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

[Dmitry Torokhov]

On Thu, May 25, 2017 at 05:16:29PM -0700, Luis R. Rodriguez wrote:
> This adds helpers for getting access to the kmod limit from
> userspace. This knob should help userspace more gracefully and
> deterministically handle module loading.

I think more details is needed before we add a new ABI to the kernel.
Why can't userspace submit as much as it wants and the kernel decide how
much it will service at once?

> 
> Cc: Tom Gundersen <teg@xxxxxxx>
> Cc: Petr Mladek <pmladek@xxxxxxxx>
> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>
> ---
>  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
> 

-- 
Dmitry