Kernel: Don't trigger page faults during profiling stack walk

The kernel sampling profiler will walk thread stacks during the timer
tick handler. Since it's not safe to trigger page faults during IRQ's,
we now avoid this by checking the page tables manually before accessing
each stack location.

1 files changed, 26 insertions, 0 deletions

diff --git a/Kernel/VM/MemoryManager.cpp b/Kernel/VM/MemoryManager.cpp
index 1e71e3172e..c9a1846733 100644
--- a/Kernel/VM/MemoryManager.cpp
+++ b/Kernel/VM/MemoryManager.cpp
@@ -191,6 +191,21 @@ void MemoryManager::parse_memory_map()
         m_user_physical_pages += region.finalize_capacity();
 }
 
+const PageTableEntry* MemoryManager::pte(const PageDirectory& page_directory, VirtualAddress vaddr)
+{
+    ASSERT_INTERRUPTS_DISABLED();
+    u32 page_directory_table_index = (vaddr.get() >> 30) & 0x3;
+    u32 page_directory_index = (vaddr.get() >> 21) & 0x1ff;
+    u32 page_table_index = (vaddr.get() >> 12) & 0x1ff;
+
+    auto* pd = quickmap_pd(const_cast<PageDirectory&>(page_directory), page_directory_table_index);
+    const PageDirectoryEntry& pde = pd[page_directory_index];
+    if (!pde.is_present())
+        return nullptr;
+
+    return &quickmap_pt(PhysicalAddress((uintptr_t)pde.page_table_base()))[page_table_index];
+}
+
 PageTableEntry& MemoryManager::ensure_pte(PageDirectory& page_directory, VirtualAddress vaddr)
 {
     ASSERT_INTERRUPTS_DISABLED();
@@ -611,6 +626,17 @@ bool MemoryManager::validate_kernel_read(const Process& process, VirtualAddress
     return validate_range<AccessSpace::Kernel, AccessType::Read>(process, vaddr, size);
 }
 
+bool MemoryManager::can_read_without_faulting(const Process& process, VirtualAddress vaddr, size_t size) const
+{
+    // FIXME: Use the size argument!
+    UNUSED_PARAM(size);
+    auto* pte = const_cast<MemoryManager*>(this)->pte(process.page_directory(), vaddr);
+    if (!pte)
+        return false;
+    return pte->is_present();
+}
+
+
 bool MemoryManager::validate_user_read(const Process& process, VirtualAddress vaddr, size_t size) const
 {
     if (!is_user_address(vaddr))