Fix setup of usermode stack and added final system call to exit.

SConstruct

@@ -14,7 +14,7 @@ AddOption(
 target  = GetOption('target')
 
 env = Environment(tools =  ['default', 'compilation_db'])
-env.Append(CCFLAGS = ['-g', '-O0'])
+env.Append(CCFLAGS = ['-g', '-O0', '-fno-stack-protector'])
 env.Append(CPPDEFINES = ['BASTL_EXTENSIONS=1'])
 # env.Append(CCFLAGS = ['-O2'])
 

targets/_any/include/os/syscall.hpp

@@ -12,7 +12,8 @@ namespace baos
 enum class Syscall : std::uint64_t
 {
     FILE_READ = 0,
-    FILE_WRITE = 1
+    FILE_WRITE = 1,
+    EXIT = 0x3C
 };
 
 void setupSyscall() noexcept;

targets/_any/src/app/main.cpp

@@ -108,7 +108,7 @@ void cmdDumpPageTable() noexcept
 }
 }
 
-extern "C" void main()
+extern "C" void usermode_main()
 {
     std::string cmd;
     while(true)
@@ -132,6 +132,10 @@ extern "C" void main()
             cmdDumpPageTable();
             continue;
         }
+        else if (cmd == "exit")
+        {
+            return;
+        }
         std::printf("Unknown command: %s.\n", cmd.c_str());
     }
 }

targets/_any/src/os/syscall.cpp

@@ -46,5 +46,7 @@ extern "C" void __baosSyscall(baos::Syscall cmd, std::uint64_t param0, std::uint
         case Syscall::FILE_WRITE:
             sysFileWrite(static_cast<unsigned>(param0), std::bit_cast<const char*>(param1), param2);
             break;
+        case Syscall::EXIT:
+            break;
     }
 }

targets/x86_64/src/kernel/boot.s

@@ -103,6 +103,12 @@ __enterUsermode:
     movw %si, %fs
     movw %si, %gs
 
+    // the ABI expects the stack pointer to be 16-byte aligned BEFORE calling the function
+    // i.e. after pushing the instruction pointer, and when arriving in the function, it should be offset by 8 byte
+    // since we're not using a regular call, manually offset the stack pointer by 8 bytes and write the "return" address
+    subq $8, %r8
+    movq $__exitUsermode, (%r8)
+
     movq %rsp, (%rdx)
     pushq %rsi // data segment
     pushq %r8
@@ -111,10 +117,16 @@ __enterUsermode:
     pushq %rcx
     iretq
 
+__exitUsermode:
+    movq $0x3C, %rdi
+    syscall
+    jmp __exitUsermode
+
 .global __handleSyscall
 .type __handleSyscall @function
 // void __handleSyscall()
 __handleSyscall:
+    enter $0, $0
     pushq %rcx // store the original instruction pointer
     pushq %rsp // also store the original stack pointer
     movq %r8, %rcx
@@ -122,6 +134,7 @@ __handleSyscall:
     call __baosSyscall
     popq %rsp
     popq %rcx
+    leave
     sysretq
 
 /*

targets/x86_64/src/kernel/startup.cpp

@@ -186,7 +186,7 @@ extern "C"
 {
 EfiBootInfo* gBootInfo;
 
-void main();
+void usermode_main();
 
 void kernel_main()
 {
@@ -284,7 +284,7 @@ void kernel_main()
         initializePCIDevice(header);
     }
 
-    static std::array<std::uint8_t, 16384> usermodeStack;
-    __enterUsermode(SEGIDX_USER_CODE, SEGIDX_USER_DATA, &tss.rsp0, &main, &usermodeStack.back());
+    static std::array<std::uint8_t, 16384> usermodeStack alignas(16);
+    __enterUsermode(SEGIDX_USER_CODE, SEGIDX_USER_DATA, &tss.rsp0, &usermode_main, &usermodeStack.back() + 1);
 }
 } // extern "C"