Problem with sending data from userspace to bpf program with maps

Question

I have problem with my bpf program. I getting error while loading this program. my bpf program is:

#include <linux/skbuff.h>
#include <linux/netdevice.h>
#include <uapi/linux/bpf.h>
#include <linux/version.h>
#include <bpf/bpf_helpers.h>
#include <bpf/bpf_tracing.h>
#include <linux/fs.h>
#include <uapi/asm-generic/errno-base.h>

struct {
    __uint(type, BPF_MAP_TYPE_HASH);
    __type(key,   int);
    __type(value, ino_t);
    __uint(max_entries, 256);
} qu SEC(".maps");


SEC("lsm/task_kill")
BPF_PROG(
    lsm__task_kill,
    struct task_struct* p, 
    struct kernel_siginfo* info,
    int sig,
    const struct cred* cred
) {
    int key = 0;
    int* my_pid = bpf_map_lookup_elem(&qu, &key);
    if (*my_pid == 3935991) {
        return -EPERM;
    }
    
    bpf_ringbuf_output(&rb, &data, sizeof(data), 0);

    return 0;
}

and this is my user program:

#include <bpf/bpf.h>
#include <unistd.h>
#include <string.h>
#include <sys/syscall.h>
#include <stdlib.h>
#include <stdio.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <linux/version.h>
#include <linux/perf_event.h>
#include <linux/hw_breakpoint.h>
#include <errno.h>
#include <stdio.h>
#include <unistd.h>
#include <bpf/libbpf.h>
#include <time.h>


int main(
    int         argc, 
    const char* argv[]
) {
    if (argc <= 1) {
        return -1;
    }
    
    struct stat stat_buf = {0};
    int ret = stat(argv[1], &stat_buf);
    if (ret == -1) {
        printf("ERROR: resolving pathname failed: %s\n", strerror(errno));
        return -1;
    }

    struct bpf_object *obj = bpf_object__open("./my_prog.o");
    if (libbpf_get_error(obj)) {
        fprintf(stderr, "ERROR: opening BPF object file failed\n");
        return 0;
    }

    bpf_object__load(obj);
    if (libbpf_get_error(obj)) {
        fprintf(stderr, "ERROR: opening BPF object file failed\n");
        return 0;
    }

    int qfd = bpf_object__find_map_fd_by_name(obj, "qu");
    if (qfd < 0) {
        fprintf(stderr, "ERROR: finding a queue in obj file failed\n");
        return -1;
    }

    int val = stat_buf.st_ino;
    int key = 0;
    ret = bpf_map_update_elem(qfd, &key, &val, BPF_ANY);
    if (ret < 0) {
        printf("ERROR: updating map failed\n");
        return -1;
    }
    
    
    struct bpf_link*    links[1];
    struct bpf_program* prog;
    int j = 0;
    bpf_object__for_each_program(prog, obj) {
        links[j] = bpf_program__attach_lsm(prog);
        if (libbpf_get_error(links[j])) {
            fprintf(stderr, "ERROR: bpf_program__attach failed\n");
            links[j] = NULL;
            return -1;
        }
        j++;
    }

   
}

there is no error while making bpf object file and compiling user program, but when i run user program i get this error while loading bpf program:

user@host:~$ sudo ./my_prog_loader ./some_file

libbpf: elf: skipping unrecognized data section(22) .eh_frame
libbpf: elf: skipping relo section(23) .rel.eh_frame for section(22) .eh_frame
libbpf: load bpf program failed: Permission denied
libbpf: -- BEGIN DUMP LOG ---
libbpf: 
R1 type=ctx expected=fp
; BPF_PROG(
0: (b7) r6 = 0
; Data data = {0};
1: (7b) *(u64 *)(r10 -8) = r6
last_idx 1 first_idx 0
regs=40 stack=0 before 0: (b7) r6 = 0
2: (7b) *(u64 *)(r10 -16) = r6
3: (7b) *(u64 *)(r10 -24) = r6
4: (7b) *(u64 *)(r10 -32) = r6
5: (bf) r1 = r10
6: (07) r1 += -16
; bpf_get_current_comm(data.str, 16);
7: (b7) r2 = 16
8: (85) call bpf_get_current_comm#16
last_idx 8 first_idx 0
regs=4 stack=0 before 7: (b7) r2 = 16
; data.pid  = 000000;
9: (63) *(u32 *)(r10 -32) = r6
; int key = 0;
10: (63) *(u32 *)(r10 -36) = r6
11: (bf) r2 = r10
; 
12: (07) r2 += -36
; ino_t* my_pid = bpf_map_lookup_elem(&qu, &key);
13: (18) r1 = 0xffff8fa64edafc00
15: (85) call bpf_map_lookup_elem#1
; if (*my_pid == 3935991) {
16: (79) r1 = *(u64 *)(r0 +0)
R0 invalid mem access 'map_value_or_null'
processed 16 insns (limit 1000000) max_states_per_insn 0 total_states 1 peak_states 1 mark_read 1

libbpf: -- END LOG --
libbpf: failed to load program 'lsm__task_kill'
libbpf: failed to load object './my_prog.o'
ERROR: finding a map in obj file failed

user@host:~$_

why I'm getting this error and what should I do?? thank you for any help!

pchaigno · Accepted Answer · 2021-06-15 07:28:36Z

2

TL;DR. You should check that the pointer returned by bpf_map_lookup_elem is not NULL.

With the following logs, the BPF verifier is telling you that, when it reaches the dereference of my_pid, the pointer may still have a NULL value. It thus contains a map value or a NULL value, i.e., map_value_or_null.

; ino_t* my_pid = bpf_map_lookup_elem(&qu, &key);
13: (18) r1 = 0xffff8fa64edafc00
15: (85) call bpf_map_lookup_elem#1
; if (*my_pid == 3935991) {
16: (79) r1 = *(u64 *)(r0 +0)
R0 invalid mem access 'map_value_or_null'

bpf_map_lookup_elem returns a NULL value whenever the looked up key isn't found.

You could simply do something like:

if (my_pid && *my_pid == 3935991) {
    return -EPERM;
}

answered Jun 15, 2021 at 7:28

pchaigno

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2 Comments

Redwan Over a year ago

Thank you. so why I'm getting null value of certain key at all?? am I wrong in updating map?

pchaigno Over a year ago

The verifier runs when you load the program, before you run the program for the first time. So it checks all possibilities, including the possibility that a map lookup returns NULL. That doesn't mean it will happen at runtime.

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