The intent is that phase 1 can be called from the syscall fast path.

In this implementation, phase1 can handle any combination of
TIF_NOHZ (RCU context tracking), TIF_SECCOMP, and TIF_SYSCALL_AUDIT,
unless seccomp requests a ptrace event, in which case phase2 is
forced.

In principle, this could yield a big speedup for TIF_NOHZ as well as
for TIF_SECCOMP if syscall exit work were similarly split up.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
 arch/x86/include/asm/ptrace.h |   5 ++
 arch/x86/kernel/ptrace.c      | 157 +++++++++++++++++++++++++++++++++++-------
 2 files changed, 138 insertions(+), 24 deletions(-)

diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 6205f0c434db..86fc2bb82287 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h

@@ -75,6 +75,11 @@ convert_ip_to_linear(struct task_struct *child, struct pt_regs *regs);
 extern void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
 			 int error_code, int si_code);
 
+
+extern unsigned long syscall_trace_enter_phase1(struct pt_regs *, u32 arch);
+extern long syscall_trace_enter_phase2(struct pt_regs *, u32 arch,
+				       unsigned long phase1_result);
+
 extern long syscall_trace_enter(struct pt_regs *);
 extern void syscall_trace_leave(struct pt_regs *);
 

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index bbf338a04a5d..29576c244699 100644
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c

@@ -1441,20 +1441,126 @@ void send_sigtrap(struct task_struct *tsk, struct pt_regs *regs,
 	force_sig_info(SIGTRAP, &info, tsk);
 }
 
+static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
+{
+#ifdef CONFIG_X86_64
+	if (arch == AUDIT_ARCH_X86_64) {
+		audit_syscall_entry(arch, regs->orig_ax, regs->di,
+				    regs->si, regs->dx, regs->r10);
+	} else
+#endif
+	{
+		audit_syscall_entry(arch, regs->orig_ax, regs->bx,
+				    regs->cx, regs->dx, regs->si);
+	}
+}
+
 /*
- * We must return the syscall number to actually look up in the table.
- * This can be -1L to skip running any syscall at all.
+ * We can return 0 to resume the syscall or anything else to go to phase
+ * 2.  If we resume the syscall, we need to put something appropriate in
+ * regs->orig_ax.
+ *
+ * NB: We don't have full pt_regs here, but regs->orig_ax and regs->ax
+ * are fully functional.
+ *
+ * For phase 2's benefit, our return value is:
+ * 0:			resume the syscall
+ * 1:			go to phase 2; no seccomp phase 2 needed
+ * anything else:	go to phase 2; pass return value to seccomp
  */
-long syscall_trace_enter(struct pt_regs *regs)
+unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
 {
-	long ret = 0;
+	unsigned long ret = 0;
+	u32 work;
+
+	BUG_ON(regs != task_pt_regs(current));
+
+	work = ACCESS_ONCE(current_thread_info()->flags) &
+		_TIF_WORK_SYSCALL_ENTRY;
 
 	/*
 	 * If TIF_NOHZ is set, we are required to call user_exit() before
 	 * doing anything that could touch RCU.
 	 */
-	if (test_thread_flag(TIF_NOHZ))
+	if (work & _TIF_NOHZ) {
 		user_exit();
+		work &= ~TIF_NOHZ;
+	}
+
+#ifdef CONFIG_SECCOMP
+	/*
+	 * Do seccomp first -- it should minimize exposure of other
+	 * code, and keeping seccomp fast is probably more valuable
+	 * than the rest of this.
+	 */
+	if (work & _TIF_SECCOMP) {
+		struct seccomp_data sd;
+
+		sd.arch = arch;
+		sd.nr = regs->orig_ax;
+		sd.instruction_pointer = regs->ip;
+#ifdef CONFIG_X86_64
+		if (arch == AUDIT_ARCH_X86_64) {
+			sd.args[0] = regs->di;
+			sd.args[1] = regs->si;
+			sd.args[2] = regs->dx;
+			sd.args[3] = regs->r10;
+			sd.args[4] = regs->r8;
+			sd.args[5] = regs->r9;
+		} else
+#endif
+		{
+			sd.args[0] = regs->bx;
+			sd.args[1] = regs->cx;
+			sd.args[2] = regs->dx;
+			sd.args[3] = regs->si;
+			sd.args[4] = regs->di;
+			sd.args[5] = regs->bp;
+		}
+
+		BUILD_BUG_ON(SECCOMP_PHASE1_OK != 0);
+		BUILD_BUG_ON(SECCOMP_PHASE1_SKIP != 1);
+
+		ret = seccomp_phase1(&sd);
+		if (ret == SECCOMP_PHASE1_SKIP) {
+			regs->orig_ax = -1;