linux-kernel - [PATCH] x86/fred: Fix 64bit identifier in fred

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Message-Id: <20250814190436.2555438-1-andrew.cooper3@citrix.com>
Date: Thu, 14 Aug 2025 20:04:36 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>,
	Xin Li <xin@...or.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...nel.org>,
	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
Subject: [PATCH] x86/fred: Fix 64bit identifier in fred_ss

FRED can only be enabled in Long Mode.  This is the 64bit mode (as opposed to
compatibility mode) identifier, rather than being something hard-wired at 1.

Signed-off-by: Andrew Cooper <andrew.cooper3@...rix.com>
---
CC: Xin Li <xin@...or.com>
CC: "H. Peter Anvin" <hpa@...or.com>
CC: Andy Lutomirski <luto@...nel.org>
CC: Thomas Gleixner <tglx@...utronix.de>
CC: Ingo Molnar <mingo@...hat.com>
CC: Borislav Petkov <bp@...en8.de>
CC: Dave Hansen <dave.hansen@...ux.intel.com>
CC: x86@...nel.org
CC: linux-kernel@...r.kernel.org

IIRC, this was wrong in an older version of the FRED spec.  I made the same
mistake in Xen, and I didn't copy Linux's structure.

I don't thinks this warrants backporting, but the fixes tag is:

Fixes: 3c77bf02d0c0 ("x86/ptrace: Add FRED additional information to the pt_regs structure")
---
 arch/x86/entry/entry_fred.c   | 4 ++--
 arch/x86/include/asm/fred.h   | 2 +-
 arch/x86/include/asm/ptrace.h | 4 ++--
 3 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
index f004a4dc74c2..94e626cc6a07 100644
--- a/arch/x86/entry/entry_fred.c
+++ b/arch/x86/entry/entry_fred.c
@@ -78,13 +78,13 @@ static noinstr void fred_intx(struct pt_regs *regs)
 static __always_inline void fred_other(struct pt_regs *regs)
 {
 	/* The compiler can fold these conditions into a single test */
-	if (likely(regs->fred_ss.vector == FRED_SYSCALL && regs->fred_ss.lm)) {
+	if (likely(regs->fred_ss.vector == FRED_SYSCALL && regs->fred_ss.l)) {
 		regs->orig_ax = regs->ax;
 		regs->ax = -ENOSYS;
 		do_syscall_64(regs, regs->orig_ax);
 		return;
 	} else if (ia32_enabled() &&
-		   likely(regs->fred_ss.vector == FRED_SYSENTER && !regs->fred_ss.lm)) {
+		   likely(regs->fred_ss.vector == FRED_SYSENTER && !regs->fred_ss.l)) {
 		regs->orig_ax = regs->ax;
 		regs->ax = -ENOSYS;
 		do_fast_syscall_32(regs);
diff --git a/arch/x86/include/asm/fred.h b/arch/x86/include/asm/fred.h
index 12b34d5b2953..2bb65677c079 100644
--- a/arch/x86/include/asm/fred.h
+++ b/arch/x86/include/asm/fred.h
@@ -79,7 +79,7 @@ static __always_inline void fred_entry_from_kvm(unsigned int type, unsigned int
 		.type   = type,
 		.vector = vector,
 		.nmi    = type == EVENT_TYPE_NMI,
-		.lm     = 1,
+		.l      = 1,
 	};
 
 	asm_fred_entry_from_kvm(ss);
diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 50f75467f73d..37370c3b1ffd 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -84,8 +84,8 @@ struct fred_ss {
 			:  4,
 		/* Event was incident to enclave execution */
 		enclave	:  1,
-		/* CPU was in long mode */
-		lm	:  1,
+		/* CPU was in 64-bit mode */
+		l	:  1,
 		/*
 		 * Nested exception during FRED delivery, not set
 		 * for #DF.

base-commit: 24ea63ea387714634813359e2c8e0e6c36952f73
-- 
2.39.5