[3/3] mini-os: mm: convert set_readonly() to use walk_pt()

Commit Message

Jürgen Groß July 31, 2024, 1 p.m. UTC

Instead of having another copy of a page table walk in set_readonly(),
just use walk_pt().

As it will be needed later anyway, split out the TLB flushing into a
dedicated function.

Signed-off-by: Juergen Gross <[e-mail]>
---
 arch/x86/mm.c | 119 +++++++++++++++++++++-----------------------------
 1 file changed, 50 insertions(+), 69 deletions(-)

Comments

Samuel Thibault July 31, 2024, 9:37 p.m. UTC | #1

Juergen Gross, le mer. 31 juil. 2024 15:00:26 +0200, a ecrit:
> +static int set_readonly_func(unsigned long va, unsigned int lvl, bool is_leaf,
> +                             pgentry_t *pte, void *par)
> +{
> +    struct set_readonly_par *ro = par;
>  
> +    mmu_updates[ro->count].ptr = virt_to_mach(pte);
> +    mmu_updates[ro->count].val = *pte & ~_PAGE_RW;
> +    ro->count++;
> +
> +    if ( (ro->count == L1_PAGETABLE_ENTRIES ||
> +          va + 2 * PAGE_SIZE > ro->etext) &&
> +         HYPERVISOR_mmu_update(mmu_updates, ro->count, NULL, DOMID_SELF) < 0 )
> +    {
> +        printk("ERROR: set_readonly(): PTE could not be updated\n");
> +        do_exit();
> +    }

Don't we also want to set ro->count to 0?
And assert that it is 0 after calling walk_pt in set_readonly, to make
sure the va + 2 * PAGE_SIZE > ro->etext test did work properly
(personally I would have rather made set_readonly call a last
HYPERVISOR_mmu_update in case ro->count is not 0, which looks more
robust that a quite magic-looking va + 2 * PAGE_SIZE > ro->etext test)

Samuel

Jürgen Groß Aug. 1, 2024, 6 a.m. UTC | #2

On 31.07.24 23:37, Samuel Thibault wrote:
> Juergen Gross, le mer. 31 juil. 2024 15:00:26 +0200, a ecrit:
>> +static int set_readonly_func(unsigned long va, unsigned int lvl, bool is_leaf,
>> +                             pgentry_t *pte, void *par)
>> +{
>> +    struct set_readonly_par *ro = par;
>>   
>> +    mmu_updates[ro->count].ptr = virt_to_mach(pte);
>> +    mmu_updates[ro->count].val = *pte & ~_PAGE_RW;
>> +    ro->count++;
>> +
>> +    if ( (ro->count == L1_PAGETABLE_ENTRIES ||
>> +          va + 2 * PAGE_SIZE > ro->etext) &&
>> +         HYPERVISOR_mmu_update(mmu_updates, ro->count, NULL, DOMID_SELF) < 0 )
>> +    {
>> +        printk("ERROR: set_readonly(): PTE could not be updated\n");
>> +        do_exit();
>> +    }
> 
> Don't we also want to set ro->count to 0?

Oh, indeed. Thanks for catching this.

> And assert that it is 0 after calling walk_pt in set_readonly, to make
> sure the va + 2 * PAGE_SIZE > ro->etext test did work properly
> (personally I would have rather made set_readonly call a last
> HYPERVISOR_mmu_update in case ro->count is not 0, which looks more
> robust that a quite magic-looking va + 2 * PAGE_SIZE > ro->etext test)

I think you are right. I'll do that.


Juergen

Patch

diff --git a/arch/x86/mm.c b/arch/x86/mm.c
index accde291..90992068 100644
--- a/arch/x86/mm.c
+++ b/arch/x86/mm.c
@@ -397,92 +397,73 @@  static void build_pagetable(unsigned long *start_pfn, unsigned long *max_pfn)
  * Mark portion of the address space read only.
  */
 extern struct shared_info shared_info;
-static void set_readonly(void *text, void *etext)
-{
-    unsigned long start_address =
-        ((unsigned long) text + PAGE_SIZE - 1) & PAGE_MASK;
-    unsigned long end_address = (unsigned long) etext;
-    pgentry_t *tab = pt_base, page;
-    unsigned long mfn = pfn_to_mfn(virt_to_pfn(pt_base));
-    unsigned long offset;
-    unsigned long page_size = PAGE_SIZE;
+
+struct set_readonly_par {
+    unsigned long etext;
 #ifdef CONFIG_PARAVIRT
-    int count = 0;
-    int rc;
+    unsigned int count;
 #endif
+};
 
-    printk("setting %p-%p readonly\n", text, etext);
+static int set_readonly_func(unsigned long va, unsigned int lvl, bool is_leaf,
+                             pgentry_t *pte, void *par)
+{
+    struct set_readonly_par *ro = par;
 
-    while ( start_address + page_size <= end_address )
-    {
-        tab = pt_base;
-        mfn = pfn_to_mfn(virt_to_pfn(pt_base));
+    if ( !is_leaf )
+        return 0;
 
-#if defined(__x86_64__)
-        offset = l4_table_offset(start_address);
-        page = tab[offset];
-        mfn = pte_to_mfn(page);
-        tab = to_virt(mfn_to_pfn(mfn) << PAGE_SHIFT);
-#endif
-        offset = l3_table_offset(start_address);
-        page = tab[offset];
-        mfn = pte_to_mfn(page);
-        tab = to_virt(mfn_to_pfn(mfn) << PAGE_SHIFT);
-        offset = l2_table_offset(start_address);        
-        if ( !(tab[offset] & _PAGE_PSE) )
-        {
-            page = tab[offset];
-            mfn = pte_to_mfn(page);
-            tab = to_virt(mfn_to_pfn(mfn) << PAGE_SHIFT);
+    if ( va + (1UL << ptdata[lvl].shift) > ro->etext )
+        return 1;
 
-            offset = l1_table_offset(start_address);
-        }
+    if ( va == (unsigned long)&shared_info )
+    {
+        printk("skipped %lx\n", va);
+        return 0;
+    }
 
-        if ( start_address != (unsigned long)&shared_info )
-        {
 #ifdef CONFIG_PARAVIRT
-            mmu_updates[count].ptr = 
-                ((pgentry_t)mfn << PAGE_SHIFT) + sizeof(pgentry_t) * offset;
-            mmu_updates[count].val = tab[offset] & ~_PAGE_RW;
-            count++;
+    mmu_updates[ro->count].ptr = virt_to_mach(pte);
+    mmu_updates[ro->count].val = *pte & ~_PAGE_RW;
+    ro->count++;
+
+    if ( (ro->count == L1_PAGETABLE_ENTRIES ||
+          va + 2 * PAGE_SIZE > ro->etext) &&
+         HYPERVISOR_mmu_update(mmu_updates, ro->count, NULL, DOMID_SELF) < 0 )
+    {
+        printk("ERROR: set_readonly(): PTE could not be updated\n");
+        do_exit();
+    }
 #else
-            tab[offset] &= ~_PAGE_RW;
+    *pte &= ~_PAGE_RW;
 #endif
-        }
-        else
-            printk("skipped %lx\n", start_address);
 
-        start_address += page_size;
+    return 0;
+}
 
 #ifdef CONFIG_PARAVIRT
-        if ( count == L1_PAGETABLE_ENTRIES || 
-             start_address + page_size > end_address )
-        {
-            rc = HYPERVISOR_mmu_update(mmu_updates, count, NULL, DOMID_SELF);
-            if ( rc < 0 )
-            {
-                printk("ERROR: set_readonly(): PTE could not be updated\n");
-                do_exit();
-            }
-            count = 0;
-        }
-#else
-        if ( start_address == (1UL << L2_PAGETABLE_SHIFT) )
-            page_size = 1UL << L2_PAGETABLE_SHIFT;
-#endif
-    }
+static void tlb_flush(void)
+{
+    mmuext_op_t op = { .cmd = MMUEXT_TLB_FLUSH_ALL };
+    int count;
 
-#ifdef CONFIG_PARAVIRT
-    {
-        mmuext_op_t op = {
-            .cmd = MMUEXT_TLB_FLUSH_ALL,
-        };
-        int count;
-        HYPERVISOR_mmuext_op(&op, 1, &count, DOMID_SELF);
-    }
+    HYPERVISOR_mmuext_op(&op, 1, &count, DOMID_SELF);
+}
 #else
+static void tlb_flush(void)
+{
     write_cr3((unsigned long)pt_base);
+}
 #endif
+
+static void set_readonly(void *text, void *etext)
+{
+    struct set_readonly_par setro = { .etext = (unsigned long)etext };
+    unsigned long start_address = PAGE_ALIGN((unsigned long)text);
+
+    printk("setting %p-%p readonly\n", text, etext);
+    walk_pt(start_address, setro.etext, set_readonly_func, &setro);
+    tlb_flush();
 }
 
 /*