linux-kernel - [for-linus][PATCH] ftrace: Fix potential warning in trace_printk_seq during ftrace

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Message-ID: <20250822121138.5e113885@gandalf.local.home>
Date: Fri, 22 Aug 2025 12:11:38 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Tengda Wu <wutengda@...weicloud.com>
Subject: [for-linus][PATCH] ftrace: Fix potential warning in
 trace_printk_seq during ftrace_dump


  git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git
trace/fixes

Head SHA1: c1e730442be2902ad5f9acc244ffc6e6400b981a


Tengda Wu (1):
      ftrace: Fix potential warning in trace_printk_seq during ftrace_dump

----
 kernel/trace/trace.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---------------------------
commit c1e730442be2902ad5f9acc244ffc6e6400b981a
Author: Tengda Wu <wutengda@...weicloud.com>
Date:   Fri Aug 22 03:33:43 2025 +0000

    ftrace: Fix potential warning in trace_printk_seq during ftrace_dump
    
    When calling ftrace_dump_one() concurrently with reading trace_pipe,
    a WARN_ON_ONCE() in trace_printk_seq() can be triggered due to a race
    condition.
    
    The issue occurs because:
    
    CPU0 (ftrace_dump)                              CPU1 (reader)
    echo z > /proc/sysrq-trigger
    
    !trace_empty(&iter)
    trace_iterator_reset(&iter) <- len = size = 0
                                                    cat /sys/kernel/tracing/trace_pipe
    trace_find_next_entry_inc(&iter)
      __find_next_entry
        ring_buffer_empty_cpu <- all empty
      return NULL
    
    trace_printk_seq(&iter.seq)
      WARN_ON_ONCE(s->seq.len >= s->seq.size)
    
    In the context between trace_empty() and trace_find_next_entry_inc()
    during ftrace_dump, the ring buffer data was consumed by other readers.
    This caused trace_find_next_entry_inc to return NULL, failing to populate
    `iter.seq`. At this point, due to the prior trace_iterator_reset, both
    `iter.seq.len` and `iter.seq.size` were set to 0. Since they are equal,
    the WARN_ON_ONCE condition is triggered.
    
    Move the trace_printk_seq() into the if block that checks to make sure the
    return value of trace_find_next_entry_inc() is non-NULL in
    ftrace_dump_one(), ensuring the 'iter.seq' is properly populated before
    subsequent operations.
    
    Cc: Masami Hiramatsu <mhiramat@...nel.org>
    Cc: Mark Rutland <mark.rutland@....com>
    Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
    Cc: Ingo Molnar <mingo@...e.hu>
    Link: https://lore.kernel.org/20250822033343.3000289-1-wutengda@huaweicloud.com
    Fixes: d769041f8653 ("ring_buffer: implement new locking")
    Signed-off-by: Tengda Wu <wutengda@...weicloud.com>
    Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8d8935ed416d..1b7db732c0b1 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -10638,10 +10638,10 @@ static void ftrace_dump_one(struct trace_array *tr, enum ftrace_dump_mode dump_m
 			ret = print_trace_line(&iter);
 			if (ret != TRACE_TYPE_NO_CONSUME)
 				trace_consume(&iter);
+
+			trace_printk_seq(&iter.seq);
 		}
 		touch_nmi_watchdog();
-
-		trace_printk_seq(&iter.seq);
 	}
 
 	if (!cnt)