Date:   Sun, 22 Oct 2017 20:18:20 +0200
From:   Christian Brauner <christian.brauner@...onical.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     ebiederm@...ssion.com, linux-kernel@...r.kernel.org,
        serge@...lyn.com, stgraber@...ntu.com, tycho@...ho.ws
Subject: Re: [PATCH 2/2 v4] user namespaces: bump idmap limits to 340

On Thu, Oct 19, 2017 at 09:11:36PM +0200, Christian Brauner wrote:
> There are quite some use cases where users run into the current limit for
> {g,u}id mappings. Consider a user requesting us to map everything but 999, and
> 1001 for a given range of 1000000000 with a sub{g,u}id layout of:
> 
> some-user:100000:1000000000
> some-user:999:1
> some-user:1000:1
> some-user:1001:1
> some-user:1002:1
> 
> This translates to:
> 
> MAPPING-TYPE | CONTAINER |    HOST |     RANGE |
> -------------|-----------|---------|-----------|
>          uid |       999 |     999 |         1 |
>          uid |      1001 |    1001 |         1 |
>          uid |         0 | 1000000 |       999 |
>          uid |      1000 | 1001000 |         1 |
>          uid |      1002 | 1001002 | 999998998 |
> ------------------------------------------------
>          gid |       999 |     999 |         1 |
>          gid |      1001 |    1001 |         1 |
>          gid |         0 | 1000000 |       999 |
>          gid |      1000 | 1001000 |         1 |
>          gid |      1002 | 1001002 | 999998998 |
> 
> which is already the current limit.
> 
> As discussed at LPC simply bumping the number of limits is not going to work
> since this would mean that struct uid_gid_map won't fit into a single cache-line
> anymore thereby regressing performance for the base-cases. The same problem
> seems to arise when using a single pointer. So the idea is to use
> 
> struct uid_gid_extent {
> 	u32 first;
> 	u32 lower_first;
> 	u32 count;
> };
> 
> struct uid_gid_map { /* 64 bytes -- 1 cache line */
> 	u32 nr_extents;
> 	union {
> 		struct uid_gid_extent extent[UID_GID_MAP_MAX_BASE_EXTENTS];
> 		struct {
> 			struct uid_gid_extent *forward;
> 			struct uid_gid_extent *reverse;
> 		};
> 	};
> };
> 
> For the base cases we will only use the struct uid_gid_extent extent member. If
> we go over UID_GID_MAP_MAX_BASE_EXTENTS mappings we perform a single 4k
> kmalloc() which means we can have a maximum of 340 mappings
> (340 * size(struct uid_gid_extent) = 4080). For the latter case we use two
> pointers "forward" and "reverse". The forward pointer points to an array sorted
> by "first" and the reverse pointer points to an array sorted by "lower_first".
> We can then perform binary search on those arrays.
> 
> Performance Testing:
> When Eric introduced the extent-based struct uid_gid_map approach he measured
> the performanc impact of his idmap changes:
> 
> > My benchmark consisted of going to single user mode where nothing else was
> > running. On an ext4 filesystem opening 1,000,000 files and looping through all
> > of the files 1000 times and calling fstat on the individuals files. This was
> > to ensure I was benchmarking stat times where the inodes were in the kernels
> > cache, but the inode values were not in the processors cache. My results:
> 
> > v3.4-rc1:         ~= 156ns (unmodified v3.4-rc1 with user namespace support disabled)
> > v3.4-rc1-userns-: ~= 155ns (v3.4-rc1 with my user namespace patches and user namespace support disabled)
> > v3.4-rc1-userns+: ~= 164ns (v3.4-rc1 with my user namespace patches and user namespace support enabled)
> 
> I used an identical approach on my laptop. Here's a thorough description of what
> I did. I built a 4.14.0-rc4 mainline kernel with my new idmap patches applied. I
> booted into single user mode and used an ext4 filesystem to open/create
> 1,000,000 files. Then I looped through all of the files calling fstat() on each
> of them 1000 times and calculated the mean fstat() time for a single file. (The
> test program can be found below.)
> 
> Here are the results. For fun, I compared the first version of my patch which
> scaled linearly with the new version of the patch:
> 
> |   # MAPPINGS |   PATCH-V1 | PATCH-NEW |
> |--------------|------------|-----------|
> |   0 mappings |     158 ns |   158 ns  |
> |   1 mappings |     164 ns |   157 ns  |
> |   2 mappings |     170 ns |   158 ns  |
> |   3 mappings |     175 ns |   161 ns  |
> |   5 mappings |     187 ns |   165 ns  |
> |  10 mappings |     218 ns |   199 ns  |
> |  50 mappings |     528 ns |   218 ns  |
> | 100 mappings |     980 ns |   229 ns  |
> | 200 mappings |    1880 ns |   239 ns  |
> | 300 mappings |    2760 ns |   240 ns  |
> | 340 mappings | not tested |   248 ns  |
> 
> Here's the test program I used. I asked Eric what he did and this is a more
> "advanced" implementation of the idea. It's pretty straight-forward:
> 
>  #define __GNU_SOURCE
>  #define __STDC_FORMAT_MACROS
>  #include <errno.h>
>  #include <dirent.h>
>  #include <fcntl.h>
>  #include <inttypes.h>
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <string.h>
>  #include <unistd.h>
>  #include <sys/stat.h>
>  #include <sys/time.h>
>  #include <sys/types.h>
> 
>  int main(int argc, char *argv[])
>  {
>  	int ret;
>  	size_t i, k;
>  	int fd[1000000];
>  	int times[1000];
>  	char pathname[4096];
>  	struct stat st;
>  	struct timeval t1, t2;
>  	uint64_t time_in_mcs;
>  	uint64_t sum = 0;
> 
>  	if (argc != 2) {
>  		fprintf(stderr, "Please specify a directory where to create "
>  				"the test files\n");
>  		exit(EXIT_FAILURE);
>  	}
> 
>  	for (i = 0; i < sizeof(fd) / sizeof(fd[0]); i++) {
>  		sprintf(pathname, "%s/idmap_test_%zu", argv[1], i);
>  		fd[i]= open(pathname, O_RDWR | O_CREAT, S_IXUSR | S_IXGRP | S_IXOTH);
>  		if (fd[i] < 0) {
>  			ssize_t j;
>  			for (j = i; j >= 0; j--)
>  				close(fd[j]);
>  			exit(EXIT_FAILURE);
>  		}
>  	}
> 
>  	for (k = 0; k < 1000; k++) {
>  		ret = gettimeofday(&t1, NULL);
>  		if (ret < 0)
>  			goto close_all;
> 
>  		for (i = 0; i < sizeof(fd) / sizeof(fd[0]); i++) {
>  			ret = fstat(fd[i], &st);
>  			if (ret < 0)
>  				goto close_all;
>  		}
> 
>  		ret = gettimeofday(&t2, NULL);
>  		if (ret < 0)
>  			goto close_all;
> 
>  		time_in_mcs = (1000000 * t2.tv_sec + t2.tv_usec) -
>  			      (1000000 * t1.tv_sec + t1.tv_usec);
>  		printf("Total time in micro seconds:       %" PRIu64 "\n",
>  		       time_in_mcs);
>  		printf("Total time in nanoseconds:         %" PRIu64 "\n",
>  		       time_in_mcs * 1000);
>  		printf("Time per file in nanoseconds:      %" PRIu64 "\n",
>  		       (time_in_mcs * 1000) / 1000000);
>  		times[k] = (time_in_mcs * 1000) / 1000000;
>  	}
> 
>  close_all:
>  	for (i = 0; i < sizeof(fd) / sizeof(fd[0]); i++)
>  		close(fd[i]);
> 
>  	if (ret < 0)
>  		exit(EXIT_FAILURE);
> 
>  	for (k = 0; k < 1000; k++) {
>  		sum += times[k];
>  	}
> 
>  	printf("Mean time per file in nanoseconds: %" PRIu64 "\n", sum / 1000);
> 
>  	exit(EXIT_SUCCESS);;
>  }
> 
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> CC: Serge Hallyn <serge@...lyn.com>
> CC: Eric Biederman <ebiederm@...ssion.com>
> ---
> Changelog 2017-10-19:
> * kernel/user_namespace.c: Remove unnecessary mutex when freeing extents.
> * non-functional: Fix typos in commit message.
> * non-functional: Add headers to test program in commit message to make tests I
>   performed easily reproducible.
> ---
>  include/linux/user_namespace.h |   7 +-
>  kernel/user_namespace.c        | 355 +++++++++++++++++++++++++++++++++++++----
>  2 files changed, 329 insertions(+), 33 deletions(-)
> 
> diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
> index 7c83d7f6289b..1c1046a60fb4 100644
> --- a/include/linux/user_namespace.h
> +++ b/include/linux/user_namespace.h
> @@ -10,7 +10,8 @@
>  #include <linux/sysctl.h>
>  #include <linux/err.h>
>  
> -#define UID_GID_MAP_MAX_EXTENTS 5
> +#define UID_GID_MAP_MAX_BASE_EXTENTS 5
> +#define UID_GID_MAP_MAX_EXTENTS 340
>  
>  struct uid_gid_extent {
>  	u32 first;
> @@ -18,10 +19,10 @@ struct uid_gid_extent {
>  	u32 count;
>  };
>  
> -struct uid_gid_map {	/* 64 bytes -- 1 cache line */
> +struct uid_gid_map { /* 64 bytes -- 1 cache line */
>  	u32 nr_extents;
>  	union {
> -		struct uid_gid_extent extent[UID_GID_MAP_MAX_EXTENTS];
> +		struct uid_gid_extent extent[UID_GID_MAP_MAX_BASE_EXTENTS];
>  		struct {
>  			struct uid_gid_extent *forward;
>  			struct uid_gid_extent *reverse;
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index c490f1e4313b..c311249b7596 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -23,6 +23,8 @@
>  #include <linux/ctype.h>
>  #include <linux/projid.h>
>  #include <linux/fs_struct.h>
> +#include <linux/bsearch.h>
> +#include <linux/sort.h>
>  
>  static struct kmem_cache *user_ns_cachep __read_mostly;
>  static DEFINE_MUTEX(userns_state_mutex);
> @@ -181,6 +183,18 @@ static void free_user_ns(struct work_struct *work)
>  	do {
>  		struct ucounts *ucounts = ns->ucounts;
>  		parent = ns->parent;
> +		if (ns->gid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
> +			kfree(ns->gid_map.forward);
> +			kfree(ns->gid_map.reverse);
> +		}
> +		if (ns->uid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
> +			kfree(ns->uid_map.forward);
> +			kfree(ns->uid_map.reverse);
> +		}
> +		if (ns->projid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
> +			kfree(ns->projid_map.forward);
> +			kfree(ns->projid_map.reverse);
> +		}
>  		retire_userns_sysctls(ns);
>  #ifdef CONFIG_PERSISTENT_KEYRINGS
>  		key_put(ns->persistent_keyring_register);
> @@ -198,7 +212,84 @@ void __put_user_ns(struct user_namespace *ns)
>  }
>  EXPORT_SYMBOL(__put_user_ns);
>  
> -static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
> +/**
> + * idmap_key struct holds the information necessary to find an idmapping in a
> + * sorted idmap array. It is passed to cmp_map_id() as first argument.
> + */
> +struct idmap_key {
> +	bool map_up; /* true  -> id from kid; false -> kid from id */
> +	u32 id; /* id to find */
> +	u32 count; /* == 0 unless used with map_id_range_down() */
> +};
> +
> +/**
> + * cmp_map_id - Function to be passed to bsearch() to find the requested
> + * idmapping. Expects struct idmap_key to be passed via @k.
> + */
> +static int cmp_map_id(const void *k, const void *e)
> +{
> +	u32 first, last, id2;
> +	const struct idmap_key *key = k;
> +	const struct uid_gid_extent *el = e;
> +
> +	/* handle map_id_range_down() */
> +	if (key->count)
> +		id2 = key->id + key->count - 1;
> +	else
> +		id2 = key->id;
> +
> +	/* handle map_id_{down,up}() */
> +	if (key->map_up)
> +		first = el->lower_first;
> +	else
> +		first = el->first;
> +
> +	last = first + el->count - 1;
> +
> +	if (key->id >= first && key->id <= last &&
> +	    (id2 >= first && id2 <= last))
> +		return 0;
> +
> +	if (key->id < first || id2 < first)
> +		return -1;
> +
> +	return 1;
> +}
> +
> +/**
> + * map_id_range_down_max - Find idmap via binary search in ordered idmap array.
> + * Can only be called if number of mappings exceeds UID_GID_MAP_MAX_BASE_EXTENTS.
> + */
> +static u32 map_id_range_down_max(struct uid_gid_map *map, u32 id, u32 count)
> +{
> +	u32 extents;
> +	struct uid_gid_extent *extent;
> +	struct idmap_key key;
> +
> +	key.map_up = false;
> +	key.count = count;
> +	key.id = id;
> +
> +	extents = map->nr_extents;
> +	smp_rmb();
> +
> +	extent = bsearch(&key, map->forward, extents,
> +			 sizeof(struct uid_gid_extent), cmp_map_id);
> +	/* Map the id or note failure */
> +	if (extent)
> +		id = (id - extent->first) + extent->lower_first;
> +	else
> +		id = (u32) -1;
> +
> +	return id;
> +}
> +
> +/**
> + * map_id_range_down_base - Find idmap via binary search in static extent array.
> + * Can only be called if number of mappings is equal or less than
> + * UID_GID_MAP_MAX_BASE_EXTENTS.
> + */
> +static u32 map_id_range_down_base(struct uid_gid_map *map, u32 id, u32 count)
>  {
>  	unsigned idx, extents;
>  	u32 first, last, id2;
> @@ -224,7 +315,23 @@ static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
>  	return id;
>  }
>  
> -static u32 map_id_down(struct uid_gid_map *map, u32 id)
> +static u32 map_id_range_down(struct uid_gid_map *map, u32 id, u32 count)
> +{
> +	u32 extents = map->nr_extents;
> +	smp_rmb();
> +
> +	if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> +		return map_id_range_down_base(map, id, count);
> +
> +	return map_id_range_down_max(map, id, count);
> +}
> +
> +/**
> + * map_id_down_base - Find idmap via binary search in static extent array.
> + * Can only be called if number of mappings is equal or less than
> + * UID_GID_MAP_MAX_BASE_EXTENTS.
> + */
> +static u32 map_id_down_base(struct uid_gid_map *map, u32 id)
>  {
>  	unsigned idx, extents;
>  	u32 first, last;
> @@ -247,7 +354,23 @@ static u32 map_id_down(struct uid_gid_map *map, u32 id)
>  	return id;
>  }
>  
> -static u32 map_id_up(struct uid_gid_map *map, u32 id)
> +static u32 map_id_down(struct uid_gid_map *map, u32 id)
> +{
> +	u32 extents = map->nr_extents;
> +	smp_rmb();
> +
> +	if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> +		return map_id_down_base(map, id);
> +
> +	return map_id_range_down_max(map, id, 0);
> +}
> +
> +/**
> + * map_id_up_base - Find idmap via binary search in static extent array.
> + * Can only be called if number of mappings is equal or less than
> + * UID_GID_MAP_MAX_BASE_EXTENTS.
> + */
> +static u32 map_id_up_base(struct uid_gid_map *map, u32 id)
>  {
>  	unsigned idx, extents;
>  	u32 first, last;
> @@ -270,6 +393,45 @@ static u32 map_id_up(struct uid_gid_map *map, u32 id)
>  	return id;
>  }
>  
> +/**
> + * map_id_up_max - Find idmap via binary search in ordered idmap array.
> + * Can only be called if number of mappings exceeds UID_GID_MAP_MAX_BASE_EXTENTS.
> + */
> +static u32 map_id_up_max(struct uid_gid_map *map, u32 id)
> +{
> +	u32 extents;
> +	struct uid_gid_extent *extent;
> +	struct idmap_key key;
> +
> +	key.map_up = true;
> +	key.count = 0;
> +	key.id = id;
> +
> +	extents = map->nr_extents;
> +	smp_rmb();
> +
> +	extent = bsearch(&key, map->reverse, extents,
> +			 sizeof(struct uid_gid_extent), cmp_map_id);
> +	/* Map the id or note failure */
> +	if (extent)
> +		id = (id - extent->lower_first) + extent->first;
> +	else
> +		id = (u32) -1;
> +
> +	return id;
> +}
> +
> +static u32 map_id_up(struct uid_gid_map *map, u32 id)
> +{
> +	u32 extents = map->nr_extents;
> +	smp_rmb();
> +
> +	if (extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> +		return map_id_up_base(map, id);
> +
> +	return map_id_up_max(map, id);
> +}
> +
>  /**
>   *	make_kuid - Map a user-namespace uid pair into a kuid.
>   *	@ns:  User namespace that the uid is in
> @@ -540,13 +702,15 @@ static int projid_m_show(struct seq_file *seq, void *v)
>  static void *m_start(struct seq_file *seq, loff_t *ppos,
>  		     struct uid_gid_map *map)
>  {
> -	struct uid_gid_extent *extent = NULL;
>  	loff_t pos = *ppos;
>  
> -	if (pos < map->nr_extents)
> -		extent = &map->extent[pos];
> +	if (pos >= map->nr_extents)
> +		return NULL;
>  
> -	return extent;
> +	if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> +		return &map->extent[pos];
> +
> +	return &map->forward[pos];
>  }
>  
>  static void *uid_m_start(struct seq_file *seq, loff_t *ppos)
> @@ -618,7 +782,10 @@ static bool mappings_overlap(struct uid_gid_map *new_map,
>  		u32 prev_upper_last, prev_lower_last;
>  		struct uid_gid_extent *prev;
>  
> -		prev = &new_map->extent[idx];
> +		if (new_map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> +			prev = &new_map->extent[idx];
> +		else
> +			prev = &new_map->forward[idx];
>  
>  		prev_upper_first = prev->first;
>  		prev_lower_first = prev->lower_first;
> @@ -638,6 +805,104 @@ static bool mappings_overlap(struct uid_gid_map *new_map,
>  	return false;
>  }
>  
> +/**
> + * insert_extent - Safely insert a new idmap extent into struct uid_gid_map.
> + * Takes care to allocate a 4K block of memory if the number of mappings exceeds
> + * UID_GID_MAP_MAX_BASE_EXTENTS.
> + */
> +static int insert_extent(struct uid_gid_map *map, struct uid_gid_extent *extent)
> +{
> +	if (map->nr_extents < UID_GID_MAP_MAX_BASE_EXTENTS) {
> +		map->extent[map->nr_extents].first = extent->first;
> +		map->extent[map->nr_extents].lower_first = extent->lower_first;
> +		map->extent[map->nr_extents].count = extent->count;
> +		return 0;
> +	}
> +
> +	if (map->nr_extents == UID_GID_MAP_MAX_BASE_EXTENTS) {
> +		struct uid_gid_extent *forward;
> +
> +		/* Allocate memory for 340 mappings. */
> +		forward = kmalloc(sizeof(struct uid_gid_extent) *
> +				 UID_GID_MAP_MAX_EXTENTS, GFP_KERNEL);
> +		if (IS_ERR(forward))
> +			return PTR_ERR(forward);
> +
> +		/* Copy over memory. Only set up memory for the forward pointer.
> +		 * Defer the memory setup for the reverse pointer.
> +		 */
> +		memcpy(forward, map->extent,
> +		       map->nr_extents * sizeof(map->extent[0]));
> +
> +		map->forward = forward;
> +		map->reverse = NULL;
> +	}
> +
> +	map->forward[map->nr_extents].first = extent->first;
> +	map->forward[map->nr_extents].lower_first = extent->lower_first;
> +	map->forward[map->nr_extents].count = extent->count;
> +	return 0;
> +}
> +
> +/* cmp function to sort() forward mappings */
> +static int cmp_extents_forward(const void *a, const void *b)
> +{
> +	const struct uid_gid_extent *e1 = a;
> +	const struct uid_gid_extent *e2 = b;
> +
> +	if (e1->first < e2->first)
> +		return -1;
> +
> +	if (e1->first > e2->first)
> +		return 1;
> +
> +	return 0;
> +}
> +
> +/* cmp function to sort() reverse mappings */
> +static int cmp_extents_reverse(const void *a, const void *b)
> +{
> +	const struct uid_gid_extent *e1 = a;
> +	const struct uid_gid_extent *e2 = b;
> +
> +	if (e1->lower_first < e2->lower_first)
> +		return -1;
> +
> +	if (e1->lower_first > e2->lower_first)
> +		return 1;
> +
> +	return 0;
> +}
> +
> +/**
> + * sort_idmaps - Sorts an array of idmap entries.
> + * Can only be called if number of mappings exceeds UID_GID_MAP_MAX_BASE_EXTENTS.
> + */
> +static int sort_idmaps(struct uid_gid_map *map)
> +{
> +	if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> +		return 0;
> +
> +	/* Sort forward array. */
> +	sort(map->forward, map->nr_extents, sizeof(struct uid_gid_extent),
> +	     cmp_extents_forward, NULL);
> +
> +	/* Only copy the memory from forward we actually need. */
> +	map->reverse = kmemdup(map->forward,
> +			       map->nr_extents * sizeof(struct uid_gid_extent),
> +			       GFP_KERNEL);
> +	if (IS_ERR(map->reverse)) {
> +		map->reverse = NULL;
> +		return PTR_ERR(map->reverse);

This is wrong. kmemdup() does not return IS_ERR(). It returns NULL. So this
check must be adapted to:

if (!map->revers)
        return -ENOMEM;

Sending a new version of the patch.

> +	}
> +
> +	/* Sort reverse array. */
> +	sort(map->reverse, map->nr_extents, sizeof(struct uid_gid_extent),
> +	     cmp_extents_reverse, NULL);
> +
> +	return 0;
> +}
> +
>  static ssize_t map_write(struct file *file, const char __user *buf,
>  			 size_t count, loff_t *ppos,
>  			 int cap_setid,
> @@ -648,7 +913,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>  	struct user_namespace *ns = seq->private;
>  	struct uid_gid_map new_map;
>  	unsigned idx;
> -	struct uid_gid_extent *extent = NULL;
> +	struct uid_gid_extent extent;
>  	char *kbuf = NULL, *pos, *next_line;
>  	ssize_t ret = -EINVAL;
>  
> @@ -673,6 +938,8 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>  	 */
>  	mutex_lock(&userns_state_mutex);
>  
> +	memset(&new_map, 0, sizeof(struct uid_gid_map));
> +
>  	ret = -EPERM;
>  	/* Only allow one successful write to the map */
>  	if (map->nr_extents != 0)
> @@ -700,9 +967,7 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>  	/* Parse the user data */
>  	ret = -EINVAL;
>  	pos = kbuf;
> -	new_map.nr_extents = 0;
>  	for (; pos; pos = next_line) {
> -		extent = &new_map.extent[new_map.nr_extents];
>  
>  		/* Find the end of line and ensure I don't look past it */
>  		next_line = strchr(pos, '\n');
> @@ -714,17 +979,17 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>  		}
>  
>  		pos = skip_spaces(pos);
> -		extent->first = simple_strtoul(pos, &pos, 10);
> +		extent.first = simple_strtoul(pos, &pos, 10);
>  		if (!isspace(*pos))
>  			goto out;
>  
>  		pos = skip_spaces(pos);
> -		extent->lower_first = simple_strtoul(pos, &pos, 10);
> +		extent.lower_first = simple_strtoul(pos, &pos, 10);
>  		if (!isspace(*pos))
>  			goto out;
>  
>  		pos = skip_spaces(pos);
> -		extent->count = simple_strtoul(pos, &pos, 10);
> +		extent.count = simple_strtoul(pos, &pos, 10);
>  		if (*pos && !isspace(*pos))
>  			goto out;
>  
> @@ -734,29 +999,33 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>  			goto out;
>  
>  		/* Verify we have been given valid starting values */
> -		if ((extent->first == (u32) -1) ||
> -		    (extent->lower_first == (u32) -1))
> +		if ((extent.first == (u32) -1) ||
> +		    (extent.lower_first == (u32) -1))
>  			goto out;
>  
>  		/* Verify count is not zero and does not cause the
>  		 * extent to wrap
>  		 */
> -		if ((extent->first + extent->count) <= extent->first)
> +		if ((extent.first + extent.count) <= extent.first)
>  			goto out;
> -		if ((extent->lower_first + extent->count) <=
> -		     extent->lower_first)
> +		if ((extent.lower_first + extent.count) <=
> +		     extent.lower_first)
>  			goto out;
>  
>  		/* Do the ranges in extent overlap any previous extents? */
> -		if (mappings_overlap(&new_map, extent))
> +		if (mappings_overlap(&new_map, &extent))
>  			goto out;
>  
> -		new_map.nr_extents++;
> -
> -		/* Fail if the file contains too many extents */
> -		if ((new_map.nr_extents == UID_GID_MAP_MAX_EXTENTS) &&
> +		if ((new_map.nr_extents + 1) == UID_GID_MAP_MAX_EXTENTS &&
>  		    (next_line != NULL))
>  			goto out;
> +
> +		ret = insert_extent(&new_map, &extent);
> +		if (ret < 0)
> +			goto out;
> +		ret = -EINVAL;
> +
> +		new_map.nr_extents++;
>  	}
>  	/* Be very certaint the new map actually exists */
>  	if (new_map.nr_extents == 0)
> @@ -767,16 +1036,26 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>  	if (!new_idmap_permitted(file, ns, cap_setid, &new_map))
>  		goto out;
>  
> +	ret = sort_idmaps(&new_map);
> +	if (ret < 0)
> +		goto out;
> +
> +	ret = -EPERM;
>  	/* Map the lower ids from the parent user namespace to the
>  	 * kernel global id space.
>  	 */
>  	for (idx = 0; idx < new_map.nr_extents; idx++) {
> +		struct uid_gid_extent *e;
>  		u32 lower_first;
> -		extent = &new_map.extent[idx];
> +
> +		if (new_map.nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
> +			e = &new_map.extent[idx];
> +		else
> +			e = &new_map.forward[idx];
>  
>  		lower_first = map_id_range_down(parent_map,
> -						extent->lower_first,
> -						extent->count);
> +						e->lower_first,
> +						e->count);
>  
>  		/* Fail if we can not map the specified extent to
>  		 * the kernel global id space.
> @@ -784,18 +1063,34 @@ static ssize_t map_write(struct file *file, const char __user *buf,
>  		if (lower_first == (u32) -1)
>  			goto out;
>  
> -		extent->lower_first = lower_first;
> +		e->lower_first = lower_first;
>  	}
>  
>  	/* Install the map */
> -	memcpy(map->extent, new_map.extent,
> -		new_map.nr_extents*sizeof(new_map.extent[0]));
> +	if (new_map.nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) {
> +		memcpy(map->extent, new_map.extent,
> +		       new_map.nr_extents * sizeof(new_map.extent[0]));
> +	} else {
> +		map->forward = new_map.forward;
> +		map->reverse = new_map.reverse;
> +	}
>  	smp_wmb();
>  	map->nr_extents = new_map.nr_extents;
>  
>  	*ppos = count;
>  	ret = count;
>  out:
> +	if (ret < 0 && new_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
> +		if (new_map.forward)
> +			kfree(new_map.forward);
> +		map->forward = NULL;
> +
> +		if (new_map.reverse)
> +			kfree(new_map.reverse);
> +		map->reverse = NULL;
> +		map->nr_extents = 0;
> +	}
> +
>  	mutex_unlock(&userns_state_mutex);
>  	kfree(kbuf);
>  	return ret;
> -- 
> 2.14.1
> 

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