From ef1763334bd3159190bfc200f2581260cf23b7af Mon Sep 17 00:00:00 2001
From: Michael Scire <SciresM@gmail.com>
Date: Mon, 13 Jul 2020 19:16:05 -0700
Subject: [PATCH] kern: patch svc tables via asm, instead of relying on
 compiler to be nice

---
 .../source/arch/arm64/svc/kern_svc_tables.cpp | 19 ++++++-------
 .../arch/arm64/svc/kern_svc_tables_asm.s      | 27 +++++++++++++++++++
 2 files changed, 37 insertions(+), 9 deletions(-)
 create mode 100644 mesosphere/kernel/source/arch/arm64/svc/kern_svc_tables_asm.s

diff --git a/libraries/libmesosphere/source/arch/arm64/svc/kern_svc_tables.cpp b/libraries/libmesosphere/source/arch/arm64/svc/kern_svc_tables.cpp
index e2a1684fa..9e7510f1f 100644
--- a/libraries/libmesosphere/source/arch/arm64/svc/kern_svc_tables.cpp
+++ b/libraries/libmesosphere/source/arch/arm64/svc/kern_svc_tables.cpp
@@ -92,6 +92,8 @@ namespace ams::kern::svc {
 
     constinit const std::array<SvcTableEntry, NumSupervisorCalls> SvcTable64From32 = SvcTable64From32Impl;
 
+    void PatchSvcTableEntry(const SvcTableEntry *table, u32 id, SvcTableEntry entry);
+
     namespace {
 
         /* NOTE: Although the SVC tables are constants, our global constructor will run before .rodata is protected R--. */
@@ -101,34 +103,33 @@ namespace ams::kern::svc {
             private:
                 static SvcTablePatcher s_instance;
             private:
-                ALWAYS_INLINE volatile SvcTableEntry *GetEditableTable(const SvcTable *table) {
+                ALWAYS_INLINE const SvcTableEntry *GetTableData(const SvcTable *table) {
                     if (table != nullptr) {
-                        return const_cast<volatile SvcTableEntry *>(table->data());
+                        return table->data();
                     } else {
                         return nullptr;
                     }
                 }
 
-                /* TODO: This should perhaps be implemented in assembly. */
-                NOINLINE void PatchTables(volatile SvcTableEntry *table_64, volatile SvcTableEntry *table_64_from_32) {
+                NOINLINE void PatchTables(const SvcTableEntry *table_64, const SvcTableEntry *table_64_from_32) {
                     /* Get the target firmware. */
                     const auto target_fw = kern::GetTargetFirmware();
 
                     /* 10.0.0 broke the ABI for QueryIoMapping. */
                     if (target_fw < TargetFirmware_10_0_0) {
-                        if (table_64)         { table_64[        svc::SvcId_QueryIoMapping] = LegacyQueryIoMapping::Call64; }
-                        if (table_64_from_32) { table_64_from_32[svc::SvcId_QueryIoMapping] = LegacyQueryIoMapping::Call64From32; }
+                        if (table_64)         { ::ams::kern::svc::PatchSvcTableEntry(table_64,         svc::SvcId_QueryIoMapping, LegacyQueryIoMapping::Call64); }
+                        if (table_64_from_32) { ::ams::kern::svc::PatchSvcTableEntry(table_64_from_32, svc::SvcId_QueryIoMapping, LegacyQueryIoMapping::Call64From32); }
                     }
 
                     /* 3.0.0 broke the ABI for ContinueDebugEvent. */
                     if (target_fw < TargetFirmware_3_0_0) {
-                        if (table_64)         { table_64[        svc::SvcId_ContinueDebugEvent] = LegacyContinueDebugEvent::Call64; }
-                        if (table_64_from_32) { table_64_from_32[svc::SvcId_ContinueDebugEvent] = LegacyContinueDebugEvent::Call64From32; }
+                        if (table_64)         { ::ams::kern::svc::PatchSvcTableEntry(table_64,         svc::SvcId_ContinueDebugEvent, LegacyContinueDebugEvent::Call64); }
+                        if (table_64_from_32) { ::ams::kern::svc::PatchSvcTableEntry(table_64_from_32, svc::SvcId_ContinueDebugEvent, LegacyContinueDebugEvent::Call64From32); }
                     }
                 }
             public:
                 SvcTablePatcher(const SvcTable *table_64, const SvcTable *table_64_from_32) {
-                    PatchTables(GetEditableTable(table_64), GetEditableTable(table_64_from_32));
+                    PatchTables(GetTableData(table_64), GetTableData(table_64_from_32));
                 }
         };
 
diff --git a/mesosphere/kernel/source/arch/arm64/svc/kern_svc_tables_asm.s b/mesosphere/kernel/source/arch/arm64/svc/kern_svc_tables_asm.s
new file mode 100644
index 000000000..324bdd5a4
--- /dev/null
+++ b/mesosphere/kernel/source/arch/arm64/svc/kern_svc_tables_asm.s
@@ -0,0 +1,27 @@
+/*
+ * Copyright (c) 2018-2020 Atmosphère-NX
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms and conditions of the GNU General Public License,
+ * version 2, as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+/* ams::kern::svc::PatchSvcTableEntry(void (* const*)(), unsigned int, void (*)()) */
+.section    .text._ZN3ams4kern3svc18PatchSvcTableEntryEPKPFvvEjS3_, "ax", %progbits
+.global     _ZN3ams4kern3svc18PatchSvcTableEntryEPKPFvvEjS3_
+.type       _ZN3ams4kern3svc18PatchSvcTableEntryEPKPFvvEjS3_, %function
+_ZN3ams4kern3svc18PatchSvcTableEntryEPKPFvvEjS3_:
+    /* This function violates const correctness by design, to patch the svc tables. */
+    /* The svc tables live in .rodata (.rel.ro), but must be patched by initial constructors */
+    /* to support firmware-specific table entries. */
+    mov w1, w1
+    str x2, [x0, x1, lsl #3]
+    ret
\ No newline at end of file