On Wed, Oct 19, 2022 at 6:57 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> Currently we allow to create kprobe multi link on function from kernel
> module, but we don't take the module reference to ensure it's not
> unloaded while we are tracing it.
>
> The multi kprobe link is based on fprobe/ftrace layer which takes
> different approach and releases ftrace hooks when module is unloaded
> even if there's tracer registered on top of it.
>
> Adding code that gathers all the related modules for the link and takes
> their references before it's attached. All kernel module references are
> released after link is unregistered.
>
> Note that we do it the same way already for trampoline probes
> (but for single address).
>
> Acked-by: Song Liu <song@xxxxxxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
> kernel/trace/bpf_trace.c | 92 ++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 92 insertions(+)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 17ae9e8336db..9a4a2388dff2 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2452,6 +2452,8 @@ struct bpf_kprobe_multi_link {
> unsigned long *addrs;
> u64 *cookies;
> u32 cnt;
> + struct module **mods;
> + u32 mods_cnt;
> };
>
> struct bpf_kprobe_multi_run_ctx {
> @@ -2507,6 +2509,14 @@ static int copy_user_syms(struct user_syms *us, unsigned long __user *usyms, u32
> return err;
> }
>
> +static void kprobe_multi_put_modules(struct module **mods, u32 cnt)
> +{
> + u32 i;
> +
> + for (i = 0; i < cnt; i++)
> + module_put(mods[i]);
> +}
> +
> static void free_user_syms(struct user_syms *us)
> {
> kvfree(us->syms);
> @@ -2519,6 +2529,7 @@ static void bpf_kprobe_multi_link_release(struct bpf_link *link)
>
> kmulti_link = container_of(link, struct bpf_kprobe_multi_link, link);
> unregister_fprobe(&kmulti_link->fp);
> + kprobe_multi_put_modules(kmulti_link->mods, kmulti_link->mods_cnt);
> }
>
> static void bpf_kprobe_multi_link_dealloc(struct bpf_link *link)
> @@ -2528,6 +2539,7 @@ static void bpf_kprobe_multi_link_dealloc(struct bpf_link *link)
> kmulti_link = container_of(link, struct bpf_kprobe_multi_link, link);
> kvfree(kmulti_link->addrs);
> kvfree(kmulti_link->cookies);
> + kfree(kmulti_link->mods);
> kfree(kmulti_link);
> }
>
> @@ -2663,6 +2675,71 @@ static void symbols_swap_r(void *a, void *b, int size, const void *priv)
> }
> }
>
> +struct module_addr_args {
> + unsigned long *addrs;
> + u32 addrs_cnt;
> + struct module **mods;
> + int mods_cnt;
> + int mods_cap;
> +};
> +
> +static int module_callback(void *data, const char *name,
> + struct module *mod, unsigned long addr)
> +{
> + struct module_addr_args *args = data;
> + struct module **mods;
> +
> + /* We iterate all modules symbols and for each we:
> + * - search for it in provided addresses array
> + * - if found we check if we already have the module pointer stored
> + * (we iterate modules sequentially, so we can check just the last
> + * module pointer)
> + * - take module reference and store it
> + */
> + if (!bsearch(&addr, args->addrs, args->addrs_cnt, sizeof(addr),
> + bpf_kprobe_multi_addrs_cmp))
> + return 0;
> +
> + if (args->mods && args->mods[args->mods_cnt - 1] == mod)
> + return 0;
> +
> + if (args->mods_cnt == args->mods_cap) {
> + args->mods_cap = max(16, args->mods_cap * 3 / 2);
> + mods = krealloc_array(args->mods, args->mods_cap, sizeof(*mods), GFP_KERNEL);
> + if (!mods)
> + return -ENOMEM;
> + args->mods = mods;
> + }
> +
> + if (!try_module_get(mod))
> + return -EINVAL;
> +
> + args->mods[args->mods_cnt] = mod;
> + args->mods_cnt++;
> + return 0;
> +}
> +
> +static int get_modules_for_addrs(struct module ***mods, unsigned long *addrs, u32 addrs_cnt)
> +{
> + struct module_addr_args args = {
> + .addrs = addrs,
> + .addrs_cnt = addrs_cnt,
> + };
> + int err;
> +
> + /* We return either err < 0 in case of error, ... */
> + err = module_kallsyms_on_each_symbol(module_callback, &args);
> + if (err) {
> + kprobe_multi_put_modules(args.mods, args.mods_cnt);
> + kfree(args.mods);
> + return err;
> + }
> +
> + /* or number of modules found if everything is ok. */
> + *mods = args.mods;
> + return args.mods_cnt;
> +}
> +
> int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> {
> struct bpf_kprobe_multi_link *link = NULL;
> @@ -2773,10 +2850,25 @@ int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
> bpf_kprobe_multi_cookie_cmp,
> bpf_kprobe_multi_cookie_swap,
> link);
> + } else {
> + /*
> + * We need to sort addrs array even if there are no cookies
> + * provided, to allow bsearch in get_modules_for_addrs.
> + */
> + sort(addrs, cnt, sizeof(*addrs),
> + bpf_kprobe_multi_addrs_cmp, NULL);
> + }
> +
> + err = get_modules_for_addrs(&link->mods, addrs, cnt);
> + if (err < 0) {
> + bpf_link_cleanup(&link_primer);
> + return err;
> }
> + link->mods_cnt = err;
>
> err = register_fprobe_ips(&link->fp, addrs, cnt);
> if (err) {
> + kprobe_multi_put_modules(link->mods, link->mods_cnt);
I don't think bpf_link_cleanup() will free link->mods, you have to do
it explicitly here
> bpf_link_cleanup(&link_primer);
> return err;
> }
> --
> 2.37.3
>
