usermode: add exit system call

The exit system call has index 0. It is used by usermode_stub() to exit back to kernel, when user task is finished. Signed-off-by: Pawel Wieczorkiewicz <[email protected]>
KernelTestFramework · Jul 22, 2022 · 874ff2f · 874ff2f
1 parent c9a195d
commit 874ff2f
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Showing 5 changed files with 68 additions and 15 deletions.
diff --git a/arch/x86/entry.S b/arch/x86/entry.S
@@ -160,7 +160,10 @@ ENTRY(usermode_stub)
     xchg %_ASM_DI, %_ASM_SI
     call *%_ASM_SI
 
-    int $X86_RET2KERN_INT
+    /* sys_exit */
+    mov %_ASM_AX, %_ASM_DI
+    xor %_ASM_AX, %_ASM_AX
+    syscall
 END_FUNC(usermode_stub)
 
 SECTION(.data.rmode, "aw", 16)

diff --git a/arch/x86/traps.c b/arch/x86/traps.c
@@ -42,14 +42,6 @@ extern void asm_interrupt_handler_uart(void);
 extern void asm_interrupt_handler_keyboard(void);
 extern void asm_interrupt_handler_timer(void);
 
-static void ret2kern_handler(void) {
-    /* clang-format off */
-    asm volatile("mov %%gs:%[sp], %%" STR(_ASM_SP) "\n"
-                 "POPF \n"
-                 :: [ sp ] "m"(ACCESS_ONCE(PERCPU_VAR(usermode_private))));
-    /* clang-format on */
-}
-
 static void init_tss(percpu_t *percpu) {
 #if defined(__i386__)
     percpu->tss_df.iopb = sizeof(percpu->tss_df);
@@ -155,10 +147,6 @@ void init_traps(const cpu_t *cpu) {
     set_intr_gate(&percpu->idt[X86_EX_DF],  __KERN_CS, _ul(entry_DF),  GATE_DPL0, GATE_PRESENT, 1);
 #endif
 
-    /* User mode return to kernel handler */
-    set_intr_gate(&percpu->idt[X86_RET2KERN_INT], __KERN_CS, _ul(ret2kern_handler), GATE_DPL3, GATE_PRESENT, 0);
-    /* clang-format on */
-
     set_intr_gate(&percpu->idt[COM1_IRQ0_OFFSET], __KERN_CS,
                   _ul(asm_interrupt_handler_uart), GATE_DPL0, GATE_PRESENT, 0);
     set_intr_gate(&percpu->idt[COM2_IRQ0_OFFSET], __KERN_CS,

diff --git a/common/usermode.c b/common/usermode.c
@@ -52,6 +52,15 @@ static inline long syscall_return(long return_code) {
     return ret;
 }
 
+static inline void stack_switch(void) {
+    /* clang-format off */
+    asm volatile(
+        "xchg %%gs:%[private], %%" STR(_ASM_SP) "\n"
+        ::[private] "m"(ACCESS_ONCE(PERCPU_VAR(usermode_private)))
+    );
+    /* clang-format on */
+}
+
 static inline void switch_address_space(const cr3_t *cr3) {
     /* clang-format off */
     asm volatile(
@@ -65,7 +74,49 @@ static inline void switch_address_space(const cr3_t *cr3) {
     /* clang-format on */
 }
 
+static inline void _sys_exit(void) {
+    /* clang-format off */
+    asm volatile (
+        "mov %%" STR(_ASM_DI)", %%gs:%[private]\n"
+        POPF()
+        RESTORE_ALL_REGS()
+        "mov %%gs:%[private], %%" STR(_ASM_AX) "\n"
+        "ret\n"
+        :: [ private ] "m"(ACCESS_ONCE(PERCPU_VAR(usermode_private)))
+        : STR(_ASM_AX)
+    );
+    /* clang-format on */
+}
+
 void __naked syscall_handler(void) {
+    register unsigned long syscall_nr asm(STR(_ASM_AX));
+    register unsigned long param1 asm(STR(_ASM_DI));
+    (void) param1;
+    asm volatile(SAVE_CLOBBERED_REGS():::);
+    switch_address_space(&cr3);
+    swapgs();
+    stack_switch();
+    syscall_save();
+
+    switch (syscall_nr) {
+    case SYSCALL_EXIT:
+        syscall_restore();
+        _sys_exit();
+        UNREACHABLE();
+
+    default:
+        printk("Unknown syscall: %lu\n", syscall_nr);
+        syscall_return(-1L);
+        break;
+    }
+
+    syscall_restore();
+    stack_switch();
+    swapgs();
+    switch_address_space(&user_cr3);
+
+    asm volatile(RESTORE_CLOBBERED_REGS():::);
+
     sysret();
 }
 
@@ -92,5 +143,15 @@ static void init_syscall(void) {
 void init_usermode(percpu_t *percpu) {
     vmap_user_4k(&cr3, virt_to_mfn(&cr3), L1_PROT);
     vmap_user_4k(&enter_usermode, virt_to_mfn(&enter_usermode), L1_PROT);
+    vmap_user_4k(&syscall_handler, virt_to_mfn(&syscall_handler), L1_PROT);
+
     init_syscall();
 }
+
+static inline void __user_text sys_exit(unsigned long exit_code) {
+    asm volatile("syscall" ::"A"(SYSCALL_EXIT), "D"(exit_code) : STR(_ASM_CX), "r11");
+}
+
+void __user_text exit(unsigned long exit_code) {
+    sys_exit(exit_code);
+}
diff --git a/include/arch/x86/traps.h b/include/arch/x86/traps.h
@@ -30,8 +30,6 @@
 
 #define MAX_INT 256
 
-#define X86_RET2KERN_INT 32
-
 #ifndef __ASSEMBLY__
 #include <cpu.h>
 

diff --git a/include/usermode.h b/include/usermode.h
@@ -25,6 +25,8 @@
 #ifndef KTF_USERMODE_H
 #define KTF_USERMODE_H
 
+#define SYSCALL_EXIT 0
+
 #ifndef __ASSEMBLY__
 #include <percpu.h>
 #include <sched.h>
@@ -43,6 +45,7 @@ extern void __naked syscall_handler(void);
 
 extern void init_usermode(percpu_t *percpu);
 
+extern void __user_text exit(unsigned long exit_code);
 #endif /* __ASSEMBLY__ */
 
 #endif /* KTF_USERMODE_H */