sys/syscalls: fix invalid value returned to userland

Since 278687c (interrupts: add and use thread_get_interrupt_frame()),
we were storing the return value inside the interrupt_frame structure
saved inside struct process when an interrupt is triggered.

Changes to this frame are not reflected inside the actual frame used
when returning to userland. Because of this, the return value for every
syscall was that contained inside the EAX register before executing the
syscall.

Retreive and modify the actual interrupt frame located at the top of the
thread's kernel stack.

Author: d4ilyrun

kernel/arch/i686/syscalls.c

@@ -1,6 +1,10 @@
 #include <kernel/interrupts.h>
+#include <kernel/process.h>
 #include <kernel/syscalls.h>
 
+/*
+ *
+ */
 void arch_syscall_get_args(interrupt_frame *frame, syscall_args_t *args)
 {
     args->nr = frame->stub.eax;
@@ -12,7 +16,17 @@ void arch_syscall_get_args(interrupt_frame *frame, syscall_args_t *args)
     args->arg6 = frame->stub.ebp;
 }
 
-void arch_syscall_set_return_value(interrupt_frame *frame, u32 value)
+/*
+ *
+ */
+void arch_syscall_set_return_value(u32 value)
 {
+    struct interrupt_frame *frame;
+
+    /*
+     * Update the value inside the **actual** interrupt frame used when
+     * returning to userland.
+     */
+    frame = (void *)current->context.esp0 - sizeof(struct interrupt_frame);
     frame->stub.eax = value;
 }

kernel/sys/syscalls.c

@@ -45,7 +45,7 @@ static inline void trace_syscall_exit(const struct syscall *syscall, int ret)
 void arch_syscall_get_args(interrupt_frame *frame, syscall_args_t *args);
 
 /** Set the value to be returned by the syscall */
-void arch_syscall_set_return_value(interrupt_frame *frame, u32 value);
+void arch_syscall_set_return_value(u32 value);
 
 #define DECLARE_SYSCALL(name, vector, argc, ...) \
     [SYS_##name] = {#name, (void *)sys_##name, argc},
@@ -109,7 +109,7 @@ static interrupt_return_t syscall(void *data)
     trace_syscall_exit(syscall, ret);
 
 syscall_exit:
-    arch_syscall_set_return_value(frame, ret);
+    arch_syscall_set_return_value(ret);
     return INTERRUPT_HANDLED;
 }