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

[Daniel Borkmann]

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

On Thu, Oct 20, 2016 at 9:58 AM, Alexei Starovoitov
 wrote:

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


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", ) == 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.


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

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

[William Tu]

> 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

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

[William Tu]

On Thu, Oct 20, 2016 at 9:58 AM, Alexei Starovoitov
 wrote:
> On Thu, Oct 20, 2016 at 06:04:38PM +0200, Daniel Borkmann wrote:
>>
>> 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", ) == 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

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

[Alexei Starovoitov]

On Thu, Oct 20, 2016 at 06:04:38PM +0200, Daniel Borkmann wrote:
> 
> 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", ) == 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!

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

[Daniel Borkmann]

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

On 10/19/2016 07:31 AM, 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.


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.


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", ) == 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 

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

[Daniel Borkmann]

On 10/19/2016 07:31 AM, 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.


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.


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.

Thanks,
Daniel

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

[Daniel Borkmann]

On 10/19/2016 07:31 AM, 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.


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.


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.

Thanks,
Daniel

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

[William Tu]

> ...
>> - 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

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

[Alexei Starovoitov]

On Sun, Oct 16, 2016 at 09:41:28AM -0700, William Tu wrote:
> 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 
> ---
>  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 */
> + };
...
> - 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.

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

[William Tu]

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 
---
 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, , );
+   ret = bpf_lookup_elem(fd, , , 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, , 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, , 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
---