bpf: Fix memleak due to fentry attach failure

In the Linux kernel, the following vulnerability has been resolved:

bpf: Fix memleak due to fentry attach failure

If it fails to attach fentry, the allocated bpf trampoline image will be left in the system. That can be verified by checking /proc/kallsyms.

This meamleak can be verified by a simple bpf program as follows:

SEC("fentry/trap_init") int fentry_run() { return 0; }

It will fail to attach trap_init because this function is freed after kernel init, and then we can find the trampoline image is left in the system by checking /proc/kallsyms.

$ tail /proc/kallsyms ffffffffc0613000 t bpf_trampoline_6442453466_1 [bpf] ffffffffc06c3000 t bpf_trampoline_6442453466_1 [bpf]

$ bpftool btf dump file /sys/kernel/btf/vmlinux | grep "FUNC 'trap_init'" [2522] FUNC 'trap_init' type_id=119 linkage=static

$ echo $((6442453466 & 0x7fffffff)) 2522

Note that there are two left bpf trampoline images, that is because the libbpf will fallback to raw tracepoint if -EINVAL is returned.