From: John Ogness <john.ogness@linutronix.de>
Date: Fri, 11 Dec 2020 00:55:25 +0106
Subject: [PATCH 16/28] printk: track/limit recursion

Limit printk() recursion to 1 level. This is enough to print a
stacktrace for the printk call, should a WARN or BUG occur.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/printk/printk.c |   74 +++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 71 insertions(+), 3 deletions(-)

--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1940,6 +1940,65 @@ static void call_console_drivers(const c
 	}
 }
 
+#ifdef CONFIG_PRINTK_NMI
+#define NUM_RECURSION_CTX 2
+#else
+#define NUM_RECURSION_CTX 1
+#endif
+
+struct printk_recursion {
+	char	count[NUM_RECURSION_CTX];
+};
+
+static DEFINE_PER_CPU(struct printk_recursion, percpu_printk_recursion);
+static char printk_recursion_count[NUM_RECURSION_CTX];
+
+static char *printk_recursion_counter(void)
+{
+	struct printk_recursion *rec;
+	char *count;
+
+	if (!printk_percpu_data_ready()) {
+		count = &printk_recursion_count[0];
+	} else {
+		rec = this_cpu_ptr(&percpu_printk_recursion);
+
+		count = &rec->count[0];
+	}
+
+#ifdef CONFIG_PRINTK_NMI
+	if (in_nmi())
+		count++;
+#endif
+
+	return count;
+}
+
+static bool printk_enter_irqsave(unsigned long *flags)
+{
+	char *count;
+
+	local_irq_save(*flags);
+	count = printk_recursion_counter();
+	/* Only 1 level of recursion allowed. */
+	if (*count > 1) {
+		local_irq_restore(*flags);
+		return false;
+	}
+	(*count)++;
+
+	return true;
+}
+
+static void printk_exit_irqrestore(unsigned long flags)
+{
+	char *count;
+
+	count = printk_recursion_counter();
+	(*count)--;
+	local_irq_restore(flags);
+}
+
 int printk_delay_msec __read_mostly;
 
 static inline void printk_delay(void)
@@ -2040,11 +2099,13 @@ int vprintk_store(int facility, int leve
 	struct prb_reserved_entry e;
 	enum log_flags lflags = 0;
 	struct printk_record r;
+	unsigned long irqflags;
 	u16 trunc_msg_len = 0;
 	char prefix_buf[8];
 	u16 reserve_size;
 	va_list args2;
 	u16 text_len;
+	int ret = 0;
 	u64 ts_nsec;
 
 	/*
@@ -2055,6 +2116,9 @@ int vprintk_store(int facility, int leve
 	 */
 	ts_nsec = local_clock();
 
+	if (!printk_enter_irqsave(&irqflags))
+		return 0;
+
 	/*
 	 * The sprintf needs to come first since the syslog prefix might be
 	 * passed in as a parameter. An extra byte must be reserved so that
@@ -2092,7 +2156,8 @@ int vprintk_store(int facility, int leve
 				prb_commit(&e);
 			}
 
-			return text_len;
+			ret = text_len;
+			goto out;
 		}
 	}
 
@@ -2108,7 +2173,7 @@ int vprintk_store(int facility, int leve
 
 		prb_rec_init_wr(&r, reserve_size + trunc_msg_len);
 		if (!prb_reserve(&e, prb, &r))
-			return 0;
+			goto out;
 	}
 
 	/* fill message */
@@ -2130,7 +2195,10 @@ int vprintk_store(int facility, int leve
 	else
 		prb_final_commit(&e);
 
-	return (text_len + trunc_msg_len);
+	ret = text_len + trunc_msg_len;
+out:
+	printk_exit_irqrestore(irqflags);
+	return ret;
 }
 
 asmlinkage int vprintk_emit(int facility, int level,