Re: [PATCH 2/3] bpf: Optimize get_modules_for_addrs()

[Song Liu <song@kernel.org>]

On Wed, Jan 4, 2023 at 8:25 AM Petr Mladek <pmladek@suse.com> wrote:
>
> On Fri 2022-12-30 19:27:28, Zhen Lei wrote:
> > Function __module_address() can quickly return the pointer of the module
> > to which an address belongs. We do not need to traverse the symbols of all
> > modules to check whether each address in addrs[] is the start address of
> > the corresponding symbol, because register_fprobe_ips() will do this check
> > later.
> >
> > Assuming that there are m modules, each module has n symbols on average,
> > and the number of addresses 'addrs_cnt' is abbreviated as K. Then the time
> > complexity of the original method is O(K * log(K)) + O(m * n * log(K)),
> > and the time complexity of current method is O(K * (log(m) + M)), M <= m.
> > (m * n * log(K)) / (K * m) ==> n / log2(K). Even if n is 10 and K is 128,
> > the ratio is still greater than 1. Therefore, the new method will
> > generally have better performance.
> >
> > Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> > ---
> >  kernel/trace/bpf_trace.c | 101 ++++++++++++++++-----------------------
> >  1 file changed, 40 insertions(+), 61 deletions(-)
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index 5f3be4bc16403a5..0ff9037098bd241 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2684,69 +2684,55 @@ 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)
> > +static int get_modules_for_addrs(struct module ***out_mods, unsigned long *addrs, u32 addrs_cnt)
> >  {
> > -     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;
> > +     int i, j, err;
> > +     int mods_cnt = 0;
> > +     int mods_cap = 0;
> > +     struct module *mod;
> > +     struct module **mods = NULL;
> >
> > -     if (args->mods && args->mods[args->mods_cnt - 1] == mod)
> > -             return 0;
> > +     for (i = 0; i < addrs_cnt; i++) {
> > +             mod = __module_address(addrs[i]);
>
> This must be called under module_mutex to make sure that the module
> would not disappear.

module_mutex is not available outside kernel/module/. The common
practice is to disable preempt before calling __module_address().
CONFIG_LOCKDEP should catch this.

Thanks,
Song

[...]