Re: [PATCH 3/8] bpf: Add bpf_cookie support to fprobe

Andrii Nakryiko
2022-02-07T19:04:36Z

On Wed, Feb 2, 2022 at 5:54 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> Adding support to call bpf_get_attach_cookie helper from
> kprobe program attached by fprobe link.
>
> The bpf_cookie is provided by array of u64 values, where
> each value is paired with provided function address with
> the same array index.
>
> Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
> include/linux/bpf.h | 2 +
> include/uapi/linux/bpf.h | 1 +
> kernel/bpf/syscall.c | 83 +++++++++++++++++++++++++++++++++-
> kernel/trace/bpf_trace.c | 16 ++++++-
> tools/include/uapi/linux/bpf.h | 1 +
> 5 files changed, 100 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 6eb0b180d33b..7b65f05c0487 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1301,6 +1301,8 @@ static inline void bpf_reset_run_ctx(struct bpf_run_ctx *old_ctx)
> #endif
> }
>
> +u64 bpf_fprobe_cookie(struct bpf_run_ctx *ctx, u64 ip);
> +
> /* BPF program asks to bypass CAP_NET_BIND_SERVICE in bind. */
> #define BPF_RET_BIND_NO_CAP_NET_BIND_SERVICE (1 << 0)
> /* BPF program asks to set CN on the packet. */
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index c0912f0a3dfe..0dc6aa4f9683 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1484,6 +1484,7 @@ union bpf_attr {
> __aligned_u64 addrs;
> __u32 cnt;
> __u32 flags;
> + __aligned_u64 bpf_cookies;

maybe put it right after addrs, they are closely related and cnt
describes all of syms/addrs/cookies.

> } fprobe;
> };
> } link_create;
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 0cfbb112c8e1..6c5e74bc43b6 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -33,6 +33,8 @@
> #include <linux/rcupdate_trace.h>
> #include <linux/memcontrol.h>
> #include <linux/fprobe.h>
> +#include <linux/bsearch.h>
> +#include <linux/sort.h>
>
> #define IS_FD_ARRAY(map) ((map)->map_type == BPF_MAP_TYPE_PERF_EVENT_ARRAY || \
> (map)->map_type == BPF_MAP_TYPE_CGROUP_ARRAY || \
> @@ -3025,10 +3027,18 @@ static int bpf_perf_link_attach(const union bpf_attr *attr, struct bpf_prog *pro
>
> #ifdef CONFIG_FPROBE
>
> +struct bpf_fprobe_cookie {
> + unsigned long addr;
> + u64 bpf_cookie;
> +};
> +
> struct bpf_fprobe_link {
> struct bpf_link link;
> struct fprobe fp;
> unsigned long *addrs;
> + struct bpf_run_ctx run_ctx;
> + struct bpf_fprobe_cookie *bpf_cookies;

you already have all the addrs above, why keeping a second copy of
each addrs in bpf_fprobe_cookie. Let's have two arrays: addrs
(unsigned long) and cookies (u64) and make sure that they are sorted
together. Then lookup addrs, calculate index, use that index to fetch
cookie.

Seems like sort_r() provides exactly the interface you'd need to do
this very easily. Having addrs separate from cookies also a bit
advantageous in terms of TLB misses (if you need any more persuasion
;)

> + u32 cnt;
> };
>
> static void bpf_fprobe_link_release(struct bpf_link *link)
> @@ -3045,6 +3055,7 @@ static void bpf_fprobe_link_dealloc(struct bpf_link *link)
>
> fprobe_link = container_of(link, struct bpf_fprobe_link, link);
> kfree(fprobe_link->addrs);
> + kfree(fprobe_link->bpf_cookies);
> kfree(fprobe_link);
> }
>
> @@ -3053,9 +3064,37 @@ static const struct bpf_link_ops bpf_fprobe_link_lops = {
> .dealloc = bpf_fprobe_link_dealloc,
> };
>
> +static int bpf_fprobe_cookie_cmp(const void *_a, const void *_b)
> +{
> + const struct bpf_fprobe_cookie *a = _a;
> + const struct bpf_fprobe_cookie *b = _b;
> +
> + if (a->addr == b->addr)
> + return 0;
> + return a->addr < b->addr ? -1 : 1;
> +}
> +
> +u64 bpf_fprobe_cookie(struct bpf_run_ctx *ctx, u64 ip)
> +{
> + struct bpf_fprobe_link *fprobe_link;
> + struct bpf_fprobe_cookie *val, key = {
> + .addr = (unsigned long) ip,
> + };
> +
> + if (!ctx)
> + return 0;

is it allowed to have ctx == NULL?

> + fprobe_link = container_of(ctx, struct bpf_fprobe_link, run_ctx);
> + if (!fprobe_link->bpf_cookies)
> + return 0;
> + val = bsearch(&key, fprobe_link->bpf_cookies, fprobe_link->cnt,
> + sizeof(key), bpf_fprobe_cookie_cmp);
> + return val ? val->bpf_cookie : 0;
> +}
> +
> static int fprobe_link_prog_run(struct bpf_fprobe_link *fprobe_link,
> struct pt_regs *regs)
> {
> + struct bpf_run_ctx *old_run_ctx;
> int err;
>
> if (unlikely(__this_cpu_inc_return(bpf_prog_active) != 1)) {
> @@ -3063,12 +3102,16 @@ static int fprobe_link_prog_run(struct bpf_fprobe_link *fprobe_link,
> goto out;
> }
>
> + old_run_ctx = bpf_set_run_ctx(&fprobe_link->run_ctx);
> +
> rcu_read_lock();
> migrate_disable();
> err = bpf_prog_run(fprobe_link->link.prog, regs);
> migrate_enable();
> rcu_read_unlock();
>
> + bpf_reset_run_ctx(old_run_ctx);
> +
> out:
> __this_cpu_dec(bpf_prog_active);
> return err;
> @@ -3161,10 +3204,12 @@ static int fprobe_resolve_syms(const void *usyms, u32 cnt,
>
> static int bpf_fprobe_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> {
> + struct bpf_fprobe_cookie *bpf_cookies = NULL;
> struct bpf_fprobe_link *link = NULL;
> struct bpf_link_primer link_primer;
> + void __user *ubpf_cookies;
> + u32 flags, cnt, i, size;
> unsigned long *addrs;
> - u32 flags, cnt, size;
> void __user *uaddrs;
> void __user *usyms;
> int err;
> @@ -3205,6 +3250,37 @@ static int bpf_fprobe_link_attach(const union bpf_attr *attr, struct bpf_prog *p
> goto error;
> }
>
> + ubpf_cookies = u64_to_user_ptr(attr->link_create.fprobe.bpf_cookies);

nit: let's call all this "cookies", this bpf_ prefix feels a bit
redundant (I know about perf_event.bpf_cookie, but still).

> + if (ubpf_cookies) {
> + u64 *tmp;
> +
> + err = -ENOMEM;
> + tmp = kzalloc(size, GFP_KERNEL);

kvmalloc?

> + if (!tmp)
> + goto error;
> +
> + if (copy_from_user(tmp, ubpf_cookies, size)) {
> + kfree(tmp);
> + err = -EFAULT;
> + goto error;
> + }
> + …