objtool, lkdtm: Obfuscate the do_nothing() pointer

If execute_location()'s memcpy of do_nothing() gets inlined and unrolled
by the compiler, it copies one word at a time:

    mov    0x0(%rip),%rax    R_X86_64_PC32    .text+0x1374
    mov    %rax,0x38(%rbx)
    mov    0x0(%rip),%rax    R_X86_64_PC32    .text+0x136c
    mov    %rax,0x30(%rbx)
    ...

Those .text references point to the middle of the function, causing
objtool to complain about their lack of ENDBR.

Prevent that by resolving the function pointer at runtime rather than
build time.  This fixes the following warning:

  drivers/misc/lkdtm/lkdtm.o: warning: objtool: execute_location+0x23: relocation to !ENDBR: .text+0x1378

Reported-by: kernel test robot <[email protected]>
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/30b9abffbddeb43c4f6320b1270fa9b4d74c54ed.1742852847.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/oe-kbuild-all/[email protected]/

Author: jpoimboe

drivers/misc/lkdtm/perms.c

@@ -28,6 +28,13 @@ static const unsigned long rodata = 0xAA55AA55;
 /* This is marked __ro_after_init, so it should ultimately be .rodata. */
 static unsigned long ro_after_init __ro_after_init = 0x55AA5500;
 
+/*
+ * This is a pointer to do_nothing() which is initialized at runtime rather
+ * than build time to avoid objtool IBT validation warnings caused by an
+ * inlined unrolled memcpy() in execute_location().
+ */
+static void __ro_after_init *do_nothing_ptr;
+
 /*
  * This just returns to the caller. It is designed to be copied into
  * non-executable memory regions.
@@ -65,13 +72,12 @@ static noinline __nocfi void execute_location(void *dst, bool write)
 {
 	void (*func)(void);
 	func_desc_t fdesc;
-	void *do_nothing_text = dereference_function_descriptor(do_nothing);
 
-	pr_info("attempting ok execution at %px\n", do_nothing_text);
+	pr_info("attempting ok execution at %px\n", do_nothing_ptr);
 	do_nothing();
 
 	if (write == CODE_WRITE) {
-		memcpy(dst, do_nothing_text, EXEC_SIZE);
+		memcpy(dst, do_nothing_ptr, EXEC_SIZE);
 		flush_icache_range((unsigned long)dst,
 				   (unsigned long)dst + EXEC_SIZE);
 	}
@@ -267,6 +273,8 @@ static void lkdtm_ACCESS_NULL(void)
 
 void __init lkdtm_perms_init(void)
 {
+	do_nothing_ptr = dereference_function_descriptor(do_nothing);
+
 	/* Make sure we can write to __ro_after_init values during __init */
 	ro_after_init |= 0xAA;
 }