fs/proc: Expose RSEQ configuration

For userspace checkpoint and restore (C/R) some way of getting process
state containing RSEQ configuration is needed.

There are two ways this information is going to be used:
 - to re-enable RSEQ for threads which had it enabled before C/R
 - to detect if a thread was in a critical section during C/R

Since C/R preserves TLS memory and addresses RSEQ ABI will be restored
using the address registered before C/R.

Detection whether the thread is in a critical section during C/R is
needed to enforce behavior of RSEQ abort during C/R. Attaching with
ptrace() before registers are dumped itself doesn't cause RSEQ abort.
Restoring the instruction pointer within the critical section is
problematic because rseq_cs may get cleared before the control is
passed to the migrated application code leading to RSEQ invariants not
being preserved.

Signed-off-by: Piotr Figiel <figiel@google.com>

fs/proc/base.c

@@ -662,6 +662,21 @@ static int proc_pid_syscall(struct seq_file *m, struct pid_namespace *ns,
 
 	return 0;
 }
+
+#ifdef CONFIG_RSEQ
+static int proc_pid_rseq(struct seq_file *m, struct pid_namespace *ns,
+				struct pid *pid, struct task_struct *task)
+{
+	int res = lock_trace(task);
+
+	if (res)
+		return res;
+	seq_printf(m, "%tx %08x\n", (ptrdiff_t)((uintptr_t)task->rseq),
+		   task->rseq_sig);
+	unlock_trace(task);
+	return 0;
+}
+#endif /* CONFIG_RSEQ */
 #endif /* CONFIG_HAVE_ARCH_TRACEHOOK */
 
 /************************************************************************/
@@ -3182,6 +3197,9 @@ static const struct pid_entry tgid_base_stuff[] = {
 	REG("comm",      S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
 	ONE("syscall",    S_IRUSR, proc_pid_syscall),
+#ifdef CONFIG_RSEQ
+	ONE("rseq",       S_IRUSR, proc_pid_rseq),
+#endif
 #endif
 	REG("cmdline",    S_IRUGO, proc_pid_cmdline_ops),
 	ONE("stat",       S_IRUGO, proc_tgid_stat),
@@ -3522,6 +3540,9 @@ static const struct pid_entry tid_base_stuff[] = {
 			 &proc_pid_set_comm_operations, {}),
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
 	ONE("syscall",   S_IRUSR, proc_pid_syscall),
+#ifdef CONFIG_RSEQ
+	ONE("rseq",      S_IRUSR, proc_pid_rseq),
+#endif
 #endif
 	REG("cmdline",   S_IRUGO, proc_pid_cmdline_ops),
 	ONE("stat",      S_IRUGO, proc_tid_stat),