Keep kernel GS active on userland exit fast-path

Author: pradosh-arduino

source/includes/tty.h

@@ -10,5 +10,6 @@
 void tty_init(void);
 void tty_input_char(char c);
 int tty_read(char* buf, uint64_t count);
+void tty_flush_input(void);
 
 #endif

source/kernel/C/interrupts/syscalls-x86_64.asm

@@ -1,6 +1,15 @@
 global syscall_entry
 extern syscall_handler
 extern kernel_stack_top
+extern userland_should_return_kernel
+extern userland_resume_rsp
+extern userland_resume_rip
+extern userland_resume_rbx
+extern userland_resume_rbp
+extern userland_resume_r12
+extern userland_resume_r13
+extern userland_resume_r14
+extern userland_resume_r15
 
 section .bss
 align 8
@@ -54,6 +63,21 @@ syscall_entry:
     ; Restore userland RBX prior to iretq.
     mov rbx, [rel saved_user_rbx]
 
+    cmp byte [rel userland_should_return_kernel], 0
+    je .return_to_user
+
+    mov rbx, [rel userland_resume_rbx]
+    mov rbp, [rel userland_resume_rbp]
+    mov r12, [rel userland_resume_r12]
+    mov r13, [rel userland_resume_r13]
+    mov r14, [rel userland_resume_r14]
+    mov r15, [rel userland_resume_r15]
+    mov rsp, [rel userland_resume_rsp]
+    mov rax, [rel userland_resume_rip]
+    jmp rax
+
+.return_to_user:
+
     ; Return to user
     swapgs
     iretq

source/kernel/C/shell/commands/exec.c

@@ -13,6 +13,7 @@
 #include <executables/elf.h>
 #include <userland.h>
 #include <graphics.h>
+#include <tty.h>
 
 int cmd_exec(int argc, char** argv)
 {
@@ -37,10 +38,12 @@ int cmd_exec(int argc, char** argv)
 
     user_argv[user_argc] = NULL;
 
+    tty_flush_input();
+
     if (userland_exec(path, user_argc, user_argv, NULL) != 0) {
         eprintf("exec: failed to load ELF");
         return -1;
     }
 
     return 0;
-}
+}

source/kernel/C/user-input/tty.c

@@ -86,3 +86,8 @@ int tty_read(char* buf, uint64_t count) {
 
     return (int)read;
 }
+
+void tty_flush_input(void) {
+    rb_clear(&cooked_rb);
+    line_len = 0;
+}

source/kernel/C/userland.c

@@ -13,8 +13,15 @@ static uint64_t user_heap_mapped_end = USER_HEAP_VADDR;
 static uint64_t user_mmap_cursor = USER_MMAP_VADDR;
 static uint64_t user_mmap_end = USER_MMAP_VADDR;
 static volatile bool userland_running = false;
-static volatile uint64_t userland_resume_rip = 0;
-static volatile uint64_t userland_resume_rsp = 0;
+volatile bool userland_should_return_kernel = false;
+volatile uint64_t userland_resume_rip = 0;
+volatile uint64_t userland_resume_rsp = 0;
+volatile uint64_t userland_resume_rbx = 0;
+volatile uint64_t userland_resume_rbp = 0;
+volatile uint64_t userland_resume_r12 = 0;
+volatile uint64_t userland_resume_r13 = 0;
+volatile uint64_t userland_resume_r14 = 0;
+volatile uint64_t userland_resume_r15 = 0;
 static volatile int userland_last_exit_code = 0;
 
 static void debug_dump_initial_stack(uint64_t stack_top) {
@@ -329,17 +336,13 @@ uint64_t userland_mmap_anon(uint64_t length) {
 }
 
 bool userland_prepare_exit(syscall_frame_t* frame, uint64_t exit_code) {
-    if (!frame || !userland_running || !userland_resume_rip || !userland_resume_rsp)
+    (void)frame;
+
+    if (!userland_running || !userland_resume_rip || !userland_resume_rsp)
         return false;
 
     userland_last_exit_code = (int)exit_code;
-
-    frame->rip = userland_resume_rip;
-    frame->cs = 0x08;
-    frame->rflags = 0x202;
-    frame->rsp = userland_resume_rsp;
-    frame->ss = 0x10;
-    frame->rax = exit_code;
+    userland_should_return_kernel = true;
     return true;
 }
 
@@ -413,8 +416,15 @@ int userland_exec(const char* path, int argc, const char* const* argv, const cha
 
     uint64_t kernel_rsp = 0;
     asm volatile("mov %%rsp, %0" : "=r"(kernel_rsp));
+    asm volatile("mov %%rbx, %0" : "=r"(userland_resume_rbx));
+    asm volatile("mov %%rbp, %0" : "=r"(userland_resume_rbp));
+    asm volatile("mov %%r12, %0" : "=r"(userland_resume_r12));
+    asm volatile("mov %%r13, %0" : "=r"(userland_resume_r13));
+    asm volatile("mov %%r14, %0" : "=r"(userland_resume_r14));
+    asm volatile("mov %%r15, %0" : "=r"(userland_resume_r15));
     userland_resume_rsp = kernel_rsp;
     userland_resume_rip = (uint64_t)&&userland_return_label;
+    userland_should_return_kernel = false;
     userland_last_exit_code = 0;
     userland_running = true;
 
@@ -443,12 +453,18 @@ int userland_exec(const char* path, int argc, const char* const* argv, const cha
 
 userland_return_label:
     userland_running = false;
+    userland_should_return_kernel = false;
     userland_resume_rip = 0;
     userland_resume_rsp = 0;
+    userland_resume_rbx = 0;
+    userland_resume_rbp = 0;
+    userland_resume_r12 = 0;
+    userland_resume_r13 = 0;
+    userland_resume_r14 = 0;
+    userland_resume_r15 = 0;
     wrmsr64_local(IA32_FS_BASE_MSR, 0);
     userland_unmap_all();
     userland_heap_init();
     printf(blue_color "\n[process exited with code %d]" reset_color, userland_last_exit_code);
-    hcf2();
     return 0;
 }