Re: [PATCH v2 1/4] ftrace: Add print_function_args()

Masami Hiramatsu
2024-12-25T13:16:05Z

On Mon, 23 Dec 2024 15:13:48 -0500
Steven Rostedt <rostedt@goodmis.org> wrote:

> From: Sven Schnelle <svens@linux.ibm.com>
>
> 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.
>
> It can only handle simply arguments and up to FTRACE_REGS_MAX_ARGS number
> of arguments. When it hits a max, it will print ", ...":
>
> page_to_skb(vi=0xffff8d53842dc980, rq=0xffff8d53843a0800, page=0xfffffc2e04337c00, offset=6160, len=64, truesize=1536, ...)
>
> And if it hits an argument that is not recognized, it will print the raw
> value and the type of argument it is:
>
> make_vfsuid(idmap=0xffffffff87f99db8, fs_userns=0xffffffff87e543c0, kuid=0x0 (STRUCT))
> __pti_set_user_pgtbl(pgdp=0xffff8d5384ab47f8, pgd=0x110e74067 (STRUCT))
>
> Co-developed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> ---
> Changes since v1: https://lore.kernel.org/20240904065908.1009086-5-svens@linux.ibm.com
>
> - Added Config option FUNCTION_TRACE_ARGS to this patch
> (unconditional if dependencies are met)
>
> - Changed the print_function_args() function to take an array of
> unsigned long args and not the ftrace_regs pointer. The ftrace_regs
> should be opaque from generic code.
>
> - Have the function print the name of an BTF type that is not supported.
>
> - Added FTRACE_REGS_MAX_ARGS as the number of arguments saved in
> the event and printed out.
>
> - Print "...," if the number of arguments goes past FTRACE_REGS_MAX_ARGS.
>
> include/linux/ftrace_regs.h | 5 +++
> kernel/trace/Kconfig | 6 +++
> kernel/trace/trace_output.c | 78 +++++++++++++++++++++++++++++++++++++
> kernel/trace/trace_output.h | 9 +++++
> 4 files changed, 98 insertions(+)
>
> diff --git a/include/linux/ftrace_regs.h b/include/linux/ftrace_regs.h
> index bbc1873ca6b8..15627ceea9bc 100644
> --- a/include/linux/ftrace_regs.h
> +++ b/include/linux/ftrace_regs.h
> @@ -35,4 +35,9 @@ struct ftrace_regs;
>
> #endif /* HAVE_ARCH_FTRACE_REGS */
>
> +/* This can be overridden by the architectures */
> +#ifndef FTRACE_REGS_MAX_ARGS
> +# define FTRACE_REGS_MAX_ARGS 6
> +#endif
> +
> #endif /* _LINUX_FTRACE_REGS_H */
> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index d570b8b9c0a9..60412c1012ef 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -263,6 +263,12 @@ config FUNCTION_GRAPH_RETADDR
> the function is called. This feature is off by default, and you can
> enable it via the trace option funcgraph-retaddr.
>
> +config FUNCTION_TRACE_ARGS
> + bool
> + depends on HAVE_FUNCTION_ARG_ACCESS_API
> + depends on DEBUG_INFO_BTF

For using the BTF APIs, we also needs BPF_SYSCALL (DEBUG_INFO_BTF just
compiles the BTF info into the kernel binary.)

Others looks good to me.

Revewied-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Thanks!

> + default y
> +
> config DYNAMIC_FTRACE
> bool "enable/disable function tracing dynamically"
> depends on FUNCTION_TRACER
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index da748b7cbc4d..40d6c7a9e0c4 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
> @@ -680,6 +683,81 @@ 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, unsigned long *args,
> + 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 (!args)
> + 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);
> + 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++) {
> + /* This only prints what the arch allows (6 args by default) */
> + if (i == FTRACE_REGS_MAX_ARGS) {
> + trace_seq_puts(s, "...");
> + break;
> + }
> +
> + arg = args[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);
> +
> + switch (t ? BTF_INFO_KIND(t->info) : BTF_KIND_UNKN) {
> + case BTF_KIND_UNKN:
> + trace_seq_putc(s, '?');
> + /* Still print unknown type values */
> + fallthrough;
> + 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:
> + /* This does not handle complex arguments */
> + trace_seq_printf(s, "0x%lx (%s)", arg, btf_type_str(t));
> + 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..2e305364f2a9 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, unsigned long *args,
> + unsigned long func);
> +#else
> +static inline void print_function_args(struct trace_seq *s, unsigned long *args,
> + unsigned long func) {
> + trace_seq_puts(s, "()");
> +}
> +#endif
> #endif
>
> --
> 2.45.2
>
>

--
Masami Hiramatsu (Google) <mhiramat@kernel.org>