pstore: Alert on backend write error

The pstore dump function doesn't alert at all on errors - despite
pstore is usually a last resource and if it fails users won't be
able to read the kernel log, this is not the case for server users
with serial access, for example.

So, let's at least attempt to inform such advanced users on the first
backend writing error detected during the kmsg dump - this is also
very useful for pstore debugging purposes.
Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20221013210648.137452-2-gpiccoli@igalia.com

fs/pstore/platform.c

@@ -393,6 +393,7 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 	const char	*why;
 	unsigned int	part = 1;
 	unsigned long	flags = 0;
+	int		saved_ret = 0;
 	int		ret;
 
 	why = kmsg_dump_reason_str(reason);
@@ -463,12 +464,21 @@ static void pstore_dump(struct kmsg_dumper *dumper,
 		if (ret == 0 && reason == KMSG_DUMP_OOPS) {
 			pstore_new_entry = 1;
 			pstore_timer_kick();
+		} else {
+			/* Preserve only the first non-zero returned value. */
+			if (!saved_ret)
+				saved_ret = ret;
 		}
 
 		total += record.size;
 		part++;
 	}
 	spin_unlock_irqrestore(&psinfo->buf_lock, flags);
+
+	if (saved_ret) {
+		pr_err_once("backend (%s) writing error (%d)\n", psinfo->name,
+			    saved_ret);
+	}
 }
 
 static struct kmsg_dumper pstore_dumper = {