Date:   Mon, 16 Oct 2017 17:30:54 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     linux-kernel@...r.kernel.org
Cc:     ebiederm@...ssion.com, serge@...lyn.com, stgraber@...ntu.com,
        tycho@...ho.ws, Christian Brauner <christian.brauner@...ntu.com>
Subject: [PATCH 2/2 v2] user namespaces: bump idmap limits to 340

There a 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 kalloc() of
4k which means we can have a maximum of 340 mappings
(340 * size(struct uid_gid_extent) = 4080). For the latter case we use two
pointer *forward, *reverse. The forward pointer points to an array sorted by
"first" and reverse 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 patch 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-V2 |
|--------------|------------|----------|
|   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:

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) {
			printf("asdf: %s\n", strerror(errno));
			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: Eric Biederman Hallyn <serge@...lyn.com>
CC: Serge Hallyn <ebiederm@...ssion.com>

Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
---
Changelog 2017-10-16:
* Use completely new approach which makes it pointless to go into specific
  differences.
---
 include/linux/user_namespace.h |   7 +-
 kernel/user_namespace.c        | 359 +++++++++++++++++++++++++++++++++++++----
 2 files changed, 333 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..eceb03ae597c 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 +183,22 @@ static void free_user_ns(struct work_struct *work)
 	do {
 		struct ucounts *ucounts = ns->ucounts;
 		parent = ns->parent;
+
+		mutex_lock(&userns_state_mutex);
+		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);
+		}
+		mutex_unlock(&userns_state_mutex);
+
 		retire_userns_sysctls(ns);
 #ifdef CONFIG_PERSISTENT_KEYRINGS
 		key_put(ns->persistent_keyring_register);
@@ -198,7 +216,84 @@ 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 +319,23 @@ 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 +358,23 @@ 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 +397,45 @@ 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 +706,15 @@ 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;
+
+	if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS)
+		return &map->extent[pos];
 
-	return extent;
+	return &map->forward[pos];
 }
 
 static void *uid_m_start(struct seq_file *seq, loff_t *ppos)
@@ -618,7 +786,10 @@ 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 +809,104 @@ 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);
+	}
+
+	/* 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 +917,7 @@ 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 +942,8 @@ 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 +971,7 @@ 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 +983,17 @@ 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 +1003,33 @@ 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 +1040,26 @@ 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 +1067,34 @@ 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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