Re: Stackleak vs noinstr (Was: [GIT pull] objtool/core for v5.16-rc1)

[Peter Zijlstra]

On Tue, Nov 02, 2021 at 10:05:50AM +0100, Peter Zijlstra wrote:
> On Tue, Nov 02, 2021 at 09:00:36AM +0100, Peter Zijlstra wrote:
> > On Mon, Nov 01, 2021 at 01:44:39PM -0700, Linus Torvalds wrote:
> 
> > >     do_machine_check()+0x27: call to stackleak_track_stack ...
> > >     do_syscall_64()+0x9: call to stackleak_track_stack ...
> > >     do_int80_syscall_32()+0x9: call to stackleak_track_stack ...
> > >     exc_general_protection()+0x22: call to stackleak_track_stack ...
> > >     fixup_bad_iret()+0x20: call to stackleak_track_stack ...
> > >     .entry.text+0x10e6: call to stackleak_erase ...
> > >     .entry.text+0x143: call to stackleak_erase ...
> > >     .entry.text+0x17d9: call to stackleak_erase ...
> > > 
> > > most seem to be about the stackleak thing,
> > 
> > Right, I recently ran into this and hacen't yet had time to look into
> > it. I suspect my normal build box doesn't have the GCC plugin crud
> > enabled or somesuch.
> > 
> > I think the GCC stackleak plugin needs fixing, specifically it needs a
> > function attribute such that it will not emit instrumentation in noinstr
> > functions. I'll go chase down the developer of that thing.
> 
> Alexander, is there any way to make this plugin grow a function
> attribute which we can add to noinstr ? There's a strict requirement the
> compiler doesn't add extra code to noinstr functions these days.
> 
> We'll 'soon' be running noinstr C code before switching to kernel page
> tables even.

Using my pre-release GCC-12 compiler (the only one I have with plugin
crud enabled apparently), the below seems to work.

Having the plugin gate on section name seems a lot hacky, but given it's
already doing that, one more doesn't hurt.

---
diff --git a/kernel/stackleak.c b/kernel/stackleak.c
index ce161a8e8d97..135866ca8878 100644
--- a/kernel/stackleak.c
+++ b/kernel/stackleak.c
@@ -48,7 +48,7 @@ int stack_erasing_sysctl(struct ctl_table *table, int write,
 #define skip_erasing()	false
 #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
 
-asmlinkage void notrace stackleak_erase(void)
+asmlinkage noinstr void stackleak_erase(void)
 {
 	/* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
 	unsigned long kstack_ptr = current->lowest_stack;
@@ -102,7 +102,6 @@ asmlinkage void notrace stackleak_erase(void)
 	/* Reset the 'lowest_stack' value for the next syscall */
 	current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
 }
-NOKPROBE_SYMBOL(stackleak_erase);
 
 void __used __no_caller_saved_registers notrace stackleak_track_stack(void)
 {
diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c
index e9db7dcb3e5f..07688a1c686b 100644
--- a/scripts/gcc-plugins/stackleak_plugin.c
+++ b/scripts/gcc-plugins/stackleak_plugin.c
@@ -446,6 +446,8 @@ static bool stackleak_gate(void)
 			return false;
 		if (!strncmp(TREE_STRING_POINTER(section), ".meminit.text", 13))
 			return false;
+		if (!strncmp(TREE_STRING_POINTER(section), ".noinstr.text", 13))
+			return false;
 	}
 
 	return track_frame_size >= 0;