Re: [RFC PATCH] tracing/kprobe: Support $$args for function entry

[Alan Maguire <alan.maguire@oracle.com>]

On 23/09/2021 13:39, Masami Hiramatsu wrote:

> Support $$args fetch arg for function entry. This uses
> BTF for finding the function argument. Thus it depends
> on CONFIG_BPF_SYSCALL.
>
> /sys/kernel/tracing # echo 'p vfs_read $$args' >> kprobe_events
> /sys/kernel/tracing # cat kprobe_events
> p:kprobes/p_vfs_read_0 vfs_read file=$arg1:x64 buf=$arg2:x64 count=$arg3:u64 pos=$arg4:x64
>
> Note that $$args must be used without argument name.

This looks great! Can I ask which tree you're building on

top of so I can play around with this a bit?


I also wonder if we could rework btf_show_name() to help

render full type info for the args? in kernel/bpf/btf.c:


/*
  * Populate show->state.name with type name information.
  * Format of type name is
  *
  * [.member_name = ] (type_name)
  */

The (type_name) part is what we'd want from here; no reason

we can't refactor that function to make the type name available

as a cast. It would rework the output to be something like


p:kprobes/p_vfs_read_0 vfs_read struct file *file=$arg1 , char *buf=$arg2 , size_t count=$arg3 , loff_t pos=$arg4

...if that's wanted of course (not sure what the constraints on format are here)? Thanks for pushing this along!


Alan

> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
>   kernel/trace/trace_kprobe.c |   60 ++++++++++++++++++++++++-
>   kernel/trace/trace_probe.c  |  105 +++++++++++++++++++++++++++++++++++++++++++
>   kernel/trace/trace_probe.h  |    5 ++
>   3 files changed, 168 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 3dd4fb719aa3..fe88ee8c8cd8 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -712,6 +712,58 @@ static int trace_kprobe_module_callback(struct notifier_block *nb,
>   	return NOTIFY_DONE;
>   }
>   
> +#ifdef CONFIG_BPF_SYSCALL
> +
> +static int trace_kprobe_parse_btf_args(struct trace_kprobe *tk, int i,
> +				       const char *arg, unsigned int flags)
> +{
> +	struct trace_probe *tp = &tk->tp;
> +	static struct btf *btf;
> +	const struct btf_type *t;
> +	const struct btf_param *args;
> +	s32 id, nargs;
> +	int ret;
> +
> +	if (!(flags & TPARG_FL_FENTRY))
> +		return -EINVAL;
> +	if (!tk->symbol)
> +		return -EINVAL;
> +
> +	if (!btf)
> +		btf = btf_parse_vmlinux();
> +
> +	id = btf_find_by_name_kind(btf, tk->symbol, BTF_KIND_FUNC);
> +	if (id <= 0)
> +		return -ENOENT;
> +
> +	/* Get BTF_KIND_FUNC type */
> +	t = btf_type_by_id(btf, id);
> +	if (!btf_type_is_func(t))
> +		return -ENOENT;
> +
> +	/* The type of BTF_KIND_FUNC is BTF_KIND_FUNC_PROTO */
> +	t = btf_type_by_id(btf, t->type);
> +	if (!btf_type_is_func_proto(t))
> +		return -ENOENT;
> +
> +	args = (const struct btf_param *)(t + 1);
> +	nargs = btf_type_vlen(t);
> +	for (i = 0; i < nargs; i++) {
> +		ret = traceprobe_parse_btf_arg(tp, i, btf, &args[i]);
> +		if (ret < 0)
> +			break;
> +	}
> +
> +	return ret;
> +}
> +#else
> +static int trace_kprobe_parse_btf_args(struct trace_kprobe *tk, int i,
> +				       const char *arg, unsigned int flags)
> +{
> +	return -EOPNOTSUPP;
> +}
> +#endif
> +
>   static struct notifier_block trace_kprobe_module_nb = {
>   	.notifier_call = trace_kprobe_module_callback,
>   	.priority = 1	/* Invoked after kprobe module callback */
> @@ -733,12 +785,13 @@ static int __trace_kprobe_create(int argc, const char *argv[])
>   	 *  $stack	: fetch stack address
>   	 *  $stackN	: fetch Nth of stack (N:0-)
>   	 *  $comm       : fetch current task comm
> +	 *  $$args	: fetch parameters using BTF
>   	 *  @ADDR	: fetch memory at ADDR (ADDR should be in kernel)
>   	 *  @SYM[+|-offs] : fetch memory at SYM +|- offs (SYM is a data symbol)
>   	 *  %REG	: fetch register REG
>   	 * Dereferencing memory fetch:
>   	 *  +|-offs(ARG) : fetch memory at ARG +|- offs address.
> -	 * Alias name of args:
> +	 * Alias name of args (except for $$args) :
>   	 *  NAME=FETCHARG : set NAME as alias of FETCHARG.
>   	 * Type of args:
>   	 *  FETCHARG:TYPE : use TYPE instead of unsigned long.
> @@ -877,7 +930,10 @@ static int __trace_kprobe_create(int argc, const char *argv[])
>   	/* parse arguments */
>   	for (i = 0; i < argc && i < MAX_TRACE_ARGS; i++) {
>   		trace_probe_log_set_index(i + 2);
> -		ret = traceprobe_parse_probe_arg(&tk->tp, i, argv[i], flags);
> +		if (strcmp(argv[i], "$$args") == 0)
> +			ret = trace_kprobe_parse_btf_args(tk, i, argv[i], flags);
> +		else
> +			ret = traceprobe_parse_probe_arg(&tk->tp, i, argv[i], flags);
>   		if (ret)
>   			goto error;	/* This can be -ENOMEM */
>   	}
> diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> index 2fe104109525..bbac261b1688 100644
> --- a/kernel/trace/trace_probe.c
> +++ b/kernel/trace/trace_probe.c
> @@ -765,6 +765,111 @@ static int traceprobe_conflict_field_name(const char *name,
>   	return 0;
>   }
>   
> +#ifdef CONFIG_BPF_SYSCALL
> +
> +static u32 btf_type_int(const struct btf_type *t)
> +{
> +	return *(u32 *)(t + 1);
> +}
> +
> +static const char *traceprobe_type_from_btf(struct btf *btf, s32 id)
> +{
> +	const struct btf_type *t;
> +	u32 intdata;
> +	s32 tid;
> +
> +	/* TODO: const char * could be converted as a string */
> +	t = btf_type_skip_modifiers(btf, id, &tid);
> +
> +	switch (BTF_INFO_KIND(t->info)) {
> +	case BTF_KIND_ENUM:
> +		/* enum is "int", so convert to "s32" */
> +		return "s32";
> +	case BTF_KIND_PTR:
> +		/* pointer will be converted to "x??" */
> +		if (IS_ENABLED(CONFIG_64BIT))
> +			return "x64";
> +		else
> +			return "x32";
> +	case BTF_KIND_INT:
> +		intdata = btf_type_int(t);
> +		if (BTF_INT_ENCODING(intdata) & BTF_INT_SIGNED) {
> +			switch (BTF_INT_BITS(intdata)) {
> +			case 8:
> +				return "s8";
> +			case 16:
> +				return "s16";
> +			case 32:
> +				return "s32";
> +			case 64:
> +				return "s64";
> +			}
> +		} else {	/* unsigned */
> +			switch (BTF_INT_BITS(intdata)) {
> +			case 8:
> +				return "u8";
> +			case 16:
> +				return "u16";
> +			case 32:
> +				return "u32";
> +			case 64:
> +				return "u64";
> +			}
> +		}
> +	}
> +
> +	/* Default type */
> +	if (IS_ENABLED(CONFIG_64BIT))
> +		return "x64";
> +	else
> +		return "x32";
> +}
> +
> +int traceprobe_parse_btf_arg(struct trace_probe *tp, int i, struct btf *btf,
> +			     const struct btf_param *arg)
> +{
> +	struct probe_arg *parg = &tp->args[i];
> +	const char *name, *tname;
> +	char *body;
> +	int ret;
> +
> +	tp->nr_args++;
> +	name = btf_name_by_offset(btf, arg->name_off);
> +	parg->name = kstrdup(name, GFP_KERNEL);
> +	if (!parg->name)
> +		return -ENOMEM;
> +
> +	if (!is_good_name(parg->name)) {
> +		trace_probe_log_err(0, BAD_ARG_NAME);
> +		return -EINVAL;
> +	}
> +	if (traceprobe_conflict_field_name(parg->name, tp->args, i)) {
> +		trace_probe_log_err(0, USED_ARG_NAME);
> +		return -EINVAL;
> +	}
> +
> +	/*
> +	 * Since probe event needs an appropriate command for dyn_event interface,
> +	 * convert BTF type to corresponding fetch-type string.
> +	 */
> +	tname = traceprobe_type_from_btf(btf, arg->type);
> +	if (tname)
> +		body = kasprintf(GFP_KERNEL, "$arg%d:%s", i + 1, tname);
> +	else
> +		body = kasprintf(GFP_KERNEL, "$arg%d", i + 1);
> +
> +	if (!body)
> +		return -ENOMEM;
> +	/* Parse fetch argument */
> +	ret = traceprobe_parse_probe_arg_body(body, &tp->size, parg,
> +				TPARG_FL_KERNEL | TPARG_FL_FENTRY, 0);
> +
> +	kfree(body);
> +
> +	return ret;
> +}
> +#endif
> +
>   int traceprobe_parse_probe_arg(struct trace_probe *tp, int i, const char *arg,
>   				unsigned int flags)
>   {
> diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
> index 355c78a930f8..857b946afe29 100644
> --- a/kernel/trace/trace_probe.h
> +++ b/kernel/trace/trace_probe.h
> @@ -23,6 +23,7 @@
>   #include <linux/limits.h>
>   #include <linux/uaccess.h>
>   #include <linux/bitops.h>
> +#include <linux/btf.h>
>   #include <asm/bitsperlong.h>
>   
>   #include "trace.h"
> @@ -359,6 +360,10 @@ int trace_probe_create(const char *raw_command, int (*createfn)(int, const char
>   
>   extern int traceprobe_parse_probe_arg(struct trace_probe *tp, int i,
>   				const char *argv, unsigned int flags);
> +#ifdef CONFIG_BPF_SYSCALL
> +int traceprobe_parse_btf_arg(struct trace_probe *tp, int i, struct btf *btf,
> +			     const struct btf_param *arg);
> +#endif
>   
>   extern int traceprobe_update_arg(struct probe_arg *arg);
>   extern void traceprobe_free_probe_arg(struct probe_arg *arg);
>