[RFC bpf-next] libbpf: increase rlimit before trying to create BPF maps

[Quentin Monnet <quentin.monnet@netronome.com>]

The limit for memory locked in the kernel by a process is usually set to
64 bytes by default. This can be an issue when creating large BPF maps.
A workaround is to raise this limit for the current process before
trying to create a new BPF map. Changing the hard limit requires the
CAP_SYS_RESOURCE and can usually only be done by root user (but then
only root can create BPF maps).

As far as I know there is not API to get the current amount of memory
locked for a user, therefore we cannot raise the limit only when
required. One solution, used by bcc, is to try to create the map, and on
getting a EPERM error, raising the limit to infinity before giving
another try. Another approach, used in iproute, is to raise the limit in
all cases, before trying to create the map.

Here we do the same as in iproute2: the rlimit is raised to infinity
before trying to load the map.

I send this patch as a RFC to see if people would prefer the bcc
approach instead, or the rlimit change to be in bpftool rather than in
libbpf.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
---
 tools/lib/bpf/bpf.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
index 03f9bcc4ef50..456a5a7b112c 100644
--- a/tools/lib/bpf/bpf.c
+++ b/tools/lib/bpf/bpf.c
@@ -26,6 +26,8 @@
 #include <unistd.h>
 #include <asm/unistd.h>
 #include <linux/bpf.h>
+#include <sys/resource.h>
+#include <sys/types.h>
 #include "bpf.h"
 #include "libbpf.h"
 #include <errno.h>
@@ -68,8 +70,11 @@ static inline int sys_bpf(enum bpf_cmd cmd, union bpf_attr *attr,
 int bpf_create_map_xattr(const struct bpf_create_map_attr *create_attr)
 {
 	__u32 name_len = create_attr->name ? strlen(create_attr->name) : 0;
+	struct rlimit rinf = { RLIM_INFINITY, RLIM_INFINITY };
 	union bpf_attr attr;
 
+	setrlimit(RLIMIT_MEMLOCK, &rinf);
+
 	memset(&attr, '\0', sizeof(attr));
 
 	attr.map_type = create_attr->map_type;
-- 
2.7.4