qemu: add cpu page_size call (#1433)

* qemu handy cpu page size call proposal.

* changes from feedback.

libafl_qemu/libafl_qemu_build/src/bindings.rs

@@ -120,6 +120,7 @@ pub fn generate(
         .allowlist_function("syx_snapshot_root_restore")
         .allowlist_function("syx_snapshot_dirty_list_add")
         .allowlist_function("device_list_all")
+        .allowlist_function("qemu_target_page_size")
         .blocklist_function("main_loop_wait") // bindgen issue #1313
         .parse_callbacks(Box::new(bindgen::CargoCallbacks));
 

libafl_qemu/libafl_qemu_sys/src/x86_64_stub_bindings.rs

@@ -11100,6 +11100,12 @@ impl ::std::ops::BitAndAssign for qemu_plugin_mem_rw {
         self.0 &= rhs.0;
     }
 }
+
+extern "C" {
+    #[doc = " qemu_target_page_size - return the target's page size"]
+    pub fn qemu_target_page_size() -> usize;
+}
+
 #[repr(transparent)]
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub struct qemu_plugin_mem_rw(pub ::std::os::raw::c_uint);

libafl_qemu/src/emu.rs

@@ -7,12 +7,16 @@ use core::{
     mem::MaybeUninit,
     ptr::{addr_of, copy_nonoverlapping, null},
 };
+use std::{cell::OnceCell, slice::from_raw_parts, str::from_utf8_unchecked};
 #[cfg(emulation_mode = "systemmode")]
 use std::{
     ffi::{CStr, CString},
     ptr::null_mut,
 };
-use std::{slice::from_raw_parts, str::from_utf8_unchecked};
+
+thread_local! {
+    static SNAPSHOT_PAGE_SIZE: OnceCell<usize> = OnceCell::new();
+}
 
 #[cfg(emulation_mode = "usermode")]
 use libc::c_int;
@@ -742,6 +746,25 @@ impl CPU {
     pub fn raw_ptr(&self) -> CPUStatePtr {
         self.ptr
     }
+
+    #[must_use]
+    pub fn page_size(&self) -> usize {
+        #[cfg(emulation_mode = "usermode")]
+        {
+            SNAPSHOT_PAGE_SIZE.with(|s| {
+                *s.get_or_init(|| {
+                    unsafe { libc::sysconf(libc::_SC_PAGE_SIZE) }
+                        .try_into()
+                        .expect("Invalid page size")
+                })
+            })
+        }
+        #[cfg(emulation_mode = "systemmode")]
+        {
+            SNAPSHOT_PAGE_SIZE
+                .with(|s| *s.get_or_init(|| unsafe { libafl_qemu_sys::qemu_target_page_size() }))
+        }
+    }
 }
 
 static mut EMULATOR_IS_INITIALIZED: bool = false;