[RFC PATCH 3/6] sysctl, mod_devicetable: shadow struct ctl_table.procname for file2alias

[Mauricio Faria de Oliveira <mfo@canonical.com>]

In order to expose a sysctl entry to modpost (file2alias.c, precisely)
we have to shadow 'struct ctl_table' in mod_devicetable.h, as scripts
should not access kernel headers or its types (see file2alias.c).

The required field is '.procname' (basename of '/proc/sys/.../entry').

Since 'struct ctl_table' is annotated for structure randomization and
we need a known offset for '.procname' (remember, no kernel headers),
take it out of the randomized portion (as in, eg, 'struct task_struct').

Of course, add build-time checks for struct size and .procname offset
between both structs. (This has to be done on kernel side; for headers.)

With that in place, use the regular macros in devicetable-offsets.c to
define SIZE_... and OFF_... macros for the shadow struct and the field
of interest.

Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
---
 fs/proc/proc_sysctl.c             | 19 +++++++++++++++++++
 include/linux/mod_devicetable.h   | 25 +++++++++++++++++++++++++
 include/linux/sysctl.h            | 11 ++++++++++-
 kernel/sysctl.c                   |  1 +
 scripts/mod/devicetable-offsets.c |  3 +++
 5 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
index 021e83fe831f..ebbf8702387e 100644
--- a/fs/proc/proc_sysctl.c
+++ b/fs/proc/proc_sysctl.c
@@ -19,6 +19,24 @@
 #include <linux/kmemleak.h>
 #include "internal.h"
 
+#ifdef CONFIG_MODULES
+#include <linux/mod_devicetable.h>
+
+static void check_struct_sysctl_device_id(void)
+{
+	/*
+	 * The shadow struct sysctl_device_id for file2alias.c needs
+	 * the same size of struct ctl_table and offset for procname.
+	 */
+	BUILD_BUG_ON(sizeof(struct sysctl_device_id)
+			!= sizeof(struct ctl_table));
+	BUILD_BUG_ON(offsetof(struct sysctl_device_id, procname)
+			!= offsetof(struct ctl_table, procname));
+}
+#else
+static void check_struct_sysctl_device_id(void) {}
+#endif
+
 #define list_for_each_table_entry(entry, table) \
 	for ((entry) = (table); (entry)->procname; (entry)++)
 
@@ -1779,6 +1797,7 @@ int __init proc_sys_init(void)
 	proc_sys_root->proc_dir_ops = &proc_sys_dir_file_operations;
 	proc_sys_root->nlink = 0;
 
+	check_struct_sysctl_device_id();
 	return sysctl_init_bases();
 }
 
diff --git a/include/linux/mod_devicetable.h b/include/linux/mod_devicetable.h
index 549590e9c644..9cee024d8f2f 100644
--- a/include/linux/mod_devicetable.h
+++ b/include/linux/mod_devicetable.h
@@ -20,6 +20,31 @@ enum {
 	PCI_ID_F_VFIO_DRIVER_OVERRIDE = 1,
 };
 
+/*
+ * "Device" table entry for a sysctl file (shadow of struct ctl_table).
+ *
+ * Only the procname field is reliable (known offset); all other fields
+ * are in the randomized portion of struct ctl_table, do NOT use them.
+ */
+struct sysctl_device_id {
+
+	/* This must be the first field (shadowed from struct ctl_table). */
+	const char *procname;
+
+	/* Here begins the randomizable portion of struct ctl_table. */
+
+	void *data;
+	int maxlen;
+	unsigned short mode; // umode_t in <linux/types.h>
+	void *child;
+	void *proc_handler;
+	void *poll;
+	void *extra1;
+	void *extra2;
+
+	/* Here ends the randomizable portion of struct ctl_table. */
+};
+
 /**
  * struct pci_device_id - PCI device ID structure
  * @vendor:		Vendor ID to match (or PCI_ANY_ID)
diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
index 780690dc08cd..676112fde5ff 100644
--- a/include/linux/sysctl.h
+++ b/include/linux/sysctl.h
@@ -133,7 +133,13 @@ static inline void *proc_sys_poll_event(struct ctl_table_poll *poll)
 
 /* A sysctl table is an array of struct ctl_table: */
 struct ctl_table {
+
+	/* This must be the first field (shadowed to struct sysctl_device_id) */
 	const char *procname;		/* Text ID for /proc/sys, or zero */
+
+	/* This begins the randomizable portion of the struct. */
+	randomized_struct_fields_start
+
 	void *data;
 	int maxlen;
 	umode_t mode;
@@ -142,7 +148,10 @@ struct ctl_table {
 	struct ctl_table_poll *poll;
 	void *extra1;
 	void *extra2;
-} __randomize_layout;
+
+	/* New fields go above here, so they are in the randomized portion. */
+	randomized_struct_fields_end
+};
 
 struct ctl_node {
 	struct rb_node node;
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 223376959d29..15073621cfa8 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -2487,6 +2487,7 @@ int __init sysctl_init_bases(void)
 
 	return 0;
 }
+
 #endif /* CONFIG_SYSCTL */
 /*
  * No sense putting this after each symbol definition, twice,
diff --git a/scripts/mod/devicetable-offsets.c b/scripts/mod/devicetable-offsets.c
index c0d3bcb99138..43b2549940d2 100644
--- a/scripts/mod/devicetable-offsets.c
+++ b/scripts/mod/devicetable-offsets.c
@@ -262,5 +262,8 @@ int main(void)
 	DEVID(ishtp_device_id);
 	DEVID_FIELD(ishtp_device_id, guid);
 
+	DEVID(sysctl_device_id);
+	DEVID_FIELD(sysctl_device_id, procname);
+
 	return 0;
 }
-- 
2.25.1