[RFC PATCH net-next] bpf: fix potential percpu map overcopy to user.

When running bpf_map_lookup on percpu elements, the bytes copied to
userspace depends on num_possible_cpus() * value_size, which could
potentially be larger than memory allocated from user, which depends
on sysconf(_SC_NPROCESSORS_CONF) to get the current cpu num.  As a
result, the inconsistency might corrupt the user's stack.

The fact that sysconf(_SC_NPROCESSORS_CONF) != num_possible_cpu()
happens when cpu hotadd is enabled.  For example, in Fusion when
setting vcpu.hotadd = "TRUE" or in KVM, setting
  ./qemu-system-x86_64 -smp 2, maxcpus=4 ...
the num_possible_cpu() will be 4 and sysconf() will be 2[1].
Currently the any percpu map lookup suffers this issue, try
samples/bpf/test_maps.c or tracex3.c.

Th RFC patch adds additional 'size' param from userspace so that
kernel knows the maximum memory it should copy to the user.

[1] https://www.mail-archive.com/netdev@vger.kernel.org/msg121183.html

Signed-off-by: William Tu <redacted>
---
 include/uapi/linux/bpf.h       |  5 ++++-
 kernel/bpf/syscall.c           |  5 +++--
 samples/bpf/fds_example.c      |  2 +-
 samples/bpf/libbpf.c           |  3 ++-
 samples/bpf/libbpf.h           |  2 +-
 samples/bpf/test_maps.c        | 30 +++++++++++++++---------------
 tools/include/uapi/linux/bpf.h |  5 ++++-
 7 files changed, 30 insertions(+), 22 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index f09c70b..fa0c40b 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h

@@ -123,7 +123,10 @@ union bpf_attr {
 			__aligned_u64 value;
 			__aligned_u64 next_key;
 		};
-		__u64		flags;
+		union {
+			__u64		flags;
+			__u32		size; /* number of bytes allocated in userspace */
+		};
 	};
 
 	struct { /* anonymous struct used by BPF_PROG_LOAD command */

diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index 228f962..be211ea 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c

@@ -264,13 +264,14 @@ int __weak bpf_stackmap_copy(struct bpf_map *map, void *key, void *value)
 }
 
 /* last field in 'union bpf_attr' used by this command */
-#define BPF_MAP_LOOKUP_ELEM_LAST_FIELD value
+#define BPF_MAP_LOOKUP_ELEM_LAST_FIELD size
 
 static int map_lookup_elem(union bpf_attr *attr)
 {
 	void __user *ukey = u64_to_ptr(attr->key);
 	void __user *uvalue = u64_to_ptr(attr->value);
 	int ufd = attr->map_fd;
+	u32 usize = attr->size;
 	struct bpf_map *map;
 	void *key, *value, *ptr;
 	u32 value_size;

@@ -324,7 +325,7 @@ static int map_lookup_elem(union bpf_attr *attr)
 		goto free_value;
 
 	err = -EFAULT;
-	if (copy_to_user(uvalue, value, value_size) != 0)
+	if (copy_to_user(uvalue, value, min_t(u32, usize, value_size)) != 0)
 		goto free_value;
 
 	err = 0;

diff --git a/samples/bpf/fds_example.c b/samples/bpf/fds_example.c
index 625e797..5b833d8 100644
--- a/samples/bpf/fds_example.c
+++ b/samples/bpf/fds_example.c

@@ -88,7 +88,7 @@ static int bpf_do_map(const char *file, uint32_t flags, uint32_t key,
 		       ret, strerror(errno));
 		assert(ret == 0);
 	} else if (flags & BPF_F_KEY) {
-		ret = bpf_lookup_elem(fd, &key, &value);
+		ret = bpf_lookup_elem(fd, &key, &value, sizeof(value));
 		printf("bpf: fd:%d l->(%u):%u ret:(%d,%s)\n", fd, key, value,
 		       ret, strerror(errno));
 		assert(ret == 0);

diff --git a/samples/bpf/libbpf.c b/samples/bpf/libbpf.c
index 9969e35..9f0a1c3 100644
--- a/samples/bpf/libbpf.c
+++ b/samples/bpf/libbpf.c

@@ -44,12 +44,13 @@ int bpf_update_elem(int fd, void *key, void *value, unsigned long long flags)
 	return syscall(__NR_bpf, BPF_MAP_UPDATE_ELEM, &attr, sizeof(attr));
 }
 
-int bpf_lookup_elem(int fd, void *key, void *value)
+int bpf_lookup_elem(int fd, void *key, void *value, int size)
 {
 	union bpf_attr attr = {
 		.map_fd = fd,
 		.key = ptr_to_u64(key),
 		.value = ptr_to_u64(value),
+		.size = size,
 	};
 
 	return syscall(__NR_bpf, BPF_MAP_LOOKUP_ELEM, &attr, sizeof(attr));

diff --git a/samples/bpf/libbpf.h b/samples/bpf/libbpf.h
index ac6edb6..b911185 100644
--- a/samples/bpf/libbpf.h
+++ b/samples/bpf/libbpf.h

@@ -7,7 +7,7 @@ struct bpf_insn;
 int bpf_create_map(enum bpf_map_type map_type, int key_size, int value_size,
 		   int max_entries, int map_flags);
 int bpf_update_elem(int fd, void *key, void *value, unsigned long long flags);
-int bpf_lookup_elem(int fd, void *key, void *value);
+int bpf_lookup_elem(int fd, void *key, void *value, int size);
 int bpf_delete_elem(int fd, void *key);
 int bpf_get_next_key(int fd, void *key, void *next_key);
 

diff --git a/samples/bpf/test_maps.c b/samples/bpf/test_maps.c
index cce2b59..a6a8fbe 100644
--- a/samples/bpf/test_maps.c
+++ b/samples/bpf/test_maps.c

@@ -47,11 +47,11 @@ static void test_hashmap_sanity(int i, void *data)
 	assert(bpf_update_elem(map_fd, &key, &value, -1) == -1 && errno == EINVAL);
 
 	/* check that key=1 can be found */
-	assert(bpf_lookup_elem(map_fd, &key, &value) == 0 && value == 1234);
+	assert(bpf_lookup_elem(map_fd, &key, &value, sizeof(value)) == 0 && value == 1234);
 
 	key = 2;
 	/* check that key=2 is not found */
-	assert(bpf_lookup_elem(map_fd, &key, &value) == -1 && errno == ENOENT);
+	assert(bpf_lookup_elem(map_fd, &key, &value, sizeof(value)) == -1 && errno == ENOENT);
 
 	/* BPF_EXIST means: update existing element */
 	assert(bpf_update_elem(map_fd, &key, &value, BPF_EXIST) == -1 &&

@@ -139,11 +139,11 @@ static void test_percpu_hashmap_sanity(int task, void *data)
 	 * was run from a different cpu.
 	 */
 	value[0] = 1;
-	assert(bpf_lookup_elem(map_fd, &key, value) == 0 && value[0] == 100);
+	assert(bpf_lookup_elem(map_fd, &key, value, sizeof(value)) == 0 && value[0] == 100);
 
 	key = 2;
 	/* check that key=2 is not found */
-	assert(bpf_lookup_elem(map_fd, &key, value) == -1 && errno == ENOENT);
+	assert(bpf_lookup_elem(map_fd, &key, value, sizeof(value)) == -1 && errno == ENOENT);
 
 	/* BPF_EXIST means: update existing element */
 	assert(bpf_update_elem(map_fd, &key, value, BPF_EXIST) == -1 &&

@@ -170,7 +170,7 @@ static void test_percpu_hashmap_sanity(int task, void *data)
 		assert((expected_key_mask & next_key) == next_key);
 		expected_key_mask &= ~next_key;
 
-		assert(bpf_lookup_elem(map_fd, &next_key, value) == 0);
+		assert(bpf_lookup_elem(map_fd, &next_key, value, sizeof(value)) == 0);
 		for (i = 0; i < nr_cpus; i++)
 			assert(value[i] == i + 100);
 

@@ -218,11 +218,11 @@ static void test_arraymap_sanity(int i, void *data)
 	       errno == EEXIST);
 
 	/* check that key=1 can be found */
-	assert(bpf_lookup_elem(map_fd, &key, &value) == 0 && value == 1234);
+	assert(bpf_lookup_elem(map_fd, &key, &value, sizeof(value)) == 0 && value == 1234);
 
 	key = 0;
 	/* check that key=0 is also found and zero initialized */
-	assert(bpf_lookup_elem(map_fd, &key, &value) == 0 && value == 0);
+	assert(bpf_lookup_elem(map_fd, &key, &value, sizeof(value)) == 0 && value == 0);
 
 
 	/* key=0 and key=1 were inserted, check that key=2 cannot be inserted

@@ -233,7 +233,7 @@ static void test_arraymap_sanity(int i, void *data)
 	       errno == E2BIG);
 
 	/* check that key = 2 doesn't exist */
-	assert(bpf_lookup_elem(map_fd, &key, &value) == -1 && errno == ENOENT);
+	assert(bpf_lookup_elem(map_fd, &key, &value, sizeof(value)) == -1 && errno == ENOENT);
 
 	/* iterate over two elements */
 	assert(bpf_get_next_key(map_fd, &key, &next_key) == 0 &&

@@ -274,7 +274,7 @@ static void test_percpu_arraymap_many_keys(void)
 	for (key = 0; key < nr_keys; key++) {
 		for (i = 0; i < nr_cpus; i++)
 			values[i] = 0;
-		assert(bpf_lookup_elem(map_fd, &key, values) == 0);
+		assert(bpf_lookup_elem(map_fd, &key, values, sizeof(values)) == 0);
 		for (i = 0; i < nr_cpus; i++)
 			assert(values[i] == i + 10);
 	}

@@ -307,11 +307,11 @@ static void test_percpu_arraymap_sanity(int i, void *data)
 	       errno == EEXIST);
 
 	/* check that key=1 can be found */
-	assert(bpf_lookup_elem(map_fd, &key, values) == 0 && values[0] == 100);
+	assert(bpf_lookup_elem(map_fd, &key, values, sizeof(values)) == 0 && values[0] == 100);
 
 	key = 0;
 	/* check that key=0 is also found and zero initialized */
-	assert(bpf_lookup_elem(map_fd, &key, values) == 0 &&
+	assert(bpf_lookup_elem(map_fd, &key, values, sizeof(values)) == 0 &&
 	       values[0] == 0 && values[nr_cpus - 1] == 0);
 
 

@@ -321,7 +321,7 @@ static void test_percpu_arraymap_sanity(int i, void *data)
 	       errno == E2BIG);
 
 	/* check that key = 2 doesn't exist */
-	assert(bpf_lookup_elem(map_fd, &key, values) == -1 && errno == ENOENT);
+	assert(bpf_lookup_elem(map_fd, &key, values, sizeof(values)) == -1 && errno == ENOENT);
 
 	/* iterate over two elements */
 	assert(bpf_get_next_key(map_fd, &key, &next_key) == 0 &&

@@ -371,9 +371,9 @@ static void test_map_large(void)
 	assert(bpf_get_next_key(map_fd, &key, &key) == -1 && errno == ENOENT);
 
 	key.c = 0;
-	assert(bpf_lookup_elem(map_fd, &key, &value) == 0 && value == 0);
+	assert(bpf_lookup_elem(map_fd, &key, &value, sizeof(value)) == 0 && value == 0);
 	key.a = 1;
-	assert(bpf_lookup_elem(map_fd, &key, &value) == -1 && errno == ENOENT);
+	assert(bpf_lookup_elem(map_fd, &key, &value, sizeof(value)) == -1 && errno == ENOENT);
 
 	close(map_fd);
 }

@@ -466,7 +466,7 @@ static void test_map_parallel(void)
 	/* another check for all elements */
 	for (i = 0; i < MAP_SIZE; i++) {
 		key = MAP_SIZE - i - 1;
-		assert(bpf_lookup_elem(map_fd, &key, &value) == 0 &&
+		assert(bpf_lookup_elem(map_fd, &key, &value, sizeof(value)) == 0 &&
 		       value == key);
 	}
 

diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 9e5fc16..719fa25 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h

@@ -122,7 +122,10 @@ union bpf_attr {
 			__aligned_u64 value;
 			__aligned_u64 next_key;
 		};
-		__u64		flags;
+		union {
+			__u64		flags;
+			__u32		size; /* number of bytes allocated in userspace */
+		};
 	};
 
 	struct { /* anonymous struct used by BPF_PROG_LOAD command */

-- 
2.5.0

On Sun, Oct 16, 2016 at 09:41:28AM -0700, William Tu wrote:

...

-	if (copy_to_user(uvalue, value, value_size) != 0)
+	if (copy_to_user(uvalue, value, min_t(u32, usize, value_size)) != 0)
 		goto free_value;

I think such approach won't actually fix anything. User space
may lose some of the values and won't have any idea what was lost.
I think we need to fix sample code to avoid using sysconf(_SC_NPROCESSORS_CONF)
and use /sys/devices/system/cpu/possible instead.
I would argue that glibc should be fixed as well since relying on
ls -d /sys/devices/system/cpu/cpu[0-9]*|wc -l turned out to be incorrect.

...

quoted

-     if (copy_to_user(uvalue, value, value_size) != 0)
+     if (copy_to_user(uvalue, value, min_t(u32, usize, value_size)) != 0)
              goto free_value;

I think such approach won't actually fix anything. User space
may lose some of the values and won't have any idea what was lost.
I think we need to fix sample code to avoid using sysconf(_SC_NPROCESSORS_CONF)
and use /sys/devices/system/cpu/possible instead.
I would argue that glibc should be fixed as well since relying on
ls -d /sys/devices/system/cpu/cpu[0-9]*|wc -l turned out to be incorrect.

Thanks for the feedback. I think glibc is correct. The
_SC_NPROCESSORS_CONF presents the number of processors
configured/populated and is indeed "ls
/sys/devices/system/cpu/cpu[0-9]*|wc -l". This means the actual number
of CPUs installed on your system. On the other hand, the
num_possible_cpus() includes both the installed CPUs and the empty CPU
socket/slot, in order to support CPU hotplug.

As a example, one of my dual socket motherboard with 1 CPU installed has
# /sys/devices/system/cpu/possible
0-239
# /sys/devices/system/cpu/cpu[0-9]*|wc -l
12
Note that these 12 cpus could be online/offline by
# echo 1/0 > /sys/devices/system/cpu/cpuX/online
Even if it is offline, the entry is still there.

Thinking about another solution, maybe we should use
"num_present_cpus()" which means the configured/populated CPUs and the
value is the same as sysconf(_SC_NPROCESSORS_CONF). Consider:
1) cpuX is online/offline: the num_present_cpus() remains the same.
2) new cpu is hotplug into the empty socket: the num_present_cpus()
gets updates, and also the sysconf(_SC_NPROCESSORS_CONF).

+++ b/kernel/bpf/syscall.c

@@ -297,7 +297,7 @@ static int map_lookup_elem(union bpf_attr *attr)

        if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
            map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY)
-               value_size = round_up(map->value_size, 8) * num_possible_cpus();
+               value_size = round_up(map->value_size, 8) * num_present_cpus();
        else
                value_size = map->value_size;

Thanks. Regards,
William

On 10/19/2016 07:31 AM, William Tu wrote:

quoted

...

quoted

-     if (copy_to_user(uvalue, value, value_size) != 0)
+     if (copy_to_user(uvalue, value, min_t(u32, usize, value_size)) != 0)
               goto free_value;

I think such approach won't actually fix anything. User space
may lose some of the values and won't have any idea what was lost.
I think we need to fix sample code to avoid using sysconf(_SC_NPROCESSORS_CONF)
and use /sys/devices/system/cpu/possible instead.
I would argue that glibc should be fixed as well since relying on
ls -d /sys/devices/system/cpu/cpu[0-9]*|wc -l turned out to be incorrect.

Thanks for the feedback. I think glibc is correct. The
_SC_NPROCESSORS_CONF presents the number of processors
configured/populated and is indeed "ls
/sys/devices/system/cpu/cpu[0-9]*|wc -l". This means the actual number
of CPUs installed on your system. On the other hand, the

In glibc __get_nprocs_conf() seems to try a number of things, first it
tries equivalent of /sys/devices/system/cpu/cpu[0-9]*|wc -l, if that fails,
depending on the config, it either tries to count cpus in /proc/cpuinfo,
or returns the _SC_NPROCESSORS_ONLN value instead. If /proc/cpuinfo has
some issue, it returns just 1 worst case. _SC_NPROCESSORS_ONLN will parse
/sys/devices/system/cpu/online, if that fails it looks into /proc/stat
for cpuX entries, and if also that fails for some reason, /proc/cpuinfo
is consulted (and returning 1 if unlikely all breaks down).

num_possible_cpus() includes both the installed CPUs and the empty CPU
socket/slot, in order to support CPU hotplug.

Correct.

But as you say in 2) that also has a chance of being racy on CPU hotplug
compared to num_possible_cpus() which is fixed at boot time.

Documentation/cputopology.txt +106 says /sys/devices/system/cpu/possible
outputs cpu_possible_mask. That is the same as in num_possible_cpus(), so
first step would be to fix the buggy example code, imho.

What perhaps could be done in a second step to reduce overhead is an option
for bpf(2) to pass in a cpu mask similarly as for sched_{get,set}affinity()
syscalls, where user space can construct a mask via CPU_SET(3). For the
syscall time, kernel would lock hot plugging via get_online_cpus() and
put_online_cpus(), it would check whether passed CPUs are online to query
and if so then it would copy the values into the user provided buffer. I'd
think this might be useful in a number of ways anyway.

Thanks,
Daniel

On 10/19/2016 07:31 AM, William Tu wrote:

quoted

...

quoted

-     if (copy_to_user(uvalue, value, value_size) != 0)
+     if (copy_to_user(uvalue, value, min_t(u32, usize, value_size)) != 0)
               goto free_value;

I think such approach won't actually fix anything. User space
may lose some of the values and won't have any idea what was lost.
I think we need to fix sample code to avoid using sysconf(_SC_NPROCESSORS_CONF)
and use /sys/devices/system/cpu/possible instead.
I would argue that glibc should be fixed as well since relying on
ls -d /sys/devices/system/cpu/cpu[0-9]*|wc -l turned out to be incorrect.

Thanks for the feedback. I think glibc is correct. The
_SC_NPROCESSORS_CONF presents the number of processors
configured/populated and is indeed "ls
/sys/devices/system/cpu/cpu[0-9]*|wc -l". This means the actual number
of CPUs installed on your system. On the other hand, the

In glibc __get_nprocs_conf() seems to try a number of things, first it
tries equivalent of /sys/devices/system/cpu/cpu[0-9]*|wc -l, if that fails,
depending on the config, it either tries to count cpus in /proc/cpuinfo,
or returns the _SC_NPROCESSORS_ONLN value instead. If /proc/cpuinfo has
some issue, it returns just 1 worst case. _SC_NPROCESSORS_ONLN will parse
/sys/devices/system/cpu/online, if that fails it looks into /proc/stat
for cpuX entries, and if also that fails for some reason, /proc/cpuinfo
is consulted (and returning 1 if unlikely all breaks down).

num_possible_cpus() includes both the installed CPUs and the empty CPU
socket/slot, in order to support CPU hotplug.

Correct.

But as you say in 2) that also has a chance of being racy on CPU hotplug
compared to num_possible_cpus() which is fixed at boot time.

Documentation/cputopology.txt +106 says /sys/devices/system/cpu/possible
outputs cpu_possible_mask. That is the same as in num_possible_cpus(), so
first step would be to fix the buggy example code, imho.

What perhaps could be done in a second step to reduce overhead is an option
for bpf(2) to pass in a cpu mask similarly as for sched_{get,set}affinity()
syscalls, where user space can construct a mask via CPU_SET(3). For the
syscall time, kernel would lock hot plugging via get_online_cpus() and
put_online_cpus(), it would check whether passed CPUs are online to query
and if so then it would copy the values into the user provided buffer. I'd
think this might be useful in a number of ways anyway.

Thanks,
Daniel

On 10/19/2016 05:15 PM, Daniel Borkmann wrote:

On 10/19/2016 07:31 AM, William Tu wrote:

quoted

quoted

...

quoted

-     if (copy_to_user(uvalue, value, value_size) != 0)
+     if (copy_to_user(uvalue, value, min_t(u32, usize, value_size)) != 0)
               goto free_value;

I think such approach won't actually fix anything. User space
may lose some of the values and won't have any idea what was lost.
I think we need to fix sample code to avoid using sysconf(_SC_NPROCESSORS_CONF)
and use /sys/devices/system/cpu/possible instead.
I would argue that glibc should be fixed as well since relying on
ls -d /sys/devices/system/cpu/cpu[0-9]*|wc -l turned out to be incorrect.

Thanks for the feedback. I think glibc is correct. The
_SC_NPROCESSORS_CONF presents the number of processors
configured/populated and is indeed "ls
/sys/devices/system/cpu/cpu[0-9]*|wc -l". This means the actual number
of CPUs installed on your system. On the other hand, the

In glibc __get_nprocs_conf() seems to try a number of things, first it
tries equivalent of /sys/devices/system/cpu/cpu[0-9]*|wc -l, if that fails,
depending on the config, it either tries to count cpus in /proc/cpuinfo,
or returns the _SC_NPROCESSORS_ONLN value instead. If /proc/cpuinfo has
some issue, it returns just 1 worst case. _SC_NPROCESSORS_ONLN will parse
/sys/devices/system/cpu/online, if that fails it looks into /proc/stat
for cpuX entries, and if also that fails for some reason, /proc/cpuinfo
is consulted (and returning 1 if unlikely all breaks down).

quoted

num_possible_cpus() includes both the installed CPUs and the empty CPU
socket/slot, in order to support CPU hotplug.

Correct.

quoted

As a example, one of my dual socket motherboard with 1 CPU installed has
# /sys/devices/system/cpu/possible
0-239
# /sys/devices/system/cpu/cpu[0-9]*|wc -l
12
Note that these 12 cpus could be online/offline by
# echo 1/0 > /sys/devices/system/cpu/cpuX/online
Even if it is offline, the entry is still there.

Thinking about another solution, maybe we should use
"num_present_cpus()" which means the configured/populated CPUs and the
value is the same as sysconf(_SC_NPROCESSORS_CONF). Consider:
1) cpuX is online/offline: the num_present_cpus() remains the same.
2) new cpu is hotplug into the empty socket: the num_present_cpus()
gets updates, and also the sysconf(_SC_NPROCESSORS_CONF).

+++ b/kernel/bpf/syscall.c

@@ -297,7 +297,7 @@ static int map_lookup_elem(union bpf_attr *attr)

         if (map->map_type == BPF_MAP_TYPE_PERCPU_HASH ||
             map->map_type == BPF_MAP_TYPE_PERCPU_ARRAY)
-               value_size = round_up(map->value_size, 8) * num_possible_cpus();
+               value_size = round_up(map->value_size, 8) * num_present_cpus();
         else
                 value_size = map->value_size;

But as you say in 2) that also has a chance of being racy on CPU hotplug
compared to num_possible_cpus() which is fixed at boot time.

Documentation/cputopology.txt +106 says /sys/devices/system/cpu/possible
outputs cpu_possible_mask. That is the same as in num_possible_cpus(), so
first step would be to fix the buggy example code, imho.

What perhaps could be done in a second step to reduce overhead is an option
for bpf(2) to pass in a cpu mask similarly as for sched_{get,set}affinity()
syscalls, where user space can construct a mask via CPU_SET(3). For the
syscall time, kernel would lock hot plugging via get_online_cpus() and
put_online_cpus(), it would check whether passed CPUs are online to query
and if so then it would copy the values into the user provided buffer. I'd
think this might be useful in a number of ways anyway.

Maybe something like this as mentioned first step to fix the examples. Does
this work for you, William?

  tools/testing/selftests/bpf/test_maps.c | 33 ++++++++++++++++++++++++++++++---
  1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/test_maps.c b/tools/testing/selftests/bpf/test_maps.c
index ee384f0..d4832e8 100644
--- a/tools/testing/selftests/bpf/test_maps.c
+++ b/tools/testing/selftests/bpf/test_maps.c

@@ -25,6 +25,33 @@

  static int map_flags;

+static unsigned int num_possible_cpus(void)
+{
+	static const char *fcpu = "/sys/devices/system/cpu/possible";
+	unsigned int val, possible_cpus = 0;
+	char buff[128];
+	FILE *fp;
+
+	fp = fopen(fcpu, "r");
+	if (!fp) {
+		printf("Failed to open %s: '%s'!\n", fcpu, strerror(errno));
+		exit(1);
+	}
+
+	while (fgets(buff, sizeof(buff), fp)) {
+		if (sscanf(buff, "%*u-%u", &val) == 1)
+			possible_cpus = val;
+	}
+
+	fclose(fp);
+	if (!possible_cpus) {
+		printf("Failed to retrieve # possible CPUs!\n");
+		exit(1);
+	}
+
+	return possible_cpus;
+}
+
  static void test_hashmap(int task, void *data)
  {
  	long long key, next_key, value;

@@ -110,7 +137,7 @@ static void test_hashmap(int task, void *data)

  static void test_hashmap_percpu(int task, void *data)
  {
-	unsigned int nr_cpus = sysconf(_SC_NPROCESSORS_CONF);
+	unsigned int nr_cpus = num_possible_cpus();
  	long long value[nr_cpus];
  	long long key, next_key;
  	int expected_key_mask = 0;

@@ -258,7 +285,7 @@ static void test_arraymap(int task, void *data)

  static void test_arraymap_percpu(int task, void *data)
  {
-	unsigned int nr_cpus = sysconf(_SC_NPROCESSORS_CONF);
+	unsigned int nr_cpus = num_possible_cpus();
  	int key, next_key, fd, i;
  	long values[nr_cpus];

@@ -313,7 +340,7 @@ static void test_arraymap_percpu(int task, void *data)

  static void test_arraymap_percpu_many_keys(void)
  {
-	unsigned int nr_cpus = sysconf(_SC_NPROCESSORS_CONF);
+	unsigned int nr_cpus = num_possible_cpus();
  	unsigned int nr_keys = 20000;
  	long values[nr_cpus];
  	int key, fd, i;

-- 
1.9.3

On Thu, Oct 20, 2016 at 06:04:38PM +0200, Daniel Borkmann wrote:

looks great to me.
Could you move it into bpf_sys.h or somehow make it common in libbpf
and reuse it in samples/bpf/ ?
Since quite a few samples need this fix as well.
Thanks!

On Thu, Oct 20, 2016 at 9:58 AM, Alexei Starovoitov
[off-list ref] wrote:

On Thu, Oct 20, 2016 at 06:04:38PM +0200, Daniel Borkmann wrote:

quoted

diff --git a/tools/testing/selftests/bpf/test_maps.c b/tools/testing/selftests/bpf/test_maps.c
index ee384f0..d4832e8 100644
--- a/tools/testing/selftests/bpf/test_maps.c
+++ b/tools/testing/selftests/bpf/test_maps.c

@@ -25,6 +25,33 @@

 static int map_flags;

+static unsigned int num_possible_cpus(void)
+{
+     static const char *fcpu = "/sys/devices/system/cpu/possible";
+     unsigned int val, possible_cpus = 0;
+     char buff[128];
+     FILE *fp;
+
+     fp = fopen(fcpu, "r");
+     if (!fp) {
+             printf("Failed to open %s: '%s'!\n", fcpu, strerror(errno));
+             exit(1);
+     }
+
+     while (fgets(buff, sizeof(buff), fp)) {
+             if (sscanf(buff, "%*u-%u", &val) == 1)
+                     possible_cpus = val;
+     }

looks great to me.
Could you move it into bpf_sys.h or somehow make it common in libbpf
and reuse it in samples/bpf/ ?
Since quite a few samples need this fix as well.
Thanks!

Looks good to me. I tested it and it works fine.
Thanks!
William

Documentation/cputopology.txt +106 says /sys/devices/system/cpu/possible
outputs cpu_possible_mask. That is the same as in num_possible_cpus(), so
first step would be to fix the buggy example code, imho.

What perhaps could be done in a second step to reduce overhead is an option
for bpf(2) to pass in a cpu mask similarly as for sched_{get,set}affinity()
syscalls, where user space can construct a mask via CPU_SET(3). For the
syscall time, kernel would lock hot plugging via get_online_cpus() and
put_online_cpus(), it would check whether passed CPUs are online to query
and if so then it would copy the values into the user provided buffer. I'd
think this might be useful in a number of ways anyway.

I like this idea. So in this case, the only the data at the cpu
specified by user in the CPU_SET is copied to userspace, potentially
have better performance than always copying the data *
num_possible_cpus() bytes.

Regards,
William

On 10/20/2016 08:41 PM, William Tu wrote:

On Thu, Oct 20, 2016 at 9:58 AM, Alexei Starovoitov
[off-list ref] wrote:

quoted

On Thu, Oct 20, 2016 at 06:04:38PM +0200, Daniel Borkmann wrote:

quoted

diff --git a/tools/testing/selftests/bpf/test_maps.c b/tools/testing/selftests/bpf/test_maps.c
index ee384f0..d4832e8 100644
--- a/tools/testing/selftests/bpf/test_maps.c
+++ b/tools/testing/selftests/bpf/test_maps.c

@@ -25,6 +25,33 @@

  static int map_flags;

+static unsigned int num_possible_cpus(void)
+{
+     static const char *fcpu = "/sys/devices/system/cpu/possible";
+     unsigned int val, possible_cpus = 0;
+     char buff[128];
+     FILE *fp;
+
+     fp = fopen(fcpu, "r");
+     if (!fp) {
+             printf("Failed to open %s: '%s'!\n", fcpu, strerror(errno));
+             exit(1);
+     }
+
+     while (fgets(buff, sizeof(buff), fp)) {
+             if (sscanf(buff, "%*u-%u", &val) == 1)
+                     possible_cpus = val;
+     }

looks great to me.
Could you move it into bpf_sys.h or somehow make it common in libbpf
and reuse it in samples/bpf/ ?
Since quite a few samples need this fix as well.

Ahh, true.

quoted

Thanks!

Looks good to me. I tested it and it works fine.

Okay, thanks. I'll fix that up, mid-term we should try and move most of
that over to kernel selftests/bpf, and reuse tools/lib/bpf/.

Thanks!
William