn) {
+        TRY(encode_known_good_sfn_for(entry, name));
+    } else {
+        NonnullRefPtr<SFN> sfn = TRY(create_sfn_from_lfn(name));
+        Vector<ByteBuffer> existing_sfns = TRY(collect_sfns());
+        TRY(create_unique_sfn_for(entry, move(sfn), move(existing_sfns)));
+    }
 
     // TODO: We should set the hidden attribute if the file starts with a dot or read only (the same way Linux does this).
     if (mode & S_IFDIR)
         entry.attributes |= FATAttributes::Directory;
 
     // FIXME: Set the dates
 
-    // FIXME: For some filenames lfn entries are not necessary
-    auto lfn_entries = TRY(create_lfn_entries(name, lfn_entry_checksum(entry)));
+    Vector<FATLongFileNameEntry> lfn_entries = {};
+    if (!valid_sfn)
+        auto lfn_entries = TRY(create_lfn_entries(name, lfn_entry_checksum(entry)));
 
     MutexLocker locker(m_inode_lock);
 

diff --git a/Kernel/FileSystem/FATFS/Inode.h b/Kernel/FileSystem/FATFS/Inode.h
@@ -118,6 +118,7 @@ class FATInode final : public Inode {
     static StringView byte_terminated_string(StringView, u8);
     static u8 lfn_entry_checksum(FATEntry const& entry);
     static ErrorOr<void> create_unique_sfn_for(FATEntry& entry, NonnullRefPtr<SFN> sfn, Vector<ByteBuffer> existing_sfns);
+    static ErrorOr<void> encode_known_good_sfn_for(FATEntry& entry, StringView name);
     static ErrorOr<Vector<FATLongFileNameEntry>> create_lfn_entries(StringView name, u8 checksum);
 
     ErrorOr<Vector<u32>> compute_cluster_list(FATFS&, u32 first_cluster);