Fix kernel pointer offset used by SYSCALL instruction (#90)

This PR fixes an issue introduced by #83 where the trap code for system
calls that use the `SYSCALL` instruction do not use the correct offset
in the per-CPU data structure when it retrieves the kernel stack
pointer. This was caused by a change to the `percpu_t` structure that
wasn't reflected in the offset the assembly language code uses.

include/kernel/infrastructure/i686/asm/descriptors.h

@@ -64,7 +64,7 @@
 #define GDT_USER_TLS_DATA        7
 
 /** number of descriptors in GDT */
-#define GDT_LENGTH               8
+#define GDT_NUM_ENTRIES          8
 
 /** offset of descriptor type in descriptor */
 #define SEG_FLAGS_OFFSET            40

include/kernel/infrastructure/i686/asm/percpu.h

@@ -0,0 +1,45 @@
+/*
+ * Copyright (C) 2024 Philippe Aubertin.
+ * All rights reserved.
+
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 
+ * 3. Neither the name of the author nor the names of other contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS "AS IS" AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef JINUE_KERNEL_INFRASTRUCTURE_I686_ASM_PERCPU_H
+#define JINUE_KERNEL_INFRASTRUCTURE_I686_ASM_PERCPU_H
+
+#include <kernel/infrastructure/i686/asm/descriptors.h>
+
+/* The PERCPU_OFFSET_... definitions here must match the offsets in the
+ * percpu_t struct. They are used by assembly language code that can't use the
+ * struct definition. */
+
+#define PERCPU_OFFSET_GDT  8
+
+#define PERCPU_OFFSET_TSS  (PERCPU_OFFSET_GDT + 8 * GDT_NUM_ENTRIES)
+
+#endif

include/kernel/infrastructure/i686/asm/tss.h

@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2024 Philippe Aubertin.
+ * All rights reserved.
+
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ * 
+ * 3. Neither the name of the author nor the names of other contributors
+ *    may be used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS "AS IS" AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+ * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR ANY
+ * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
+ * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
+ * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+ * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef JINUE_KERNEL_INFRASTRUCTURE_I686_ASM_TSS_H
+#define JINUE_KERNEL_INFRASTRUCTURE_I686_ASM_TSS_H
+
+#define TSS_OFFSET_ESP0 4
+
+#endif

include/kernel/infrastructure/i686/types.h

@@ -111,12 +111,11 @@ typedef struct {
 } tss_t;
 
 struct percpu_t {
+    /* Assembly language code accesses members in this structure. Make sure to
+     * update the PERCPU_OFFSET_... definitions when you change its layout. */
     struct percpu_t     *self;
     addr_space_t        *current_addr_space;
-    seg_descriptor_t     gdt[GDT_LENGTH];
-    /* The assembly-language system call entry point for the SYSCALL instruction
-     * (fast_amd_entry in trap.asm) makes assumptions regarding the location of
-     * the TSS within this structure. */
+    seg_descriptor_t     gdt[GDT_NUM_ENTRIES];
     tss_t                tss;
 };
 

kernel/infrastructure/i686/init.c

@@ -164,7 +164,7 @@ static void load_selectors(percpu_t *cpu_data, boot_alloc_t *boot_alloc) {
 
     /* load new GDT and TSS */
     pseudo->addr    = (addr_t)&cpu_data->gdt;
-    pseudo->limit   = GDT_LENGTH * 8 - 1;
+    pseudo->limit   = GDT_NUM_ENTRIES * 8 - 1;
 
     lgdt(pseudo);
 

kernel/interface/i686/trap.asm

@@ -28,6 +28,8 @@
 ; SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <kernel/infrastructure/i686/asm/descriptors.h>
+#include <kernel/infrastructure/i686/asm/percpu.h>
+#include <kernel/infrastructure/i686/asm/tss.h>
 #include <kernel/interface/i686/asm/irq.h>
 #include <kernel/machine/asm/machine.h>
 
@@ -261,9 +263,9 @@ fast_amd_entry:
     ; instruction.
     mov edx, SEG_SELECTOR(GDT_PER_CPU_DATA, RPL_KERNEL)
     mov gs, dx                          ; load gs with per-cpu data segment selector
-    mov esp, [gs:GDT_LENGTH * 8 + 4]    ; load kernel stack pointer from TSS
-                                        ; Stack pointer is at offset 4 in the TSS, and
-                                        ; the TSS follows the GDT (see percpu_t).
+    
+    ; load kernel stack pointer from TSS
+    mov esp, [gs:PERCPU_OFFSET_TSS + TSS_OFFSET_ESP0]
     
     ; For details on the stack layout, see comments in interrupt_entry above and
     ; the definition of the trapframe_t type.