[PATCH v4 32/68] cachefiles: Add some error injection support

[David Howells <dhowells@redhat.com>]

Add support for injecting ENOSPC or EIO errors.  This needs to be enabled
by CONFIG_CACHEFILES_ERROR_INJECTION=y.  Once enabled, ENOSPC on things
like write and mkdir can be triggered by:

        echo 1 >/proc/sys/cachefiles/error_injection

and EIO can be triggered on most operations by:

        echo 2 >/proc/sys/cachefiles/error_injection

Signed-off-by: David Howells <dhowells@redhat.com>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/163819624706.215744.6911916249119962943.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906925343.143852.5465695512984025812.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967134412.1823006.7354285948280296595.stgit@warthog.procyon.org.uk/ # v3
---

 fs/cachefiles/Kconfig        |    7 ++++++
 fs/cachefiles/Makefile       |    2 ++
 fs/cachefiles/error_inject.c |   46 ++++++++++++++++++++++++++++++++++++++++++
 fs/cachefiles/internal.h     |   42 +++++++++++++++++++++++++++++++++++++-
 fs/cachefiles/main.c         |   12 +++++++++++
 5 files changed, 108 insertions(+), 1 deletion(-)
 create mode 100644 fs/cachefiles/error_inject.c

diff --git a/fs/cachefiles/Kconfig b/fs/cachefiles/Kconfig
index 6827b40f7ddc..719faeeda168 100644
--- a/fs/cachefiles/Kconfig
+++ b/fs/cachefiles/Kconfig
@@ -19,3 +19,10 @@ config CACHEFILES_DEBUG
 	  caching on files module.  If this is set, the debugging output may be
 	  enabled by setting bits in /sys/modules/cachefiles/parameter/debug or
 	  by including a debugging specifier in /etc/cachefilesd.conf.
+
+config CACHEFILES_ERROR_INJECTION
+	bool "Provide error injection for cachefiles"
+	depends on CACHEFILES && SYSCTL
+	help
+	  This permits error injection to be enabled in cachefiles whilst a
+	  cache is in service.
diff --git a/fs/cachefiles/Makefile b/fs/cachefiles/Makefile
index a7f3e982e249..183fb5f3b8b1 100644
--- a/fs/cachefiles/Makefile
+++ b/fs/cachefiles/Makefile
@@ -6,4 +6,6 @@
 cachefiles-y := \
 	main.o
 
+cachefiles-$(CONFIG_CACHEFILES_ERROR_INJECTION) += error_inject.o
+
 obj-$(CONFIG_CACHEFILES) := cachefiles.o
diff --git a/fs/cachefiles/error_inject.c b/fs/cachefiles/error_inject.c
new file mode 100644
index 000000000000..58f8aec964e4
--- /dev/null
+++ b/fs/cachefiles/error_inject.c
@@ -0,0 +1,46 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/* Error injection handling.
+ *
+ * Copyright (C) 2021 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/sysctl.h>
+#include "internal.h"
+
+unsigned int cachefiles_error_injection_state;
+
+static struct ctl_table_header *cachefiles_sysctl;
+static struct ctl_table cachefiles_sysctls[] = {
+	{
+		.procname	= "error_injection",
+		.data		= &cachefiles_error_injection_state,
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0644,
+		.proc_handler	= proc_douintvec,
+	},
+	{}
+};
+
+static struct ctl_table cachefiles_sysctls_root[] = {
+	{
+		.procname	= "cachefiles",
+		.mode		= 0555,
+		.child		= cachefiles_sysctls,
+	},
+	{}
+};
+
+int __init cachefiles_register_error_injection(void)
+{
+	cachefiles_sysctl = register_sysctl_table(cachefiles_sysctls_root);
+	if (!cachefiles_sysctl)
+		return -ENOMEM;
+	return 0;
+
+}
+
+void cachefiles_unregister_error_injection(void)
+{
+	unregister_sysctl_table(cachefiles_sysctl);
+}
diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
index cff4b2a5f928..1f2fea902d3e 100644
--- a/fs/cachefiles/internal.h
+++ b/fs/cachefiles/internal.h
@@ -64,7 +64,47 @@ struct cachefiles_cache {
 
 
 /*
- * Debug tracing.
+ * error_inject.c
+ */
+#ifdef CONFIG_CACHEFILES_ERROR_INJECTION
+extern unsigned int cachefiles_error_injection_state;
+extern int cachefiles_register_error_injection(void);
+extern void cachefiles_unregister_error_injection(void);
+
+#else
+#define cachefiles_error_injection_state 0
+
+static inline int cachefiles_register_error_injection(void)
+{
+	return 0;
+}
+
+static inline void cachefiles_unregister_error_injection(void)
+{
+}
+#endif
+
+
+static inline int cachefiles_inject_read_error(void)
+{
+	return cachefiles_error_injection_state & 2 ? -EIO : 0;
+}
+
+static inline int cachefiles_inject_write_error(void)
+{
+	return cachefiles_error_injection_state & 2 ? -EIO :
+		cachefiles_error_injection_state & 1 ? -ENOSPC :
+		0;
+}
+
+static inline int cachefiles_inject_remove_error(void)
+{
+	return cachefiles_error_injection_state & 2 ? -EIO : 0;
+}
+
+
+/*
+ * Debug tracing
  */
 extern unsigned cachefiles_debug;
 #define CACHEFILES_DEBUG_KENTER	1
diff --git a/fs/cachefiles/main.c b/fs/cachefiles/main.c
index 47bc1cc078de..387d42c7185f 100644
--- a/fs/cachefiles/main.c
+++ b/fs/cachefiles/main.c
@@ -36,8 +36,18 @@ MODULE_LICENSE("GPL");
  */
 static int __init cachefiles_init(void)
 {
+	int ret;
+
+	ret = cachefiles_register_error_injection();
+	if (ret < 0)
+		goto error_einj;
+
 	pr_info("Loaded\n");
 	return 0;
+
+error_einj:
+	pr_err("failed to register: %d\n", ret);
+	return ret;
 }
 
 fs_initcall(cachefiles_init);
@@ -48,6 +58,8 @@ fs_initcall(cachefiles_init);
 static void __exit cachefiles_exit(void)
 {
 	pr_info("Unloading\n");
+
+	cachefiles_unregister_error_injection();
 }
 
 module_exit(cachefiles_exit);