x86/stacktrace: Clarify the reliable success paths

Make clear which path is for user tasks and for kthreads and idle
tasks. This will allow easier plug-in of the ORC unwinder in the next
patches.

Note that we added a check for unwind error to the top of the loop, so
that an error is returned also for user tasks (the 'goto success' would
skip the check after the loop otherwise).

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/lkml/20180518064713.26440-3-jslaby@suse.cz
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/stacktrace.c b/arch/x86/kernel/stacktrace.c
index f9dacf6d46670a5333add6eb825c7c6f0a102d70..6acf1d5ca8321277ffc5b9a2282c1f866da6c557 100644 (file)
--- a/arch/x86/kernel/stacktrace.c
+++ b/arch/x86/kernel/stacktrace.c
@@ -89,21 +89,24 @@ __save_stack_trace_reliable(struct stack_trace *trace,
        struct pt_regs *regs;
        unsigned long addr;
 
-       for (unwind_start(&state, task, NULL, NULL); !unwind_done(&state);
+       for (unwind_start(&state, task, NULL, NULL);
+            !unwind_done(&state) && !unwind_error(&state);
             unwind_next_frame(&state)) {
 
                regs = unwind_get_entry_regs(&state, NULL);
                if (regs) {
+                       /* Success path for user tasks */
+                       if (user_mode(regs))
+                               goto success;
+
                        /*
                         * Kernel mode registers on the stack indicate an
                         * in-kernel interrupt or exception (e.g., preemption
                         * or a page fault), which can make frame pointers
                         * unreliable.
                         */
-                       if (!user_mode(regs))
-                               return -EINVAL;
 
-                       break;
+                       return -EINVAL;
                }
 
                addr = unwind_get_return_address(&state);
@@ -124,6 +127,11 @@ __save_stack_trace_reliable(struct stack_trace *trace,
        if (unwind_error(&state))
                return -EINVAL;
 
+       /* Success path for non-user tasks, i.e. kthreads and idle tasks */
+       if (!(task->flags & (PF_KTHREAD | PF_IDLE)))
+               return -EINVAL;
+
+success:
        if (trace->nr_entries < trace->max_entries)
                trace->entries[trace->nr_entries++] = ULONG_MAX;