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From: Andy Lutomirski <luto@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Borislav Petkov <bp@alien8.de>, Ingo Molnar <mingo@kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andrew Lutomirski <luto@kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	"H. Peter Anvin" <hpa@zytor.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Juergen Gross <jgross@suse.com>,
	LKML <linux-kernel@vger.kernel.org>,
	He Zhe <zhe.he@windriver.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	devel@etsukata.com
Subject: Re: [PATCH v3 6/6] x86/entry/64: Remove TRACE_IRQS_*_DEBUG
Date: Thu, 11 Jul 2019 07:45:56 -0700	[thread overview]
Message-ID: <CALCETrWYZHhP80PBxOcm5sc=p=StUbuwdvjDj-E3ma7wi3w-Pg@mail.gmail.com> (raw)
In-Reply-To: <20190711114336.174080643@infradead.org>

On Thu, Jul 11, 2019 at 4:51 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> Since INT3/#BP no longer runs on an IST, this workaround is no longer
> required.
>
> Tested by running lockdep+ftrace as described in the initial commit:
>
>   5963e317b1e9 ("ftrace/x86: Do not change stacks in DEBUG when calling lockdep")
>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

I would definitely like to see this happen, but there are all kinds of
possibly nasties here.  Ideally we'd like get rid of IST for #DB, but
we can't due to the MOV SS mess.  There are a few relevant cases we
care about:

#DB from user mode -> anything that hits in C code: irrelevant --
we've exited the IST stack already.

#DB from user mode -> NMI/MCE in the asm -> #DB: The NMI code tries to
get this right.  The MCE code does not.

#DB from kernel mode -> NMI/MCE -> #DB: same as above.

MOV SS -> #DB from entry -> #DB again: ugh.  We get some protection
from shift_ist.

IMO we would ideally just clear DR7 in sensitive contexts.  Or extend
the debug_stack_set_zero(), etc hack.

All that being said, the actual _DEBUG macros shouldn't matter here, I
think.  But I'd like to sleep on it.   So not-yet-acked-by me.

  reply	other threads:[~2019-07-11 14:46 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-11 11:40 [PATCH v3 0/6] Tracing vs CR2 Peter Zijlstra
2019-07-11 11:40 ` [PATCH v3 1/6] x86/paravirt: Make read_cr2() CALLEE_SAVE Peter Zijlstra
2019-07-17 21:22   ` [tip:x86/urgent] " tip-bot for Peter Zijlstra
2019-07-11 11:40 ` [PATCH v3 2/6] x86/entry/32: Simplify common_exception Peter Zijlstra
2019-07-17 21:23   ` [tip:x86/urgent] " tip-bot for Peter Zijlstra
2019-07-11 11:40 ` [PATCH v3 3/6] x86/entry/64: Simplify idtentry a little Peter Zijlstra
2019-07-17 21:24   ` [tip:x86/urgent] " tip-bot for Peter Zijlstra
2019-07-11 11:40 ` [PATCH v3 4/6] x86/entry/64: Update comments and sanity tests for create_gap Peter Zijlstra
2019-07-17 21:25   ` [tip:x86/urgent] " tip-bot for Peter Zijlstra
2019-07-11 11:40 ` [PATCH v3 5/6] x86/mm, tracing: Fix CR2 corruption Peter Zijlstra
2019-07-17 21:25   ` [tip:x86/urgent] " tip-bot for Peter Zijlstra
2019-07-11 11:41 ` [PATCH v3 6/6] x86/entry/64: Remove TRACE_IRQS_*_DEBUG Peter Zijlstra
2019-07-11 14:45   ` Andy Lutomirski [this message]
2019-07-11 18:28     ` Peter Zijlstra
2019-07-16 19:33 ` [PATCH v3 0/6] Tracing vs CR2 Vegard Nossum
2019-07-16 21:51   ` Vegard Nossum
2019-07-17  1:02     ` Andy Lutomirski
2019-07-17  7:46       ` Vegard Nossum
2019-07-17  7:47       ` Peter Zijlstra
2019-07-17  9:37     ` Eiichi Tsukata
2019-07-18 20:27       ` Andy Lutomirski
2019-07-20  3:59         ` Eiichi Tsukata
2019-07-20 12:49           ` Andy Lutomirski
2019-07-20 14:23             ` Eiichi Tsukata
2019-07-17  8:07   ` Peter Zijlstra
2019-07-17  8:09     ` Vegard Nossum
2019-07-18  8:57       ` [PATCH] stacktrace: Force USER_DS for stack_trace_save_user() Peter Zijlstra
2019-07-18 13:21         ` Joel Fernandes
2019-07-18 14:52         ` [tip:core/urgent] " tip-bot for Peter Zijlstra

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