linux-kernel - [RFC v2 PATCH 6/7] x86/regs: syscall_get_nr() returns -1 for a non-system call

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Date:   Mon, 10 May 2021 11:53:15 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [RFC v2 PATCH 6/7] x86/regs: syscall_get_nr() returns -1 for a non-system call

syscall_get_nr() is defined to return -1 for a non-system call or a
ptrace/seccomp restart; not just any arbitrary number. See comment in
<asm-generic/syscall.h> for the official definition of this function.

Signed-off-by: H. Peter Anvin <hpa@...or.com>
---
 arch/x86/kernel/ptrace.c | 2 +-
 arch/x86/kernel/signal.c | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 87a4143aa7d7..4c208ea3bd9f 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -911,7 +911,7 @@ static int putreg32(struct task_struct *child, unsigned regno, u32 value)
 		 * syscall with TS_COMPAT still set.
 		 */
 		regs->orig_ax = value;
-		if (syscall_get_nr(child, regs) >= 0)
+		if (syscall_get_nr(child, regs) != -1)
 			child->thread_info.status |= TS_I386_REGS_POKED;
 		break;
 
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index a06cb107c0e8..e12779a2714d 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -713,7 +713,7 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
 		save_v86_state((struct kernel_vm86_regs *) regs, VM86_SIGNAL);
 
 	/* Are we from a system call? */
-	if (syscall_get_nr(current, regs) >= 0) {
+	if (syscall_get_nr(current, regs) != -1) {
 		/* If so, check system call restarting.. */
 		switch (syscall_get_error(current, regs)) {
 		case -ERESTART_RESTARTBLOCK:
@@ -793,7 +793,7 @@ void arch_do_signal_or_restart(struct pt_regs *regs, bool has_signal)
 	}
 
 	/* Did we come from a system call? */
-	if (syscall_get_nr(current, regs) >= 0) {
+	if (syscall_get_nr(current, regs) != -1) {
 		/* Restart the system call - no handlers present */
 		switch (syscall_get_error(current, regs)) {
 		case -ERESTARTNOHAND:
-- 
2.31.1