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Date:   Wed, 03 Nov 2021 10:18:22 +0300
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Subject: Re: Stackleak vs noinstr (Was: [GIT pull] objtool/core for v5.16-rc1)
To:     Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
CC:     Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>
From:   Alexander Popov <alex.popov@linux.com>
Message-ID: <A4522E50-B426-4588-A1F8-B27DB579C100@linux.com>
Precedence: bulk


On November 2, 2021 1:03:44 PM GMT+03:00, Peter Zijlstra <peterz@infradead.org> wrote:
>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.

Hello Peter!

Yes, this is a correct approach.

But I'm not sure about removing NOKPROBE_SYMBOL and notrace for stackleak_erase. Does the code in noinstr.text disable all those?

Best regards,
Alexander

>---
>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;