1 files changed, 480 insertions, 0 deletions

diff --git a/src/mods.rs b/src/mods.rs
new file mode 100644
index 0000000..a3f782b
--- /dev/null
+++ b/src/mods.rs
@@ -0,0 +1,480 @@
+//! Mod validation: two-layer inspection of Forge/Fabric mod JARs.
+//!
+//! 1. Metadata check — reads `META-INF/mods.toml` (Forge) or `fabric.mod.json` (Fabric).
+//! 2. Class scan — walks `.class` entries looking for `net/minecraft/server`
+//!    vs `net/minecraft/client` references.
+//!
+//! **Rejection policy:** only mods that explicitly declare themselves client-only
+//! in their metadata (`mods.toml` side=CLIENT, `fabric.mod.json` environment=client)
+//! are rejected. The class scan is purely informational — many Forge mods that
+//! work on servers reference `net/minecraft/client/…` for rendering/GUI but do
+//! their server work through common API classes (`net/minecraft/block/Block`, etc.)
+//! without ever referencing `net/minecraft/server/…` directly.
+
+use std::io::{Cursor, Read};
+use zip::ZipArchive;
+
+/// The canonical side for a mod, derived from metadata or class scan.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum ModSide {
+    Both,
+    Client,
+    Server,
+    Unknown,
+}
+
+impl std::fmt::Display for ModSide {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            ModSide::Both => write!(f, "BOTH"),
+            ModSide::Client => write!(f, "CLIENT"),
+            ModSide::Server => write!(f, "SERVER"),
+            ModSide::Unknown => write!(f, "UNKNOWN"),
+        }
+    }
+}
+
+/// The result of validating a single mod file.
+#[derive(Debug)]
+pub struct ValidationResult {
+    pub accepted: bool,
+    pub rejection_reason: Option<String>,
+    pub display_name: Option<String>,
+    pub side: ModSide,
+}
+
+/// Validate a mod JAR (raw bytes, filename for error messages).
+///
+/// Returns a `ValidationResult` indicating whether the mod is safe to install.
+///
+/// **Rejection policy:**
+/// - Only mods that explicitly declare themselves client-only **in their metadata**
+///   (`mods.toml` side=CLIENT, `fabric.mod.json` environment=client) are rejected
+///   outright. The class-file scan is purely informational — many Forge mods that
+///   work perfectly on a server reference `net/minecraft/client/…` classes for
+///   rendering/GUI while doing their server logic through common API classes
+///   (`net/minecraft/block/Block`, `net/minecraft/item/Item`, …) without ever
+///   referencing `net/minecraft/server/…` directly.
+pub fn validate_mod(data: &[u8], filename: &str) -> ValidationResult {
+    // Phase 1: metadata check
+    let (meta_side, meta_display_name) = read_metadata(data);
+
+    // Only reject based on metadata — the mod author's explicit declaration.
+    if meta_side == Some(ModSide::Client) {
+        return ValidationResult {
+            accepted: false,
+            rejection_reason: Some(format!(
+                "Mod '{}' declares itself as client-only in its metadata. \
+                 It would crash the server.",
+                meta_display_name.as_deref().unwrap_or(filename)
+            )),
+            display_name: meta_display_name,
+            side: ModSide::Client,
+        };
+    }
+
+    // Phase 2: class scan (informational only — never causes rejection)
+    let class_side = scan_jar_for_side(data);
+
+    let side = if meta_side.is_some() {
+        meta_side.unwrap()
+    } else {
+        match class_side {
+            ScanResult::HasServer => ModSide::Both,
+            ScanResult::OnlyClient => ModSide::Client,
+            ScanResult::NoClearSide => ModSide::Unknown,
+        }
+    };
+
+    ValidationResult {
+        accepted: true,
+        rejection_reason: if side == ModSide::Client && meta_side.is_none() {
+            Some("Class scan suggests this mod may be client-side only, \
+                 but it has been accepted anyway (no metadata declaration).".to_string())
+        } else {
+            None
+        },
+        display_name: meta_display_name,
+        side,
+    }
+}
+
+// ── Metadata parsing ──────────────────────────────────────────────
+
+/// Read side and display-name from Forge's mods.toml or Fabric's fabric.mod.json.
+/// Returns `(side, display_name)` where side is `None` if no metadata found.
+fn read_metadata(data: &[u8]) -> (Option<ModSide>, Option<String>) {
+    let cursor = Cursor::new(data);
+    let mut archive = match ZipArchive::new(cursor) {
+        Ok(a) => a,
+        Err(_) => return (None, None),
+    };
+
+    // Try Forge mods.toml
+    if let Ok(mut file) = archive.by_name("META-INF/mods.toml") {
+        let mut content = String::new();
+        if file.read_to_string(&mut content).is_ok()
+            && let Some((side, name)) = parse_forge_metadata(&content) {
+                return (Some(side), name);
+            }
+    }
+
+    // Try Fabric fabric.mod.json
+    if let Ok(mut file) = archive.by_name("fabric.mod.json") {
+        let mut content = String::new();
+        if file.read_to_string(&mut content).is_ok()
+            && let Some((side, name)) = parse_fabric_metadata(&content) {
+                return (Some(side), name);
+            }
+    }
+
+    (None, None)
+}
+
+/// Parse `[[mods]]` entries from Forge's mods.toml.
+fn parse_forge_metadata(content: &str) -> Option<(ModSide, Option<String>)> {
+    let value: toml::Value = content.parse().ok()?;
+    let mods = value.get("mods")?.as_array()?;
+
+    let mut has_both = false;
+    let mut has_server = false;
+    let mut has_client = false;
+    let mut display_name = None;
+
+    for entry in mods {
+        let entry = entry.as_table()?;
+        // Use displayName or modId
+        let name = entry
+            .get("displayName")
+            .and_then(|v| v.as_str())
+            .or_else(|| entry.get("modId").and_then(|v| v.as_str()))
+            .map(|s| s.to_string());
+        if display_name.is_none() {
+            display_name = name;
+        }
+
+        match entry.get("side").and_then(|v| v.as_str()) {
+            Some("BOTH") => has_both = true,
+            Some("SERVER") => has_server = true,
+            Some("CLIENT") => has_client = true,
+            _ => {}
+        }
+    }
+
+    if has_both || (has_server && has_client) {
+        Some((ModSide::Both, display_name))
+    } else if has_server {
+        Some((ModSide::Server, display_name))
+    } else if has_client {
+        Some((ModSide::Client, display_name))
+    } else {
+        None
+    }
+}
+
+/// Parse Fabric's fabric.mod.json.
+fn parse_fabric_metadata(content: &str) -> Option<(ModSide, Option<String>)> {
+    let value: serde_json::Value = content.parse().ok()?;
+    let env = value.get("environment").and_then(|v| v.as_str());
+    let name = value
+        .get("name")
+        .and_then(|v| v.as_str())
+        .or_else(|| value.get("id").and_then(|v| v.as_str()))
+        .map(|s| s.to_string());
+
+    match env {
+        Some("*") | Some("server") => Some((ModSide::Both, name)),
+        Some("client") => Some((ModSide::Client, name)),
+        _ => None,
+    }
+}
+
+// ── Class-file scan ───────────────────────────────────────────────
+
+#[derive(Debug, Clone, Copy, PartialEq)]
+enum ScanResult {
+    /// At least one class references server paths (mod is safe).
+    HasServer,
+    /// Every class that references Minecraft packages only references client paths.
+    OnlyClient,
+    /// No class gave a clear indication either way.
+    NoClearSide,
+}
+
+/// Walk all `.class` entries in the JAR and look for client vs server references.
+///
+/// Two independent signal layers:
+/// 1. **Server-content**: `net/minecraft/block/`, `net/minecraft/tileentity/`,
+///    `net/minecraft/command/`, `net/minecraft/world/gen/`, `net/minecraft/entity/ai/`,
+///    `net/minecraft/pathfinding/`, `net/minecraft/server/`.
+/// 2. **Client-API**: `net/minecraft/client/`, `net/minecraftforge/client/`,
+///    `net/minecraftforge/fml/client/`.
+///
+/// The `@Mod` annotation (`net/minecraftforge/fml/common/Mod`) is deliberately
+/// excluded from both sets — it is mandatory boilerplate for EVERY Forge mod
+/// and says nothing about server compatibility.
+fn scan_jar_for_side(data: &[u8]) -> ScanResult {
+    let cursor = Cursor::new(data);
+    let mut archive = match ZipArchive::new(cursor) {
+        Ok(a) => a,
+        Err(_) => return ScanResult::NoClearSide,
+    };
+
+    let mut has_server_content = false;
+    let mut has_client_api = false;
+
+    for i in 0..archive.len() {
+        let mut file = match archive.by_index(i) {
+            Ok(f) => f,
+            Err(_) => continue,
+        };
+
+        let name = file.name().to_string();
+        if !name.ends_with(".class") {
+            continue;
+        }
+
+        let mut bytes = Vec::with_capacity(file.size() as usize);
+        if file.read_to_end(&mut bytes).is_err() {
+            continue;
+        }
+
+        let result = scan_single_class(&bytes);
+        match result {
+            ClassScan::HasServerContent => has_server_content = true,
+            ClassScan::HasClientApi => has_client_api = true,
+            ClassScan::Both => {
+                has_server_content = true;
+                has_client_api = true;
+            }
+            ClassScan::Neither => {}
+        }
+
+        if has_server_content && has_client_api {
+            return ScanResult::HasServer;
+        }
+    }
+
+    if has_server_content {
+        ScanResult::HasServer
+    } else if has_client_api {
+        ScanResult::OnlyClient
+    } else {
+        ScanResult::NoClearSide
+    }
+}
+
+#[derive(Debug, PartialEq)]
+enum ClassScan {
+    /// Found server-side content API refs (`net/minecraft/block/`, `net/minecraft/item/Item`,
+    /// `net/minecraft/tileentity/`, etc.) and/or direct `net/minecraft/server/` refs.
+    HasServerContent,
+    /// Found only client-side API refs (`net/minecraft/client/gui/`,
+    /// `net/minecraftforge/client/`, `net/minecraftforge/fml/client/`) —
+    /// no server or content API refs.
+    HasClientApi,
+    /// Found both server-content AND client-api refs in the same class.
+    Both,
+    /// No Minecraft references at all.
+    Neither,
+}
+
+/// Server-side content API path prefixes — mods referencing these
+/// provide actual world content (blocks, tile entities, commands,
+/// worldgen, AI/pathfinding).
+///
+/// `net/minecraft/item/Item` is deliberately excluded — the Item class is
+/// referenced by both client-only mods (item tooltips, inventory rendering)
+/// and server mods (item registration), so it's not a reliable indicator.
+/// The `ItemStack` substring makes it even worse.
+///
+/// Forge-specific packages (`net/minecraftforge/common/`, etc.) are also
+/// excluded — they are shared client+server APIs referenced by every
+/// Forge mod including purely client-side ones.
+const SERVER_CONTENT_PREFIXES: &[&[u8]] = &[
+    b"net/minecraft/block/",
+    b"net/minecraft/tileentity/",
+    b"net/minecraft/command/",
+    b"net/minecraft/world/gen/",
+    b"net/minecraft/entity/ai/",         // entity AI (server-side only)
+    b"net/minecraft/pathfinding/",       // pathfinding (server-side only)
+];
+
+/// Client-side API path prefixes — mods referencing ONLY these (without
+/// any SERVER_CONTENT_PREFIXES) are likely client-only.
+///
+/// `net/minecraft/client/` covers everything under the client package
+/// (Minecraft, gui, renderer, resources, settings, network handlers, etc.)
+/// — all are client-only classes that don't exist on the server.
+const CLIENT_API_PREFIXES: &[&[u8]] = &[
+    b"net/minecraft/client/",
+    b"net/minecraftforge/client/",
+    b"net/minecraftforge/fml/client/",
+];
+
+/// Check a single `.class` file's raw bytes for server-side content API vs
+/// client-side API references in the constant pool.
+///
+/// We parse the constant pool minimally to find `CONSTANT_Utf8` entries,
+/// which store the actual string literals (class names, method signatures, etc.)
+/// used by the bytecode. This avoids false positives from raw byte scanning.
+fn scan_single_class(data: &[u8]) -> ClassScan {
+    if data.len() < 10 {
+        return ClassScan::Neither;
+    }
+    if data[0..4] != [0xCA, 0xFE, 0xBA, 0xBE] {
+        return ClassScan::Neither;
+    }
+
+    let cp_count = read_u16_be(&data[8..10]) as usize;
+    if cp_count < 1 {
+        return ClassScan::Neither;
+    }
+
+    let mut pos: usize = 10;
+    let mut has_server = false;
+    let mut has_client = false;
+    let mut i = 1;
+
+    while i < cp_count && pos < data.len() {
+        if pos >= data.len() {
+            break;
+        }
+        let tag = data[pos];
+        pos += 1;
+
+        match tag {
+            1 => {
+                if pos + 2 > data.len() {
+                    break;
+                }
+                let len = read_u16_be(&data[pos..pos + 2]) as usize;
+                pos += 2;
+                if pos + len > data.len() {
+                    break;
+                }
+                let slice = &data[pos..pos + len];
+
+                if !has_server
+                    && (   bytes_contains(slice, b"net/minecraft/server/")
+                        || SERVER_CONTENT_PREFIXES.iter().any(|p| bytes_contains(slice, p)))
+                {
+                    has_server = true;
+                }
+                if !has_client
+                    && CLIENT_API_PREFIXES.iter().any(|p| bytes_contains(slice, p))
+                {
+                    has_client = true;
+                }
+                pos += len;
+
+                if has_server && has_client {
+                    return ClassScan::Both;
+                }
+            }
+            // CONSTANT_Integer, CONSTANT_Float, CONSTANT_Fieldref,
+            // CONSTANT_Methodref, CONSTANT_InterfaceMethodref,
+            // CONSTANT_Dynamic, CONSTANT_InvokeDynamic
+            3 | 4 | 9 | 10 | 11 | 12 | 17 | 18 => {
+                if pos + 4 > data.len() {
+                    break;
+                }
+                pos += 4;
+            }
+            // CONSTANT_Long, CONSTANT_Double (take 2 cp slots)
+            5 | 6 => {
+                if pos + 8 > data.len() {
+                    break;
+                }
+                pos += 8;
+                i += 1; // extra slot consumed
+            }
+            // CONSTANT_Class, CONSTANT_String, CONSTANT_MethodType,
+            // CONSTANT_Module, CONSTANT_Package
+            7 | 8 | 16 | 19 | 20 => {
+                if pos + 2 > data.len() {
+                    break;
+                }
+                pos += 2;
+            }
+            // CONSTANT_MethodHandle
+            15 => {
+                if pos + 3 > data.len() {
+                    break;
+                }
+                pos += 3;
+            }
+            // Unknown/invalid tag — bail to avoid misparse
+            _ => break,
+        }
+        i += 1;
+    }
+
+    if has_server && has_client {
+        ClassScan::Both
+    } else if has_server {
+        ClassScan::HasServerContent
+    } else if has_client {
+        ClassScan::HasClientApi
+    } else {
+        ClassScan::Neither
+    }
+}
+
+// ── Helpers ───────────────────────────────────────────────────────
+
+#[inline]
+fn read_u16_be(bytes: &[u8]) -> u16 {
+    u16::from_be_bytes([bytes[0], bytes[1]])
+}
+
+/// Check if `haystack` contains `needle` at any position.
+#[inline]
+fn bytes_contains(haystack: &[u8], needle: &[u8]) -> bool {
+    haystack.windows(needle.len()).any(|w| w == needle)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_read_u16_be() {
+        assert_eq!(read_u16_be(&[0x00, 0x01]), 1);
+        assert_eq!(read_u16_be(&[0x01, 0x00]), 256);
+        assert_eq!(read_u16_be(&[0xFF, 0xFF]), 65535);
+    }
+
+    #[test]
+    fn test_bytes_contains() {
+        assert!(bytes_contains(b"hello world", b"world"));
+        assert!(!bytes_contains(b"hello world", b"foo"));
+        assert!(bytes_contains(b"net/minecraft/server/MinecraftServer", b"net/minecraft/server/"));
+        assert!(bytes_contains(b"net/minecraft/client/Minecraft", b"net/minecraft/client/"));
+    }
+
+    #[test]
+    fn test_invalid_class_file() {
+        assert_eq!(scan_single_class(&[0x00; 4]), ClassScan::Neither);
+        assert_eq!(scan_single_class(&[]), ClassScan::Neither);
+    }
+
+    #[test]
+    fn test_minimal_class_file() {
+        // Valid magic + zero constant pool count — should not crash
+        let data = [0xCA, 0xFE, 0xBA, 0xBE, 0x00, 0x00, 0x00, 0x3D, 0x00, 0x00];
+        assert_eq!(scan_single_class(&data), ClassScan::Neither);
+    }
+
+    #[test]
+    fn test_empty_jar() {
+        // Build an empty valid zip
+        let buf = {
+            let mut cursor = Cursor::new(Vec::new());
+            let zip = zip::ZipWriter::new(&mut cursor);
+            zip.finish().unwrap();
+            cursor.into_inner()
+        };
+        assert_eq!(scan_jar_for_side(&buf), ScanResult::NoClearSide);
+    }
+}