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Subject[PATCH v2] fs/proc: Expose RSEQ configuration
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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>

---

v2:
- fixed string formatting for 32-bit architectures

v1:
- https://lkml.kernel.org/r/20210113174127.2500051-1-figiel@google.com

---
fs/proc/base.c | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b3422cda2a91..7cc36a224b8b 100644
--- a/fs/proc/base.c
+++ b/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),
--
2.30.0.284.gd98b1dd5eaa7-goog
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