One way of supporting a guard page below kstack: allocate kstacks in

procinit() and map them high up (below TRAMPOLNE) with an empty
mapping below each stack.  Never free a kernel stack.

Another way would be to allocate and map them dynamically, but then we
need to reload page table when switching processes in scheduler()
and/or have a kernel pagetable per proc (if we want k->stack to be the
same virtual address in each process).

One gotcha: kernel addresses are not equal to physical addresses for
stack addresses.  A stack address must be translated if we need its
physical address (e.g., virtio passes a stack address to the disk).

1 files changed, 7 insertions, 13 deletions

diff --git a/kernel/proc.c b/kernel/proc.c
index 6ba3fec..087d504 100644
--- a/kernel/proc.c
+++ b/kernel/proc.c
@@ -26,8 +26,14 @@ procinit(void)
   struct proc *p;
   
   initlock(&pid_lock, "nextpid");
-  for(p = proc; p < &proc[NPROC]; p++)
+  for(p = proc; p < &proc[NPROC]; p++) {
       initlock(&p->lock, "proc");
+      // Allocate a page for the kernel stack.
+      char *kstack = (char *) KSTACK((int) (p - proc));
+      if((p->kstack = map_kstack(kstack)) == 0) {
+        panic("procinit");
+      }
+  }
 }
 
 // Must be called with interrupts disabled,
@@ -94,16 +100,8 @@ allocproc(void)
 found:
   p->pid = allocpid();
 
-  // Allocate a page for the kernel stack.
-  if((p->kstack = kalloc()) == 0){
-    release(&p->lock);
-    return 0;
-  }
-
   // Allocate a trapframe page.
   if((p->tf = (struct trapframe *)kalloc()) == 0){
-    kfree(p->kstack);
-    p->kstack = 0;
     release(&p->lock);
     return 0;
   }
@@ -126,9 +124,6 @@ found:
 static void
 freeproc(struct proc *p)
 {
-  if(p->kstack)
-    kfree(p->kstack);
-  p->kstack = 0;
   if(p->tf)
     kfree((void*)p->tf);
   p->tf = 0;
@@ -651,4 +646,3 @@ procdump(void)
     printf("\n");
   }
 }
-