Re: [PATCH 4/7] Add print_function_args()

[Donglin Peng]

On Wed, Sep 4, 2024 at 2:59 PM Sven Schnelle <svens@xxxxxxxxxxxxx> wrote:
>
> Add a function to decode argument types with the help of BTF. Will
> be used to display arguments in the function and function graph
> tracer.
>
> Signed-off-by: Sven Schnelle <svens@xxxxxxxxxxxxx>
> ---
>  kernel/trace/trace_output.c | 68 +++++++++++++++++++++++++++++++++++++
>  kernel/trace/trace_output.h |  9 +++++
>  2 files changed, 77 insertions(+)
>
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index d8b302d01083..70405c4cceb6 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -12,8 +12,11 @@
>  #include <linux/sched/clock.h>
>  #include <linux/sched/mm.h>
>  #include <linux/idr.h>
> +#include <linux/btf.h>
> +#include <linux/bpf.h>
>
>  #include "trace_output.h"
> +#include "trace_btf.h"
>
>  /* must be a power of 2 */
>  #define EVENT_HASHSIZE 128
> @@ -669,6 +672,71 @@ int trace_print_lat_context(struct trace_iterator *iter)
>         return !trace_seq_has_overflowed(s);
>  }
>
> +#ifdef CONFIG_FUNCTION_TRACE_ARGS
> +void print_function_args(struct trace_seq *s, struct ftrace_regs *fregs,
> +                        unsigned long func)
> +{
> +       const struct btf_param *param;
> +       const struct btf_type *t;
> +       const char *param_name;
> +       char name[KSYM_NAME_LEN];
> +       unsigned long arg;
> +       struct btf *btf;
> +       s32 tid, nr = 0;
> +       int i;
> +
> +       trace_seq_printf(s, "(");
> +
> +       if (!ftrace_regs_has_args(fregs))
> +               goto out;
> +       if (lookup_symbol_name(func, name))
> +               goto out;
> +
> +       btf = bpf_get_btf_vmlinux();
> +       if (IS_ERR_OR_NULL(btf))
> +               goto out;
> +
> +       t = btf_find_func_proto(name, &btf);

This is an excellent feature, and I am crafting a series of patches aimed
 at enhancing the search performance for locating the btf by its name.

> +       if (IS_ERR_OR_NULL(t))
> +               goto out;
> +
> +       param = btf_get_func_param(t, &nr);
> +       if (!param)
> +               goto out_put;
> +
> +       for (i = 0; i < nr; i++) {
> +               arg = ftrace_regs_get_argument(fregs, i);
> +
> +               param_name = btf_name_by_offset(btf, param[i].name_off);
> +               if (param_name)
> +                       trace_seq_printf(s, "%s = ", param_name);
> +               t = btf_type_skip_modifiers(btf, param[i].type, &tid);
> +               if (!t)
> +                       continue;
> +               switch (BTF_INFO_KIND(t->info)) {
> +               case BTF_KIND_PTR:
> +                       trace_seq_printf(s, "0x%lx", arg);
> +                       break;
> +               case BTF_KIND_INT:
> +                       trace_seq_printf(s, "%ld", arg);
> +                       break;
> +               case BTF_KIND_ENUM:
> +                       trace_seq_printf(s, "%ld", arg);
> +                       break;
> +               default:
> +                       trace_seq_printf(s, "0x%lx (%d)", arg, BTF_INFO_KIND(param[i].type));
> +                       break;
> +               }
> +               if (i < nr - 1)
> +                       trace_seq_printf(s, ", ");
> +       }
> +out_put:
> +       btf_put(btf);
> +out:
> +       trace_seq_printf(s, ")");
> +}
> +#endif
> +
>  /**
>   * ftrace_find_event - find a registered event
>   * @type: the type of event to look for
> diff --git a/kernel/trace/trace_output.h b/kernel/trace/trace_output.h
> index dca40f1f1da4..a21d8ce606f7 100644
> --- a/kernel/trace/trace_output.h
> +++ b/kernel/trace/trace_output.h
> @@ -41,5 +41,14 @@ extern struct rw_semaphore trace_event_sem;
>  #define SEQ_PUT_HEX_FIELD(s, x)                                \
>         trace_seq_putmem_hex(s, &(x), sizeof(x))
>
> +#ifdef CONFIG_FUNCTION_TRACE_ARGS
> +void print_function_args(struct trace_seq *s, struct ftrace_regs *fregs,
> +                        unsigned long func);
> +#else
> +static inline void print_function_args(struct trace_seq *s, struct ftrace_regs *fregs,
> +                                      unsigned long func) {
> +       trace_seq_puts(s, "()");
> +}
> +#endif
>  #endif
>
> --
> 2.43.0
>
>