[tip:core/urgent] stacktrace: Unbreak stack_trace_save_tsk_reliable()

[tip-bot for Joe Lawrence <tipbot@zytor.com>]

Commit-ID:  7eaf51a2e094229b75cc0c315f1cbbe2f3960058
Gitweb:     https://git.kernel.org/tip/7eaf51a2e094229b75cc0c315f1cbbe2f3960058
Author:     Joe Lawrence <joe.lawrence@redhat.com>
AuthorDate: Fri, 17 May 2019 14:51:17 -0400
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sun, 19 May 2019 11:43:22 +0200

stacktrace: Unbreak stack_trace_save_tsk_reliable()

Miroslav reported that the livepatch self-tests were failing, specifically
a case in which the consistency model ensures that a current executing
function is not allowed to be patched, "TEST: busy target module".

Recent renovations of stack_trace_save_tsk_reliable() left it returning
only an -ERRNO success indication in some configuration combinations:

  klp_check_stack()
    ret = stack_trace_save_tsk_reliable()
      #ifdef CONFIG_ARCH_STACKWALK && CONFIG_HAVE_RELIABLE_STACKTRACE
        stack_trace_save_tsk_reliable()
          ret = arch_stack_walk_reliable()
            return 0
            return -EINVAL
          ...
          return ret;
    ...
    if (ret < 0)
      /* stack_trace_save_tsk_reliable error */
    nr_entries = ret;                               << 0

Previously (and currently for !CONFIG_ARCH_STACKWALK &&
CONFIG_HAVE_RELIABLE_STACKTRACE) stack_trace_save_tsk_reliable() returned
the number of entries that it consumed in the passed storage array.

In the case of the above config and trace, be sure to return the
stacktrace_cookie.len on stack_trace_save_tsk_reliable() success.

Fixes: 25e39e32b0a3f ("livepatch: Simplify stack trace retrieval")
Reported-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: live-patching@vger.kernel.org
Cc: jikos@kernel.org
Cc: pmladek@suse.com
Link: https://lkml.kernel.org/r/20190517185117.24642-1-joe.lawrence@redhat.com

---
 kernel/stacktrace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c
index 27bafc1e271e..90d3e0bf0302 100644
--- a/kernel/stacktrace.c
+++ b/kernel/stacktrace.c
@@ -206,7 +206,7 @@ int stack_trace_save_tsk_reliable(struct task_struct *tsk, unsigned long *store,
 
 	ret = arch_stack_walk_reliable(consume_entry, &c, tsk);
 	put_task_stack(tsk);
-	return ret;
+	return ret ? ret : c.len;
 }
 #endif