[Edited Message Follows]

Thanks Song and Andrii for the response.

Use-case is global rate-limiting for incoming TCP connections. And we want to implement the token bucket algorithm using XDP for this purpose.

So we are planning to have a map that holds a token counter which gets two kinds of updates:

1. Periodic increments with 'x' number of tokens per second
2. Decrements as and when we get a new TCP connection request.

Most of our systems are 64 core machines. Since every core would try to update the counter in parallel as the packets arrive each of them, the problem I am imagining is that I might miss few updates of the counter as one core update can overwrite other’s.

I guess it is still ok to lose the case 2 type of updates as that might just allow a small fraction of more or less connections than what is configured.

But I cannot afford to lose case 1 kind of updates as that could mean that I cannot process bunch of connections until the next second.

So if I use "__sync_fetch_and_add" for incrementing the counter (for case 1), would it guarantee that this update is never missed(though some other core is trying to update the map to decrement the counter to account the incoming connection at the same time)?

My understanding is that __sync_fetch_and_add translates to BPF_XADD internally. And it looks like spin locks are being supported from 5.x kernel versions, we are on lower version, so can’t try this one atm.

Regards,
Kanthi

P.S there has been some problem sending the reply, which resulted in multiple edits and deletes, please bear with me

On Wed, May 27, 2020 at 1:29 AM Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

On Fri, May 22, 2020 at 1:07 PM Kanthi P <Pavuluri.kanthi@gmail.com> wrote:

Hi,


I’ve been reading that hash map’s update element is atomic and also that we can use BPF_XADD to make the entire map update atomically.


But I think that doesn’t guarantee that these updates are thread safe, meaning one cpu core can overwrite other core’s update.


Is there a clean way of keeping them thread safe. Unfortunately I can’t use per-cpu maps as I need global counters.


And spin locks sounds a costly operation. Can you please throw some light?

Stating that spin locks are costly without empirical data seems
premature. What's the scenario? What's the number of CPUs? What's the
level of contention? Under light contention, spin locks in practice
would be almost as fast as atomic increments. Under heavy contention,
spin locks would probably be even better than atomics because they
will not waste as much CPU, as a typical atomic retry loop would.

But basically, depending on your use case (which you should probably
describe to get a better answer), you can either:
- do atomic increment/decrement if you need to update a counter (see
examples in kernel selftests using __sync_fetch_and_add);
- use map with bpf_spin_lock (there are also examples in selftests).

Regards,

Kanthi

I am using libbpf from here https://github.com/libbpf/libbpf I'm not
using ebpf. I just linked to the ebpf issue because it seems like the
only thing related to this problem when I googled it.

On Thu, Jun 11, 2020 at 9:34 AM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:

On Thu, Jun 11, 2020 at 4:00 AM Jesper Dangaard Brouer
<brouer@redhat.com> wrote:

(Cross-posting to iovisor-dev)

Seeking input from BPF-llvm developers. How come Clang/LLVM 10+ is
generating incompatible BTF-info in ELF file, and downgrading to LLVM-9
fixes the issue ?


On Wed, 10 Jun 2020 14:50:27 -0700 Elerion <elerion1000@gmail.com> wrote:

Never mind, I fixed it by downgrading to Clang 9.

It appears to be an issue with Clang/LLVM 10+

https://github.com/cilium/ebpf/issues/43

This is newer Clang recording that function is global, not static.
libbpf is sanitizing BTF to remove this flag, if kernel doesn't
support this. But given this is re-implementation of libbpf, that's
probably not happening, right?

On Wed, Jun 10, 2020 at 2:38 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:

Elerion <elerion1000@gmail.com> writes:

[69] FUNC xdp_program type_id=68 vlen != 0

'vlen != 0' is the error. Not sure why you hit that; what's the output
of 'bpftool btf dump file yourprog.o' ?

-Toke

--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer

On Thu, Jun 11, 2020 at 4:00 AM Jesper Dangaard Brouer
<brouer@redhat.com> wrote:

(Cross-posting to iovisor-dev)

Seeking input from BPF-llvm developers. How come Clang/LLVM 10+ is
generating incompatible BTF-info in ELF file, and downgrading to LLVM-9
fixes the issue ?


On Wed, 10 Jun 2020 14:50:27 -0700 Elerion <elerion1000@gmail.com> wrote:

Never mind, I fixed it by downgrading to Clang 9.

It appears to be an issue with Clang/LLVM 10+

https://github.com/cilium/ebpf/issues/43

This is newer Clang recording that function is global, not static.
libbpf is sanitizing BTF to remove this flag, if kernel doesn't
support this. But given this is re-implementation of libbpf, that's
probably not happening, right?

(Cross-posting to iovisor-dev)

Seeking input from BPF-llvm developers. How come Clang/LLVM 10+ is
generating incompatible BTF-info in ELF file, and downgrading to LLVM-9
fixes the issue ?


On Wed, 10 Jun 2020 14:50:27 -0700 Elerion <elerion1000@gmail.com> wrote:

Never mind, I fixed it by downgrading to Clang 9.

It appears to be an issue with Clang/LLVM 10+

https://github.com/cilium/ebpf/issues/43

On Wed, Jun 10, 2020 at 2:38 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:

Elerion <elerion1000@gmail.com> writes:

[69] FUNC xdp_program type_id=68 vlen != 0

'vlen != 0' is the error. Not sure why you hit that; what's the output
of 'bpftool btf dump file yourprog.o' ?

-Toke

--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer

Hi,


I’ve been reading that hash map’s update element is atomic and also that we can use BPF_XADD to make the entire map update atomically.


But I think that doesn’t guarantee that these updates are thread safe, meaning one cpu core can overwrite other core’s update.


Is there a clean way of keeping them thread safe. Unfortunately I can’t use per-cpu maps as I need global counters.


And spin locks sounds a costly operation. Can you please throw some light?

Regards,

Kanthi

Hi,


I’ve been reading that hash map’s update element is atomic and also that we can use BPF_XADD to make the entire map update atomically.

But I think that doesn’t guarantee that these updates are thread safe, meaning one cpu core can overwrite other core’s update.

Is there a clean way of keeping them thread safe. Unfortunately I can’t use per-cpu maps as I need global counters.


And spin locks sounds a costly operation. Can you please throw some light?


Regards,

Kanthi

Hello,


I am trying to introduce USDT probe to an application and my bcc script is failing with following error:


<snip>

Traceback (most recent call last):

File "test.py", line 40, in <module>

b = BPF(text=prog, usdt_contexts=[u])

File "/usr/lib/python3.6/dist-packages/bcc/__init__.py", line 318, in __init__

"locations")

Exception: can't generate USDT probe arguments; possible cause is missing pid when a probe in a shared object has multiple locations

<end snip>


I am trying to run bcc script based on path to binary and not pid. The closest discussion I found related to this error is:

https://github.com/iovisor/bcc/issues/1774

Application that I am trying to instrument is not a shared object as discussed in issue #1774. The code that I have instrumented is in the application. Since there are more than

Thanks.

-Vallish

Hello,

I am trying to introduce USDT probe to an application and my bcc script is failing with following error:

<snip>

Traceback (most recent call last):

  File "test.py", line 40, in <module>

    b = BPF(text=prog, usdt_contexts=[u])

  File "/usr/lib/python3.6/dist-packages/bcc/__init__.py", line 318, in __init__

    "locations")

Exception: can't generate USDT probe arguments; possible cause is missing pid when a probe in a shared object has multiple locations

<end snip>

I am trying to run bcc script based on path to binary and not pid.  The closest discussion I found related to this error is:

https://github.com/iovisor/bcc/issues/1774

Thanks for the reply Andrii.

Managed to get a build working outside of the kernel tree for BPF programs.
The two major things that I learned were that first, the order in which files are
included in the build command is more important than I previously thought.
The second thing was learning how clang deals with asm differently than gcc.
I had to use samples/bpf/asm_goto_workaround.h to fix those errors.
The meat of the makefile is as follows:

CLANGINC := /usr/lib/llvm-10/lib/clang/10.0.0/include
INC_FLAGS := -nostdinc -isystem $(CLANGINC)
EXTRA_FLAGS := -O3 -emit-llvm

I suspect that these warnings come from my aggressive warning flags during
compilation rather than from actual issues in the kernel.

Right, pinning map or program doesn't ensure that program is still
attached to whatever BPF hook you attached to it. As you mentioned,
XDP, tc, cgroup-bpf programs are persistent. We are actually moving
towards the model of auto-detachment for those as well. See recent
activity around bpf_link. The solution with bpf_link to make such
attachments persistent is through pinning **link** itself, not program
or map. bpf_link is relatively recent addition, so on older kernels
you'd have to make sure you still have some process around that would
keep BPF attachment FD around.

I have been looking at the commits surrounding the pinning of bpf_link. It looks like it's only
working in kernel 5.7? I did actually go through and attempt to attach links for kprobes,
tracepoints, and raw_tracepoints in kernel 5.4 but, as you suggested, it seems unsupported.
I have yet to try on kernel 5.5-5.7 so I'll take a look this week or next.

As I mentioned before, with basic functionality in place here, I'm interested in working on
some sort of BPF tutorial similar to the XDP tutorial (https://github.com/xdp-project/xdp-tutorial)
with perhaps a more in-depth look at the technology included as well.

I'm still fuzzy on the relationship between bpf(2) and perf(1). Would it be correct to say that for
tracepoints, kprobes, and uprobes BPF leverages perf "under the hood" while for XDP and tc,
this is more like classic BPF in that it's implementation doesn't involve perf?

If that's the case then is the bpf_link object the tool to bridge BPF and perf? I noticed that when
checking for pinned BPF programs with bpftool in kernel 5.4 that unless a kprobe, tracepoint,
or uprobe is listed in "bpftool perf list", the program doesn't seem to be running. Is the use of
perf to load BPF programs potentially a way to make them "headless" instead of pinning the bpf_link objects?

Regardless, I'm excited to have a more reliable build system than I have in the past. I think I'll start looking more into CO-RE and libbpf on kernels 5.5-5.7.

Hope everyone is staying healthy out there,
Tristan

Thanks for the suggestion, now I feel more confident about this solution.

However, I have still problems with the map-in-map type: is it possible to use a map which has as key the 4 tcp-session identifier {srcIp, dstIp, srcPort, dstPort} and as value a BPF_ARRAY which is a list of some packets' headers belonging to that session?
As far as I undestood, a BPF_HASH_OF_MAPS key is coded as integer, and the value retrieved with lookup is the inner table fileDescriptor. Although, how do you initialize those inner arrays? I've tried to insert something, but what should I put as value? The inner map's fileDescriptor (how do I know it)?

Hi all, hope everyone is staying healthy out there.

I've been working on building BPF programs, and have run into a few issues that I think might be clang (vs gcc) based.
It seems that either clang isn't the most friendly of compilers when it comes to building Linux-native programs, or my lack of experience makes it seem so.
I've been trying to build the simple BPF program below:


#include "bpf_helpers.h"
#include <linux/bpf.h>
#include <linux/version.h>
#include <linux/types.h>
#include <linux/tcp.h>
#include <net/sock.h>

struct inet_sock_set_state_args {
long long pad;
const void * skaddr;
int oldstate;
int newstate;
u16 sport;
u16 dport;
u16 family;
u8 protocol;
u8 saddr[4];
u8 daddr[4];
u8 saddr_v6[16];
u8 daddr_v6[16];
};


SEC("tracepoint/sock/inet_sock_set_state")
int bpf_prog(struct inet_sock_set_state_args *args) {

struct sock *sk = (struct sock *)args->skaddr;
short lport = args->sport;

char msg[] = "lport: %d\n";
bpf_trace_printk(msg, sizeof(msg), lport);

return 0;
}

char _license[] SEC("license") = "GPL";



I've been looking through selftests/bpf/, samples/bpf/, and examples on various blogs and articles.
From this, I've come up with the following makefile:


## Build tools
LLC := llc
CC := clang
HOSTCC := clang
CLANGINC := /usr/lib/llvm-10/lib/clang/10.0.0/include

## Some useful flags
INC_FLAGS := -nostdinc -isystem $(CLANGINC)
EXTRA_FLAGS := -O3 -emit-llvm

## Includes
linuxhdrs := /usr/src/linux-headers-$(shell uname -r)
LINUXINCLUDE := -include $(linuxhdrs)/include/linux/kconfig.h \
-include /usr/include/linux/bpf.h \
-I$(linuxhdrs)/arch/x86/include/ \
-I$(linuxhdrs)/arch/x86/include/uapi \
-I$(linuxhdrs)/arch/x86/include/generated \
-I$(linuxhdrs)/arch/x86/include/generated/uapi \
-I$(linuxhdrs)/include \
-I$(linuxhdrs)/include/uapi \
-I$(linuxhdrs)/include/generated/uapi \
LIBBPF := -I/home/vagrant/libbpf/src/
OBJS := tcptest.bpf.o

$(OBJS): %.o:%.c
$(CC) $(INC_FLAGS) \
-target bpf -D__KERNEL__ -D __ASM_SYSREG_H \
-D__BPF_TRACING__ -D__TARGET_ARCH_$(ARCH) \
-Wno-unused-value -Wno-pointer-sign \
-Wno-compare-distinct-pointer-types \
-Wno-gnu-variable-sized-type-not-at-end \
-Wno-address-of-packed-member \
-Wno-tautological-compare \
-Wno-unknown-warning-option \
-Wall -v \
$(LINUXINCLUDE) $(LIBBPF) \
$(EXTRA_FLAGS) -c $< -o - | $(LLC) -march=bpf -filetype obj -o $


Unfortunately, I keep running into what seems to be asm errors. I've tried reorganizing the list of include statements, taking out "-target bpf", not including some files, including other files, etc etc.
This stackoverflow post suggests that it's a kconfig.h error, but I seem to be including the file just fine (https://stackoverflow.com/questions/56975861/error-compiling-ebpf-c-code-out-of-kernel-tree/56990939#56990939).
I'm not really sure where to go from here with building BPF programs and including files that have the kernel datatypes. Maybe I'm missing something that's obvious that I'm just ignorant of?

As additional information, and regarding kernel persistence, I am working on a monitoring project that uses BPF programs to continuously monitor the system without the bulky dependencies that BCC includes. I'm concurrently working on a BTF/CO-RE solution but I'm emphasizing a non-CO-RE approach at the moment. I can load and run BPF programs but upon termination of my userspace loader the BPF programs themselves also terminate.



I would like to have the BPF program persist in the kernel even after the user space loader has completed its execution. I read in various documentation and in a 2015 LWN article that persistent BPF programs can be created by pinning programs and maps to the BPF vfs so as to keep the fds open. I have attempted pinning the entire BPF object, various programs and various maps, and no matter what I've tried the kernel BPF program terminates when the userspace process terminates. Using bpftool I have verified that the BPF files are pinned to the location and that BPF programs themselves all work. I know that persistent BPF programs are a part of projects like XDP and tc. Is there a way to do this for a generic BPF loader without having to implement customized kernel functions? Below I have included a simplified version of my code. In which I outline the basic steps I take to load the compiled bpf programs and attempt to make persistent instances of them.

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <errno.h>

#include <getopt.h>

#include <dirent.h>

#include <sys/stat.h>

#include <unistd.h>

#include <assert.h>

#include <linux/version.h>



#include "libbpf.h"

#include "bpf.h"

#include "loader_helpers.h"



#include <stdbool.h>

#include <fcntl.h>

#include <poll.h>

#include <linux/perf_event.h>

#include <assert.h>

#include <sys/syscall.h>

#include <sys/ioctl.h>

#include <sys/mman.h>

#include <time.h>

#include <signal.h>

#include <linux/ptrace.h>



int main(int argc, char **argv) {



struct bpf_object *bpf_obj;

struct bpf_program *bpf_prog;

struct bpf_map *map;

char * license = "GPL";

__u32 kernelvers = LINUX_VERSION_CODE;

struct bpf_link * link;

int err;

int prog_fd;



bpf_obj = bpf_object__open("test_file.bpf.o");



bpf_prog = bpf_program__next(NULL, bpf_obj);



err = bpf_program__set_tracepoint(bpf_prog);

if(err) {

fprintf(stderr, "ERR couldn't setup program type\n");

return -1;

}

err = bpf_program__load(bpf_prog, license, kernelvers);

if(err) {

fprintf(stderr, "ERR couldn't setup program phase\n");

return -1;

}

prog_fd = bpf_program__fd(bpf_prog);



link = bpf_program__attach_tracepoint(bpf_prog, "syscalls", "sys_enter_openat");

if(!link) {

fprintf(stderr, "ERROR ATTACHING TRACEPOINT\n");

return -1;

}



assert(bpf_program__is_tracepoint(bpf_prog));



pin:

err = bpf_program__pin(bpf_prog, "/sys/fs/bpf/tpprogram");

if(err) {

if(err == -17) {

printf("Program exists...trying to unpin and retry!\n");

err = bpf_program__unpin(bpf_prog, "/sys/fs/bpf/tpprogram");

if(!err) {

goto pin;

}

printf("The pining already exists but it couldn't be removed...\n");

return -1;

}

printf("We couldn't pin...%d\n", err);

return -1;

}



printf("Program pinned and working...\n");



return 0;

}




Thanks for having a look and I hope these issues can be cleared up. Seems like building is the last major hurdle I have to get rolling with better engineering solutions than manually including structs in my files.
Hope everyone stays well!

Thanks for the suggestion, now I feel more confident about this solution.

However, I have still problems with the map-in-map type: is it possible to use a map which has as key the 4 tcp-session identifier {srcIp, dstIp, srcPort, dstPort} and as value a BPF_ARRAY which is a list of some packets' headers belonging to that session?
As far as I undestood, a BPF_HASH_OF_MAPS key is coded as integer, and the value retrieved with lookup is the inner table fileDescriptor. Although, how do you initialize those inner arrays? I've tried to insert something, but what should I put as value? The inner map's fileDescriptor (how do I know it)?

#include "bpf_helpers.h"
#include <linux/bpf.h>
#include <linux/version.h>
#include <linux/types.h>
#include <linux/tcp.h>
#include <net/sock.h>

struct inet_sock_set_state_args {
        long long pad;
	const void * skaddr;
	int oldstate;
	int newstate;
	u16 sport;
 	u16 dport;
	u16 family;
	u8 protocol;
	u8 saddr[4];
	u8 daddr[4];
	u8 saddr_v6[16];
	u8 daddr_v6[16];
};


SEC("tracepoint/sock/inet_sock_set_state")
int bpf_prog(struct inet_sock_set_state_args *args) {

  struct sock *sk = (struct sock *)args->skaddr;
  short lport = args->sport;

  char msg[] = "lport: %d\n";
  bpf_trace_printk(msg, sizeof(msg), lport);

  return 0;
}

char _license[] SEC("license") = "GPL";

## Build tools
LLC := llc
CC := clang
HOSTCC := clang
CLANGINC := /usr/lib/llvm-10/lib/clang/10.0.0/include

## Some useful flags
INC_FLAGS := -nostdinc -isystem $(CLANGINC)
EXTRA_FLAGS := -O3 -emit-llvm

## Includes
linuxhdrs := /usr/src/linux-headers-$(shell uname -r)
LINUXINCLUDE := -include $(linuxhdrs)/include/linux/kconfig.h \
				-include /usr/include/linux/bpf.h \
				-I$(linuxhdrs)/arch/x86/include/ \
				-I$(linuxhdrs)/arch/x86/include/uapi \
				-I$(linuxhdrs)/arch/x86/include/generated \
				-I$(linuxhdrs)/arch/x86/include/generated/uapi \
				-I$(linuxhdrs)/include \
				-I$(linuxhdrs)/include/uapi \
				-I$(linuxhdrs)/include/generated/uapi \
LIBBPF :=  -I/home/vagrant/libbpf/src/
OBJS := tcptest.bpf.o

$(OBJS): %.o:%.c
	$(CC) $(INC_FLAGS) \
		-target bpf -D__KERNEL__ -D __ASM_SYSREG_H \
		-D__BPF_TRACING__ -D__TARGET_ARCH_$(ARCH) \
		-Wno-unused-value -Wno-pointer-sign \
		-Wno-compare-distinct-pointer-types \
		-Wno-gnu-variable-sized-type-not-at-end \
		-Wno-address-of-packed-member \
		-Wno-tautological-compare \
		-Wno-unknown-warning-option \
		-Wall -v \
		$(LINUXINCLUDE) $(LIBBPF) \
		$(EXTRA_FLAGS) -c $< -o - | $(LLC) -march=bpf -filetype obj -o $