linux-kernel - [PATCH] x86/nmi: Fix "in NMI handler" check

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Date: Wed,  7 Feb 2024 08:52:35 -0800
From: Breno Leitao <leitao@...ian.org>
To: Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	"Paul E. McKenney" <paulmck@...nel.org>
Cc: leit@...a.com,
	Mark Rutland <mark.rutland@....com>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel@...r.kernel.org (open list:X86 ARCHITECTURE (32-BIT AND 64-BIT))
Subject: [PATCH] x86/nmi: Fix "in NMI handler" check

Commit 344da544f177 ("x86/nmi: Print reasons why backtrace NMIs are
ignored") creates a super nice framework to diagnose NMIs.

Every time nmi_exc() is called, it increments a per_cpu counter
(nsp->idt_nmi_seq). At its exit, it also increments the same counter.
Looking at this counter, you can see how many times that function was
called (dividing by 2), and, if the function is still being executed, by
checking the idt_nmi_seq's last bit.

On the check side (nmi_backtrace_stall_check()), that variable is
queried to check if the NMI is still being executed, but, there is a
mistake in the bitwise operation. That code wants to check if the last
bit of the idt_nmi_seq is set or not, but, does the opposite, and check
for all the other bits, which will always be true after the first
exc_nmi() executed successfully.

This appends the misleading string to the dump "(CPU currently in NMI
handler function)"

Fix it by checking the last bit, and if it is set, append the string.

Fixes: 344da544f177 ("x86/nmi: Print reasons why backtrace NMIs are ignored")
Signed-off-by: Breno Leitao <leitao@...ian.org>
---
 arch/x86/kernel/nmi.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index 17e955ab69fe..6e738ad474dc 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -639,7 +639,7 @@ void nmi_backtrace_stall_check(const struct cpumask *btp)
 			msgp = nmi_check_stall_msg[idx];
 			if (nsp->idt_ignored_snap != READ_ONCE(nsp->idt_ignored) && (idx & 0x1))
 				modp = ", but OK because ignore_nmis was set";
-			if (nmi_seq & ~0x1)
+			if (nmi_seq & 0x1)
 				msghp = " (CPU currently in NMI handler function)";
 			else if (nsp->idt_nmi_seq_snap + 1 == nmi_seq)
 				msghp = " (CPU exited one NMI handler function)";
-- 
2.39.3