On 2021-10-27, Petr Mladek [off-list ref] wrote:

quoted

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 8a7b7362c0dd..24f47fbefbb5 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c

@@ -474,8 +474,10 @@ static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
 	 * state has been re-checked. A memcpy() for all of @desc
 	 * cannot be used because of the atomic_t @state_var field.
 	 */
-	memcpy(&desc_out->text_blk_lpos, &desc->text_blk_lpos,
-	       sizeof(desc_out->text_blk_lpos)); /* LMM(desc_read:C) */
+	if (desc_out) {
+		memcpy(&desc_out->text_blk_lpos, &desc->text_blk_lpos,
+		       sizeof(desc_out->text_blk_lpos)); /* LMM(desc_read:C) */
+	}
 	if (seq_out)
 		*seq_out = info->seq; /* also part of desc_read:C */
 	if (caller_id_out)

You also need this hunk:

@@ -530,7 +530,8 @@ static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
 	state_val = atomic_long_read(state_var); /* LMM(desc_read:E) */
 	d_state = get_desc_state(id, state_val);
 out:
-	atomic_long_set(&desc_out->state_var, state_val);
+	if (desc_out)
+		atomic_long_set(&desc_out->state_var, state_val);
 	return d_state;
 }

Sorry, I missed it in my last review. This time I build+boot tested the patch.