Add cache to `is_linux` call (#2978)

* Add cache to avoid calling is_linux dozens of times

* Move variable out of loop

* Replace another usage of 'not .is_linux()' to .is_baremetal()

* Rename is_baremetal() -> is_pagefault_supported() (and invert meaning)

* Update pwndbg/aglib/memory.py

---------

Co-authored-by: Disconnect3d <dominik.b.czarnota@gmail.com>

pwndbg/aglib/memory.py

@@ -414,3 +414,20 @@ def resolve_renamed_struct_field(struct_name: str, possible_field_names: Set[str
             return field_name
 
     raise ValueError(f"Field name did not match any of {possible_field_names}.")
+
+
+@pwndbg.lib.cache.cache_until("start", "objfile")
+def is_pagefault_supported() -> bool:
+    """
+    This function should be called before stray memory dereferences to protect against the following situations:
+
+    1. On embedded systems, it's not uncommon for MMIO regions to exist where memory reads might mutate the hardware/process state.
+    2. On baremetal/embedded, paging doesn't always exist, so all memory is "valid" (and often initialized to zero) - this makes every value appear to be a pointer.
+
+    As such, we disable dereferencing by default for bare metal targets.
+
+    See more discussion here: https://github.com/pwndbg/pwndbg/pull/385
+    """
+
+    # TODO: use a better detection method
+    return pwndbg.dbg.selected_inferior().is_linux()

pwndbg/aglib/stack.py

@@ -47,7 +47,7 @@ def find_upper_stack_boundary(stack_ptr: int, max_pages: int = 1024) -> int:
 
     # We can't get the stack size from stack layout and page fault on bare metal mode,
     # so we return current page as a walkaround.
-    if not pwndbg.dbg.selected_inferior().is_linux():
+    if not pwndbg.aglib.memory.is_pagefault_supported():
         return stack_ptr + pwndbg.aglib.memory.PAGE_SIZE
 
     return pwndbg.aglib.memory.find_upper_boundary(stack_ptr, max_pages)

pwndbg/aglib/vmmap_custom.py

@@ -84,7 +84,7 @@ def explore(address_maybe: int) -> pwndbg.lib.memory.Page | None:
 
         Also assumes the entire contiguous section has the same permission.
     """
-    if not pwndbg.dbg.selected_inferior().is_linux():
+    if not pwndbg.aglib.memory.is_pagefault_supported():
         return None
 
     if auto_explore.value == "warn":

pwndbg/chain.py

@@ -59,6 +59,9 @@ def get(
     limit = int(limit)
 
     result = [address] if include_start else []
+
+    is_pagefault_supported = pwndbg.aglib.memory.is_pagefault_supported()
+
     for _ in range(limit):
         # Don't follow cycles, except to stop at the second occurrence.
         if result.count(address) >= 2:
@@ -71,11 +74,10 @@ def get(
         try:
             address = address + offset
 
-            # Avoid redundant dereferences in bare metal mode by checking
-            # if address is in any of vmmap pages
-            if not pwndbg.dbg.selected_inferior().is_linux() and not pwndbg.aglib.vmmap.find(
-                address
-            ):
+            # On embedded systems, it's non uncommon for MMIO regions to exist where memory reads might mutate the hardware/process state.
+            # This check prevents the memory dereferences to protect against this case.
+            # See discussion here: https://github.com/pwndbg/pwndbg/pull/385
+            if not is_pagefault_supported and not pwndbg.aglib.vmmap.find(address):
                 break
 
             next_address = int(