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

Masami Hiramatsu
2025-01-08T04:52:20Z

On Wed, 8 Jan 2025 10:30:08 +0800
Donglin Peng <dolinux.peng@gmail.com> wrote:

> Steven Rostedt <rostedt@goodmis.org>于2024年12月24日 周二04:14写道：
>
> > 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
> > + 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;
>
>
> There is no need to the retrieve the BTF of vmlinux, as btf_find_func_proto
> will return the correct BTF via its second parameter.

Good catch! The second parameter of btf_find_func_proto() is output.

Thank you,

>
> — donglin
>
>
> > +
> > + 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>