[ips_layer] read magic from IPS file and fix EOF check (#4469)

- [x] I have read and followed the [Contribution Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/CONTRIBUTING.md#code-contributions).
- [x] I have read and followed the [AI Policy](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/AI.md)
- [x] I have read and followed the [Coding Guidelines](https://git.eden-emu.dev/eden-emu/eden/src/branch/master/docs/policies/Coding.md) to the best of my ability.

-------------------
PatchIPS was reading the magic bytes from the target file instead
of the IPS patch, and IsEOF used a strict > comparison that never
matched for 3-byte IPS EOF markers. This caused IPS patches to be
silently skipped, breaking the Russian localization mod for
Persona 5.

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4469
Reviewed-by: lizzie <lizzie@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/src/core/file_sys/ips_layer.cpp b/src/core/file_sys/ips_layer.cpp
index 5a0ddb44f8..32e9041017 100644
--- a/src/core/file_sys/ips_layer.cpp
+++ b/src/core/file_sys/ips_layer.cpp
@@ -35,19 +35,22 @@ static IPSFileType IdentifyMagic(std::span<const u8> magic) {
 }
 
 static bool IsEOF(IPSFileType type, std::span<const u8> magic) {
-    return (type == IPSFileType::IPS && magic.size() > 3 && std::memcmp(magic.data(), "EOF", 3) == 0)
-        || (type == IPSFileType::IPS32 && magic.size() > 4 && std::memcmp(magic.data(), "EEOF", 4) == 0);
+    return (type == IPSFileType::IPS && magic.size() >= 3 && std::memcmp(magic.data(), "EOF", 3) == 0)
+        || (type == IPSFileType::IPS32 && magic.size() >= 4 && std::memcmp(magic.data(), "EEOF", 4) == 0);
 }
 
 VirtualFile PatchIPS(const VirtualFile& in, const VirtualFile& ips) {
     if (in == nullptr || ips == nullptr)
         return nullptr;
 
-    auto in_data = in->ReadAllBytes();
-    auto const type = IdentifyMagic(in_data);
+    const auto type = IdentifyMagic(ips->ReadBytes(0x5));
     if (type == IPSFileType::Error)
         return nullptr;
 
+    auto in_data = in->ReadAllBytes();
+    if (in_data.size() == 0)
+        return nullptr;
+
     std::vector<u8> temp(type == IPSFileType::IPS ? 3 : 4);
     u64 offset = 5; // After header
     while (ips->Read(temp.data(), temp.size(), offset) == temp.size()) {