usermode: add simple printf() system call

The printf() system call accepts only 4 parameters: format string and for parameters for it. There is no variadic parameters support and there is also no checks for the sanity of the format string vs parameters list. Use with care. Signed-off-by: Pawel Wieczorkiewicz <[email protected]>
KernelTestFramework · Jul 22, 2022 · d120274 · d120274
1 parent 067f393
commit d120274
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Showing 2 changed files with 34 additions and 0 deletions.
diff --git a/common/usermode.c b/common/usermode.c
@@ -95,6 +95,11 @@ void __naked syscall_handler(void) {
     (void) param1;
     register unsigned long param2 asm(STR(_ASM_SI));
     (void) param2;
+    register unsigned long param3 asm(STR(_ASM_BX));
+    (void) param3;
+    register unsigned long param4 asm(STR(_ASM_DX));
+    (void) param4;
+
     SAVE_CLOBBERED_REGS();
     switch_address_space(&cr3);
     swapgs();
@@ -107,6 +112,11 @@ void __naked syscall_handler(void) {
         _sys_exit();
         UNREACHABLE();
 
+    case SYSCALL_PRINTF:
+        printk(_ptr(param1), param2, param3, param4);
+        syscall_return(0);
+        break;
+
     case SYSCALL_MMAP: {
         void *va = _ptr(param1);
         unsigned int order = _u(param2);
@@ -176,6 +186,22 @@ static inline void __user_text sys_exit(unsigned long exit_code) {
     asm volatile("syscall" ::"A"(SYSCALL_EXIT), "D"(exit_code) : STR(_ASM_CX), "r11");
 }
 
+static inline long __user_text sys_printf(const char *fmt, unsigned long arg1,
+                                          unsigned long arg2, unsigned long arg3) {
+    register unsigned long rax asm(STR(_ASM_AX));
+
+    /* clang-format off */
+    asm volatile(
+        "syscall"
+        : "=A"(rax)
+        : "0"(SYSCALL_PRINTF), "D"(fmt), "S"(arg1), "b"(arg2), "d"(arg3)
+        : STR(_ASM_CX), "r11"
+    );
+    /* clang-format on */
+
+    return rax;
+}
+
 static inline long __user_text sys_mmap(void *va, unsigned long order) {
     register unsigned long rax asm(STR(_ASM_AX));
 
@@ -209,6 +235,11 @@ void __user_text exit(unsigned long exit_code) {
     sys_exit(exit_code);
 }
 
+void __user_text printf(const char *fmt, unsigned long arg1, unsigned long arg2,
+                        unsigned long arg3) {
+    sys_printf(fmt, arg1, arg2, arg3);
+}
+
 void *__user_text mmap(void *va, unsigned long order) {
     return _ptr(sys_mmap(va, order));
 }

diff --git a/include/usermode.h b/include/usermode.h
@@ -26,6 +26,7 @@
 #define KTF_USERMODE_H
 
 #define SYSCALL_EXIT   0
+#define SYSCALL_PRINTF 1
 #define SYSCALL_MMAP   2
 #define SYSCALL_MUNMAP 3
 
@@ -53,6 +54,8 @@ extern void __naked syscall_handler(void);
 extern void init_usermode(percpu_t *percpu);
 
 extern void __user_text exit(unsigned long exit_code);
+extern void __user_text printf(const char *fmt, unsigned long arg1, unsigned long arg2,
+                               unsigned long arg3);
 extern void *__user_text mmap(void *va, unsigned long order);
 extern void __user_text munmap(void *va, unsigned long order);