Re: [PATCH RFC] ftrace: Show all functions with addresses in available_filter_functions_addrs

[Andrii Nakryiko]

On Thu, Jun 8, 2023 at 2:26 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
>
> hi,
> when ftrace based tracers we need to cross check available_filter_functions
> with /proc/kallsyms. For example for kprobe_multi bpf link (based on fprobe)
> we need to make sure that symbol regex resolves to traceable symbols and
> that we get proper addresses for them.
>
> Looks like on the last last LSF/MM/BPF there was an agreement to add new
> file that will have available_filter_functions symbols plus addresses.
>
> This RFC is to kick off the discussion, I'm not sure Steven wants to do
> that differently ;-)
>
> thanks,
> jirka
>
>
> ---
> Adding new available_filter_functions_addrs file that shows all available
> functions (same as available_filter_functions) together with addresses,
> like:
>
>   # cat available_filter_functions_addrs | head

nit: can we have some more succinct name, like "traceable_funcs" or
something? And btw, does this have to be part of tracefs/debugfs
(never knew the difference, sorry). E.g., can it be instead exposed
through sysfs?

Either than these minor things, yep, I think this is something that
would be extremely useful, thanks, Jiri, for taking a stab at it!

>   ffffffff81000770 __traceiter_initcall_level
>   ffffffff810007c0 __traceiter_initcall_start
>   ffffffff81000810 __traceiter_initcall_finish
>   ffffffff81000860 trace_initcall_finish_cb
>   ...
>
> It's useful to have address avilable for traceable symbols, so we don't
> need to allways cross check kallsyms with available_filter_functions
> (or the other way around) and have all the data in single file.
>
> For backwards compatibility reasons we can't change the existing
> available_filter_functions file output, but we need to add new file.
>
> Suggested-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
> Suggested-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> ---
>  include/linux/ftrace.h |  1 +
>  kernel/trace/ftrace.c  | 52 ++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 48 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index b23bdd414394..6e372575a8e9 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -633,6 +633,7 @@ enum {
>         FTRACE_ITER_MOD         = (1 << 5),
>         FTRACE_ITER_ENABLED     = (1 << 6),
>         FTRACE_ITER_TOUCHED     = (1 << 7),
> +       FTRACE_ITER_ADDRS       = (1 << 8),
>  };
>
>  void arch_ftrace_update_code(int command);
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 764668467155..1f33e1f04834 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -3804,7 +3804,7 @@ static int __init ftrace_check_sync(void)
>  late_initcall_sync(ftrace_check_sync);
>  subsys_initcall(ftrace_check_for_weak_functions);
>
> -static int print_rec(struct seq_file *m, unsigned long ip)
> +static int print_rec(struct seq_file *m, unsigned long ip, bool print_addr)
>  {
>         unsigned long offset;
>         char str[KSYM_SYMBOL_LEN];
> @@ -3819,7 +3819,11 @@ static int print_rec(struct seq_file *m, unsigned long ip)
>                 ret = NULL;
>         }
>
> -       seq_puts(m, str);
> +       if (print_addr)
> +               seq_printf(m, "%lx %s", ip, str);
> +       else
> +               seq_puts(m, str);
> +
>         if (modname)
>                 seq_printf(m, " [%s]", modname);
>         return ret == NULL ? -1 : 0;
> @@ -3830,9 +3834,13 @@ static inline int test_for_valid_rec(struct dyn_ftrace *rec)
>         return 1;
>  }
>
> -static inline int print_rec(struct seq_file *m, unsigned long ip)
> +static inline int print_rec(struct seq_file *m, unsigned long ip, bool print_addr)
>  {
> -       seq_printf(m, "%ps", (void *)ip);
> +       if (print_addr)
> +               seq_printf(m, "%lx %ps", ip, (void *)ip);
> +       else
> +               seq_printf(m, "%ps", (void *)ip);
> +
>         return 0;
>  }
>  #endif
> @@ -3861,7 +3869,7 @@ static int t_show(struct seq_file *m, void *v)
>         if (!rec)
>                 return 0;
>
> -       if (print_rec(m, rec->ip)) {
> +       if (print_rec(m, rec->ip, iter->flags & FTRACE_ITER_ADDRS)) {
>                 /* This should only happen when a rec is disabled */
>                 WARN_ON_ONCE(!(rec->flags & FTRACE_FL_DISABLED));
>                 seq_putc(m, '\n');
> @@ -3996,6 +4004,30 @@ ftrace_touched_open(struct inode *inode, struct file *file)
>         return 0;
>  }
>
> +static int
> +ftrace_avail_addrs_open(struct inode *inode, struct file *file)
> +{
> +       struct ftrace_iterator *iter;
> +       int ret;
> +
> +       ret = security_locked_down(LOCKDOWN_TRACEFS);
> +       if (ret)
> +               return ret;
> +
> +       if (unlikely(ftrace_disabled))
> +               return -ENODEV;
> +
> +       iter = __seq_open_private(file, &show_ftrace_seq_ops, sizeof(*iter));
> +       if (!iter)
> +               return -ENOMEM;
> +
> +       iter->pg = ftrace_pages_start;
> +       iter->flags = FTRACE_ITER_ADDRS;
> +       iter->ops = &global_ops;
> +
> +       return 0;
> +}
> +
>  /**
>   * ftrace_regex_open - initialize function tracer filter files
>   * @ops: The ftrace_ops that hold the hash filters
> @@ -5916,6 +5948,13 @@ static const struct file_operations ftrace_touched_fops = {
>         .release = seq_release_private,
>  };
>
> +static const struct file_operations ftrace_avail_addrs_fops = {
> +       .open = ftrace_avail_addrs_open,
> +       .read = seq_read,
> +       .llseek = seq_lseek,
> +       .release = seq_release_private,
> +};
> +
>  static const struct file_operations ftrace_filter_fops = {
>         .open = ftrace_filter_open,
>         .read = seq_read,
> @@ -6377,6 +6416,9 @@ static __init int ftrace_init_dyn_tracefs(struct dentry *d_tracer)
>         trace_create_file("available_filter_functions", TRACE_MODE_READ,
>                         d_tracer, NULL, &ftrace_avail_fops);
>
> +       trace_create_file("available_filter_functions_addrs", TRACE_MODE_READ,
> +                       d_tracer, NULL, &ftrace_avail_addrs_fops);
> +
>         trace_create_file("enabled_functions", TRACE_MODE_READ,
>                         d_tracer, NULL, &ftrace_enabled_fops);
>
> --
> 2.40.1
>