tile: avoid recursive backtrace faults

This change adds support for avoiding recursive backtracer crashes;
we haven't seen this in practice other than when things are seriously
corrupt, but it may help avoid losing the root cause of a crash.

Also, don't abort kernel backtracers for invalid userspace PC's.
If we do, we lose the ability to backtrace through a userspace
call to a bad address above PAGE_OFFSET, even though that it can
be perfectly reasonable to continue the backtrace in such a case.

Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

1 files changed, 28 insertions, 2 deletions

diff --git a/arch/tile/kernel/stack.c b/arch/tile/kernel/stack.c
index af8dfc9665f6..c972689231ef 100644
--- a/arch/tile/kernel/stack.c
+++ b/arch/tile/kernel/stack.c
@@ -103,8 +103,7 @@ static struct pt_regs *valid_fault_handler(struct KBacktraceIterator* kbt)
                 if (kbt->verbose)
                         pr_err("  <%s while in kernel mode>\n", fault);
         } else if (EX1_PL(p->ex1) == USER_PL &&
-            p->pc < PAGE_OFFSET &&
-            p->sp < PAGE_OFFSET) {
+                   p->sp < PAGE_OFFSET && p->sp != 0) {
                 if (kbt->verbose)
                         pr_err("  <%s while in user mode>\n", fault);
         } else if (kbt->verbose) {
@@ -352,6 +351,26 @@ static void describe_addr(struct KBacktraceIterator *kbt,
 }
 
 /*
+ * Avoid possible crash recursion during backtrace.  If it happens, it
+ * makes it easy to lose the actual root cause of the failure, so we
+ * put a simple guard on all the backtrace loops.
+ */
+static bool start_backtrace(void)
+{
+        if (current->thread.in_backtrace) {
+                pr_err("Backtrace requested while in backtrace!\n");
+                return false;
+        }
+        current->thread.in_backtrace = true;
+        return true;
+}
+
+static void end_backtrace(void)
+{
+        current->thread.in_backtrace = false;
+}
+
+/*
  * This method wraps the backtracer's more generic support.
  * It is only invoked from the architecture-specific code; show_stack()
  * and dump_stack() (in entry.S) are architecture-independent entry points.
@@ -361,6 +380,8 @@ void tile_show_stack(struct KBacktraceIterator *kbt, int headers)
         int i;
         int have_mmap_sem = 0;
 
+        if (!start_backtrace())
+                return;
         if (headers) {
                 /*
                  * Add a blank line since if we are called from panic(),
@@ -402,6 +423,7 @@ void tile_show_stack(struct KBacktraceIterator *kbt, int headers)
                 pr_err("Stack dump complete\n");
         if (have_mmap_sem)
                 up_read(&kbt->task->mm->mmap_sem);
+        end_backtrace();
 }
 EXPORT_SYMBOL(tile_show_stack);
 
@@ -463,6 +485,8 @@ void save_stack_trace_tsk(struct task_struct *task, struct stack_trace *trace)
         int skip = trace->skip;
         int i = 0;
 
+        if (!start_backtrace())
+                goto done;
         if (task == NULL || task == current)
                 KBacktraceIterator_init_current(&kbt);
         else
@@ -476,6 +500,8 @@ void save_stack_trace_tsk(struct task_struct *task, struct stack_trace *trace)
                         break;
                 trace->entries[i++] = kbt.it.pc;
         }
+        end_backtrace();
+done:
         trace->nr_entries = i;
 }
 EXPORT_SYMBOL(save_stack_trace_tsk);