Re: [RFC bpf-next 3/4] bpf: Resolve symbols with kallsyms_lookup_names for kprobe multi link

[Andrii Nakryiko]

On Thu, Apr 7, 2022 at 5:53 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> Using kallsyms_lookup_names function to speed up symbols lookup in
> kprobe multi link attachment and replacing with it the current
> kprobe_multi_resolve_syms function.
>
> This speeds up bpftrace kprobe attachment:
>
>   # perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* {  } i:ms:1 { exit(); }'
>   ...
>   6.5681 +- 0.0225 seconds time elapsed  ( +-  0.34% )
>
> After:
>
>   # perf stat -r 5 -e cycles ./src/bpftrace -e 'kprobe:x* {  } i:ms:1 { exit(); }'
>   ...
>   0.5661 +- 0.0275 seconds time elapsed  ( +-  4.85% )
>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  kernel/trace/bpf_trace.c | 123 +++++++++++++++++++++++----------------
>  1 file changed, 73 insertions(+), 50 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index b26f3da943de..2602957225ba 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2226,6 +2226,72 @@ struct bpf_kprobe_multi_run_ctx {
>         unsigned long entry_ip;
>  };
>
> +struct user_syms {
> +       const char **syms;
> +       char *buf;
> +};
> +
> +static int copy_user_syms(struct user_syms *us, void __user *usyms, u32 cnt)
> +{
> +       const char __user **usyms_copy = NULL;
> +       const char **syms = NULL;
> +       char *buf = NULL, *p;
> +       int err = -EFAULT;
> +       unsigned int i;
> +       size_t size;
> +
> +       size = cnt * sizeof(*usyms_copy);
> +
> +       usyms_copy = kvmalloc(size, GFP_KERNEL);
> +       if (!usyms_copy)
> +               return -ENOMEM;

do you really need usyms_copy? why not just read one pointer at a time?

> +
> +       if (copy_from_user(usyms_copy, usyms, size))
> +               goto error;
> +
> +       err = -ENOMEM;
> +       syms = kvmalloc(size, GFP_KERNEL);
> +       if (!syms)
> +               goto error;
> +
> +       /* TODO this potentially allocates lot of memory (~6MB in my tests
> +        * with attaching ~40k functions). I haven't seen this to fail yet,
> +        * but it could be changed to allocate memory gradually if needed.
> +        */
> +       size = cnt * KSYM_NAME_LEN;

this reassignment of size is making it hard to follow the code, you
can just do cnt * KSYM_NAME_LEN inside kvmalloc, you don't ever use it
anywhere else

> +       buf = kvmalloc(size, GFP_KERNEL);
> +       if (!buf)
> +               goto error;
> +
> +       for (p = buf, i = 0; i < cnt; i++) {

like here, before doing strncpy_from_user() you can read usyms[i] from
user-space into temporary variable, no need for extra kvmalloc?

> +               err = strncpy_from_user(p, usyms_copy[i], KSYM_NAME_LEN);
> +               if (err == KSYM_NAME_LEN)
> +                       err = -E2BIG;
> +               if (err < 0)
> +                       goto error;
> +               syms[i] = p;
> +               p += err + 1;
> +       }
> +
> +       err = 0;
> +       us->syms = syms;
> +       us->buf = buf;
> +

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