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Date: Fri, 24 Jul 2020 16:53:47 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>,
davem@...emloft.net
Cc: torvalds@...ux-foundation.org, netdev@...r.kernel.org,
bpf@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH v3 bpf-next 3/4] bpf: Add kernel module with user mode
driver that populates bpffs.
On 7/24/20 7:58 AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@...nel.org>
>
> Add kernel module with user mode driver that populates bpffs with
> BPF iterators.
>
> $ mount bpffs /my/bpffs/ -t bpf
> $ ls -la /my/bpffs/
> total 4
> drwxrwxrwt 2 root root 0 Jul 2 00:27 .
> drwxr-xr-x 19 root root 4096 Jul 2 00:09 ..
> -rw------- 1 root root 0 Jul 2 00:27 maps.debug
> -rw------- 1 root root 0 Jul 2 00:27 progs.debug
>
> The user mode driver will load BPF Type Formats, create BPF maps, populate BPF
> maps, load two BPF programs, attach them to BPF iterators, and finally send two
> bpf_link IDs back to the kernel.
> The kernel will pin two bpf_links into newly mounted bpffs instance under
> names "progs.debug" and "maps.debug". These two files become human readable.
>
> $ cat /my/bpffs/progs.debug
> id name pages attached
> 11 dump_bpf_map 1 bpf_iter_bpf_map
> 12 dump_bpf_prog 1 bpf_iter_bpf_prog
> 27 test_pkt_access 1
> 32 test_main 1 test_pkt_access test_pkt_access
> 33 test_subprog1 1 test_pkt_access_subprog1 test_pkt_access
> 34 test_subprog2 1 test_pkt_access_subprog2 test_pkt_access
> 35 test_subprog3 1 test_pkt_access_subprog3 test_pkt_access
> 36 new_get_skb_len 1 get_skb_len test_pkt_access
> 37 new_get_skb_ifi 1 get_skb_ifindex test_pkt_access
> 38 new_get_constan 1 get_constant test_pkt_access
>
> The BPF program dump_bpf_prog() in iterators.bpf.c is printing this data about
> all BPF programs currently loaded in the system. This information is unstable
> and will change from kernel to kernel as ".debug" suffix conveys.
>
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
> init/Kconfig | 2 +
> kernel/bpf/Makefile | 1 +
> kernel/bpf/inode.c | 132 +++++++++++++++++-
> kernel/bpf/preload/Kconfig | 18 +++
> kernel/bpf/preload/Makefile | 21 +++
> kernel/bpf/preload/bpf_preload.h | 16 +++
> kernel/bpf/preload/bpf_preload_kern.c | 83 +++++++++++
> kernel/bpf/preload/bpf_preload_umd_blob.S | 7 +
> .../preload/iterators/bpf_preload_common.h | 13 ++
> kernel/bpf/preload/iterators/iterators.c | 94 +++++++++++++
> 10 files changed, 384 insertions(+), 3 deletions(-)
> create mode 100644 kernel/bpf/preload/Kconfig
> create mode 100644 kernel/bpf/preload/Makefile
> create mode 100644 kernel/bpf/preload/bpf_preload.h
> create mode 100644 kernel/bpf/preload/bpf_preload_kern.c
> create mode 100644 kernel/bpf/preload/bpf_preload_umd_blob.S
> create mode 100644 kernel/bpf/preload/iterators/bpf_preload_common.h
> create mode 100644 kernel/bpf/preload/iterators/iterators.c
[...]
>
> +struct bpf_preload_ops bpf_preload_ops = { .info.driver_name = "bpf_preload" };
> +EXPORT_SYMBOL_GPL(bpf_preload_ops);
> +
> +#if !IS_BUILTIN(CONFIG_BPF_PRELOAD_UMD)
> +static struct module *bpf_preload_mod;
> +#endif
> +
> +static bool bpf_preload_mod_get(void)
> +{
> + bool ret = true;
> +
> +#if IS_BUILTIN(CONFIG_BPF_PRELOAD_UMD)
> + return ret;
> +#else
> + /* if bpf_preload.ko wasn't loaded earlier then load it now */
> + if (!bpf_preload_ops.do_preload) {
> + request_module("bpf_preload");
> + if (!bpf_preload_ops.do_preload) {
> + pr_err("bpf_preload module is missing.\n"
> + "bpffs will not have iterators.\n");
> + return false;
> + }
> + }
> + /* and grab the reference, so it doesn't disappear while the kernel
> + * is interacting with kernel module and its UMD
> + */
> + preempt_disable();
> + bpf_preload_mod = __module_address((long)bpf_preload_ops.do_preload);
> + if (!bpf_preload_mod || !try_module_get(bpf_preload_mod)) {
Set looks good overall, but this combination looks a bit odd. Meaning, we request the
module via request_module(), in its init fn, it will set bpf_preload_ops.do_preload
callback, and here we need to search kallsyms on __module_address(bpf_preload_ops.do_preload)
just to get the module struct in order to place a ref on it via try_module_get().
Why can't the bpf_preload module simply do:
static const struct bpf_preload_umd_ops umd_ops = {
.preload = do_preload,
.finish = do_finish,
.owner = THIS_MODULE,
};
And then in load_umd():
static int __init load_umd(void)
{
int err;
err = umd_load_blob(&bpf_preload_ops.info, &bpf_preload_umd_start,
&bpf_preload_umd_end - &bpf_preload_umd_start);
if (!err)
bpf_preload_umd_ops = &umd_ops;
return err;
}
Then later in bpf_preload_mod_get() you just do ...
try_module_get(bpf_preload_umd_ops->owner)
... and can avoid this whole detour with symbol address search which looks odd and
unneeded for this case.
Thanks,
Daniel
> + bpf_preload_mod = NULL;
> + pr_err("bpf_preload module get failed.\n");
> + ret = false;
> + }
> + preempt_enable();
> + return ret;
> +#endif
> +}
> +
> +static void bpf_preload_mod_put(void)
> +{
> +#if !IS_BUILTIN(CONFIG_BPF_PRELOAD_UMD)
> + if (bpf_preload_mod) {
> + /* now user can "rmmod bpf_preload" if necessary */
> + module_put(bpf_preload_mod);
> + bpf_preload_mod = NULL;
> + }
> +#endif
> +}
> +
> +static int populate_bpffs(struct dentry *parent)
> +{
> + struct bpf_preload_info objs[BPF_PRELOAD_LINKS] = {};
> + struct bpf_link *links[BPF_PRELOAD_LINKS] = {};
> + int err = 0, i;
> +
> + /* grab the mutex to make sure the kernel interactions with bpf_preload
> + * UMD are serialized
> + */
> + mutex_lock(&bpf_preload_ops.lock);
> +
> + /* if bpf_preload.ko wasn't built into vmlinux then load it */
> + if (!bpf_preload_mod_get())
> + goto out;
> +
> + if (!bpf_preload_ops.info.tgid) {
> + /* do_preload will start UMD that will load BPF iterator programs */
> + err = bpf_preload_ops.do_preload(objs);
> + if (err)
> + goto out_put;
> + for (i = 0; i < BPF_PRELOAD_LINKS; i++) {
> + links[i] = bpf_link_by_id(objs[i].link_id);
> + if (IS_ERR(links[i])) {
> + err = PTR_ERR(links[i]);
> + goto out_put;
> + }
> + }
> + for (i = 0; i < BPF_PRELOAD_LINKS; i++) {
> + err = bpf_iter_link_pin_kernel(parent,
> + objs[i].link_name, links[i]);
> + if (err)
> + goto out_put;
> + /* do not unlink successfully pinned links even
> + * if later link fails to pin
> + */
> + links[i] = NULL;
> + }
> + /* do_finish() will tell UMD process to exit */
> + err = bpf_preload_ops.do_finish();
> + if (err)
> + goto out_put;
> + }
> +out_put:
> + bpf_preload_mod_put();
> +out:
> + mutex_unlock(&bpf_preload_ops.lock);
> + for (i = 0; i < BPF_PRELOAD_LINKS && err; i++)
> + if (!IS_ERR_OR_NULL(links[i]))
> + bpf_link_put(links[i]);
[...]
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