stacktrace: Don't skip first entry on noncurrent tasks

BugLink: https://bugs.launchpad.net/bugs/1855306
[ Upstream commit b0c51f158455e31d5024100cf3580fcd88214b0e ]

When doing cat /proc/<PID>/stack, the output is missing the first entry.
When the current code walks the stack starting in stack_trace_save_tsk,
it skips all scheduler functions (that's OK) plus one more function. But
this one function should be skipped only for the 'current' task as it is
stack_trace_save_tsk proper.

The original code (before the common infrastructure) skipped one
function only for the 'current' task -- see save_stack_trace_tsk before
3599fe12a125. So do so also in the new infrastructure now.

Fixes: 214d8ca6ee85 ("stacktrace: Provide common infrastructure")
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Michal Suchanek <msuchanek@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20191030072545.19462-1-jslaby@suse.cz
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c
index f5440abb753291df445a1ce17559fd476d810a15..9bbfbdb96ae5156ae9eb5d98b0114f46fcb5020c 100644 (file)
--- a/kernel/stacktrace.c
+++ b/kernel/stacktrace.c
@@ -141,7 +141,8 @@ unsigned int stack_trace_save_tsk(struct task_struct *tsk, unsigned long *store,
        struct stacktrace_cookie c = {
                .store  = store,
                .size   = size,
-               .skip   = skipnr + 1,
+               /* skip this function if they are tracing us */
+               .skip   = skipnr + !!(current == tsk),
        };
 
        if (!try_get_task_stack(tsk))
@@ -298,7 +299,8 @@ unsigned int stack_trace_save_tsk(struct task_struct *task,
        struct stack_trace trace = {
                .entries        = store,
                .max_entries    = size,
-               .skip           = skipnr + 1,
+               /* skip this function if they are tracing us */
+               .skip   = skipnr + !!(current == task),
        };
 
        save_stack_trace_tsk(task, &trace);