[PATCH AUTOSEL 4.19 4/6] tracing: Have large events show up as '[LINE TOO BIG]' instead of nothing

[Sasha Levin]

From: "Steven Rostedt (Google)" <rost...@goodmis.org>

[ Upstream commit b55b0a0d7c4aa2dac3579aa7e6802d1f57445096 ]

If a large event was added to the ring buffer that is larger than what the
trace_seq can handle, it just drops the output:

 ~# cat /sys/kernel/tracing/trace
 # tracer: nop
 #
 # entries-in-buffer/entries-written: 2/2   #P:8
 #
 #                                _-----=> irqs-off/BH-disabled
 #                               / _----=> need-resched
 #                              | / _---=> hardirq/softirq
 #                              || / _--=> preempt-depth
 #                              ||| / _-=> migrate-disable
 #                              |||| /     delay
 #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
 #              | |         |   |||||     |         |
            <...>-859     [001] .....   141.118951: tracing_mark_write          
 <...>-859     [001] .....   141.148201: tracing_mark_write: 78901234

Instead, catch this case and add some context:

 ~# cat /sys/kernel/tracing/trace
 # tracer: nop
 #
 # entries-in-buffer/entries-written: 2/2   #P:8
 #
 #                                _-----=> irqs-off/BH-disabled
 #                               / _----=> need-resched
 #                              | / _---=> hardirq/softirq
 #                              || / _--=> preempt-depth
 #                              ||| / _-=> migrate-disable
 #                              |||| /     delay
 #           TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
 #              | |         |   |||||     |         |
            <...>-852     [001] .....   121.550551: tracing_mark_write[LINE TOO 
BIG]
            <...>-852     [001] .....   121.550581: tracing_mark_write: 78901234

This now emulates the same output as trace_pipe.

Link: 
https://lore.kernel.org/linux-trace-kernel/20231209171058.78c1a...@gandalf.local.home

Cc: Mark Rutland <mark.rutl...@arm.com>
Cc: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
Reviewed-by: Masami Hiramatsu (Google) <mhira...@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org>
Signed-off-by: Sasha Levin <sas...@kernel.org>
---
 kernel/trace/trace.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index b43d681b072fa..e6b2d443bab9e 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3828,7 +3828,11 @@ static int s_show(struct seq_file *m, void *v)
                iter->leftover = ret;
 
        } else {
-               print_trace_line(iter);
+               ret = print_trace_line(iter);
+               if (ret == TRACE_TYPE_PARTIAL_LINE) {
+                       iter->seq.full = 0;
+                       trace_seq_puts(&iter->seq, "[LINE TOO BIG]\n");
+               }
                ret = trace_print_seq(m, &iter->seq);
                /*
                 * If we overflow the seq_file buffer, then it will
-- 
2.43.0