[PATCH v2 4/5] kmod: add helpers for getting kmod limit

["Luis R. Rodriguez" <mcgrof@kernel.org>]

This adds helpers for getting access to the kmod limit from
userspace. This knob should help userspace more gracefully and
deterministically handle module loading.

Cc: Tom Gundersen <teg@jklm.no>
Cc: Petr Mladek <pmladek@suse.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
---
 Documentation/sysctl/kernel.txt | 20 ++++++++++++++++++++
 include/linux/kmod.h            |  5 +++++
 kernel/kmod.c                   | 33 +++++++++++++++++++++++++++++++++
 kernel/sysctl.c                 |  7 +++++++
 4 files changed, 65 insertions(+)

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index bac23c198360..080ccdca1f10 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -370,6 +370,26 @@ with the "modules_disabled" sysctl.
 
 ==============================================================
 
+kmod-limit:
+
+Get the max amount of concurrent requests (kmod_concurrent) the kernel can
+make out to userspace to call 'modprobe'. This limit is known internally to the
+kernel as max_modprobes. This interface is designed to enable userspace to
+query the kernel for the max_modprobes limit so userspace can more
+deterministically handle module loading by only enabling max_modprobes
+'modprobe' calls at a time.
+
+Dependencies are resolved in userspace through depmod, so one modprobe
+call only bumps the number of concurrent threads (kmod_concurrent) by one.
+Dependencies for a module then are loaded directly in userspace using
+init_module() / finit_module() skipping bumping kmod_concurrent or being
+affected by max_modprobes.
+
+The max_modprobes value is set at build time with CONFIG_MAX_KMOD_CONCURRENT.
+You can override at initialization with the module parameter max_modprobes.
+
+==============================================================
+
 kptr_restrict:
 
 This toggle indicates whether restrictions are placed on
diff --git a/include/linux/kmod.h b/include/linux/kmod.h
index 8e2f302b214a..bbc6d59190aa 100644
--- a/include/linux/kmod.h
+++ b/include/linux/kmod.h
@@ -39,13 +39,18 @@ int __request_module(bool wait, const char *name, ...);
 #define try_then_request_module(x, mod...) \
 	((x) ?: (__request_module(true, mod), (x)))
 void init_kmod_umh(void);
+unsigned int get_kmod_umh_limit(void);
+int sysctl_kmod_limit(struct ctl_table *table, int write,
+		      void __user *buffer, size_t *lenp, loff_t *ppos);
 #else
 static inline int request_module(const char *name, ...) { return -ENOSYS; }
 static inline int request_module_nowait(const char *name, ...) { return -ENOSYS; }
 #define try_then_request_module(x, mod...) (x)
 static inline void init_kmod_umh(void) { }
+static inline unsigned int get_kmod_umh_limit(void) { return 0; }
 #endif
 
+#define get_kmod_umh_limit get_kmod_umh_limit
 
 struct cred;
 struct file;
diff --git a/kernel/kmod.c b/kernel/kmod.c
index cafd27b92d19..17de776cf368 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -111,6 +111,17 @@ static int call_modprobe(char *module_name, int wait)
 }
 
 /**
+ * get_kmod_umh_limit - get concurrent modprobe thread limit
+ *
+ * Returns the number of allowed concurrent modprobe calls.
+ */
+unsigned int get_kmod_umh_limit(void)
+{
+	return max_modprobes;
+}
+EXPORT_SYMBOL_GPL(get_kmod_umh_limit);
+
+/**
  * __request_module - try to load a kernel module
  * @wait: wait (or not) for the operation to complete
  * @fmt: printf style format string for the name of the module
@@ -194,6 +205,28 @@ void __init init_kmod_umh(void)
 	atomic_set(&kmod_concurrent_max, max_modprobes);
 }
 
+int sysctl_kmod_limit(struct ctl_table *table, int write,
+		      void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct ctl_table t;
+	int ret;
+	unsigned int local_max_modprobes = max_modprobes;
+	unsigned int min = 0;
+	unsigned int max = max_threads/2;
+
+	t = *table;
+	t.data = &local_max_modprobes;
+	t.extra1 = &min;
+	t.extra2 = &max;
+
+	if (write)
+		return -EPERM;
+
+	ret = proc_douintvec_minmax(&t, write, buffer, lenp, ppos);
+
+	return ret;
+}
+
 #endif /* CONFIG_MODULES */
 
 static void call_usermodehelper_freeinfo(struct subprocess_info *info)
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index df9f2a367882..d1c1d1999bb1 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -682,6 +682,13 @@ static struct ctl_table kern_table[] = {
 		.extra1		= &one,
 		.extra2		= &one,
 	},
+	{
+		.procname	= "kmod-limit",
+		.data		= NULL, /* filled in by handler */
+		.maxlen		= sizeof(unsigned int),
+		.mode		= 0444,
+		.proc_handler	= sysctl_kmod_limit,
+	},
 #endif
 #ifdef CONFIG_UEVENT_HELPER
 	{
-- 
2.11.0