Experiment with being more precise setting permissions for user pages.

Growing adds R|W pages (without X).  Exec() marks the stack only R|W.

Probably could setup permissions for text and data better if we call
ld with --no-omagic instead of -N.

kernel/defs.h

@@ -160,7 +160,7 @@ void            kvmmap(pagetable_t, uint64, uint64, uint64, int);
 int             mappages(pagetable_t, uint64, uint64, uint64, int);
 pagetable_t     uvmcreate(void);
 void            uvmfirst(pagetable_t, uchar *, uint);
-uint64          uvmalloc(pagetable_t, uint64, uint64);
+uint64          uvmalloc(pagetable_t, uint64, uint64, int);
 uint64          uvmdealloc(pagetable_t, uint64, uint64);
 int             uvmcopy(pagetable_t, pagetable_t, uint64);
 void            uvmfree(pagetable_t, uint64);

kernel/exec.c

@@ -49,7 +49,7 @@ exec(char *path, char **argv)
     if(ph.vaddr + ph.memsz < ph.vaddr)
       goto bad;
     uint64 sz1;
-    if((sz1 = uvmalloc(pagetable, sz, ph.vaddr + ph.memsz)) == 0)
+    if((sz1 = uvmalloc(pagetable, sz, ph.vaddr + ph.memsz, PTE_X|PTE_W)) == 0)
       goto bad;
     sz = sz1;
     if((ph.vaddr % PGSIZE) != 0)
@@ -69,7 +69,7 @@ exec(char *path, char **argv)
   // Use the second as the user stack.
   sz = PGROUNDUP(sz);
   uint64 sz1;
-  if((sz1 = uvmalloc(pagetable, sz, sz + 2*PGSIZE)) == 0)
+  if((sz1 = uvmalloc(pagetable, sz, sz + 2*PGSIZE, PTE_W)) == 0)
     goto bad;
   sz = sz1;
   uvmclear(pagetable, sz-2*PGSIZE);

kernel/proc.c

@@ -264,7 +264,7 @@ growproc(int n)
 
   sz = p->sz;
   if(n > 0){
-    if((sz = uvmalloc(p->pagetable, sz, sz + n)) == 0) {
+    if((sz = uvmalloc(p->pagetable, sz, sz + n, PTE_W)) == 0) {
       return -1;
     }
   } else if(n < 0){

kernel/vm.c

@@ -218,7 +218,7 @@ uvmfirst(pagetable_t pagetable, uchar *src, uint sz)
 // Allocate PTEs and physical memory to grow process from oldsz to
 // newsz, which need not be page aligned.  Returns new size or 0 on error.
 uint64
-uvmalloc(pagetable_t pagetable, uint64 oldsz, uint64 newsz)
+uvmalloc(pagetable_t pagetable, uint64 oldsz, uint64 newsz, int xperm)
 {
   char *mem;
   uint64 a;
@@ -234,7 +234,7 @@ uvmalloc(pagetable_t pagetable, uint64 oldsz, uint64 newsz)
       return 0;
     }
     memset(mem, 0, PGSIZE);
-    if(mappages(pagetable, a, PGSIZE, (uint64)mem, PTE_W|PTE_X|PTE_R|PTE_U) != 0){
+    if(mappages(pagetable, a, PGSIZE, (uint64)mem, PTE_R|PTE_U|xperm) != 0){
       kfree(mem);
       uvmdealloc(pagetable, a, oldsz);
       return 0;