Re: [RFC PATCH] tracing/kprobes: Avoid parsing symbol+offset when updating arguments

[Steven Rostedt]

On Sun,  4 Nov 2018 01:03:34 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> Introduce symbol_offset data structure for avoiding symbol+offset
> parsing when updating arguments.
> 
> For kprobe events, "@symbol+offset" is supported, that requires
> to be updated when new module is loaded because @symbol address
> can be solved at that point. Currently kprobe events saves
> the "symbol+offset" string and parse it repeatedly, but that
> is inefficient.

Do we care if it's inefficient? It's only done on module load and at
the beginning. That's far from a fast path.

If anything, the original solution saves memory, which is a bigger
concern than speed in this case.

-- Steve


> This introduces symbol_offset data structure which can save the
> offset and symbol separated, so that we don't need to parse it
> anymore.
> 
> Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> ---
>  kernel/trace/trace_probe.c |   49 +++++++++++++++++++++++++-------------------
>  kernel/trace/trace_probe.h |    7 +++++-
>  2 files changed, 34 insertions(+), 22 deletions(-)
> 
> diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> index bd30e9398d2a..0a3af7d6e133 100644
> --- a/kernel/trace/trace_probe.c
> +++ b/kernel/trace/trace_probe.c
> @@ -201,6 +201,26 @@ static int parse_probe_vars(char *arg, const struct fetch_type *t,
>  	return ret;
>  }
>  
> +static struct symbol_offset *allocate_symbol_offset(char *sym_offs_str)
> +{
> +	int ret;
> +	long offset = 0;
> +	struct symbol_offset *sof;
> +
> +	ret = traceprobe_split_symbol_offset(sym_offs_str, &offset);
> +	if (ret)
> +		return ERR_PTR(ret);
> +
> +	sof = kzalloc(sizeof(struct symbol_offset) + strlen(sym_offs_str) + 1,
> +			GFP_KERNEL);
> +	if (!sof)
> +		return ERR_PTR(-ENOMEM);
> +
> +	sof->offset = offset;
> +	strcpy(sof->symbol, sym_offs_str);
> +	return sof;
> +}
> +
>  /* Recursive argument parser */
>  static int
>  parse_probe_arg(char *arg, const struct fetch_type *type,
> @@ -253,9 +273,9 @@ parse_probe_arg(char *arg, const struct fetch_type *type,
>  
>  			/* Preserve symbol for updating */
>  			code->op = FETCH_NOP_SYMBOL;
> -			code->data = kstrdup(arg + 1, GFP_KERNEL);
> -			if (!code->data)
> -				return -ENOMEM;
> +			code->symoffs = allocate_symbol_offset(arg + 1);
> +			if (IS_ERR(code->symoffs))
> +				return PTR_ERR(code->symoffs);
>  			if (++code == end)
>  				return -E2BIG;
>  
> @@ -483,7 +503,7 @@ int traceprobe_parse_probe_arg(char *arg, ssize_t *size,
>  	if (ret) {
>  		for (code = tmp; code < tmp + FETCH_INSN_MAX; code++)
>  			if (code->op == FETCH_NOP_SYMBOL)
> -				kfree(code->data);
> +				kfree(code->symoffs);
>  	}
>  	kfree(tmp);
>  
> @@ -513,7 +533,7 @@ void traceprobe_free_probe_arg(struct probe_arg *arg)
>  
>  	while (code && code->op != FETCH_OP_END) {
>  		if (code->op == FETCH_NOP_SYMBOL)
> -			kfree(code->data);
> +			kfree(code->symoffs);
>  		code++;
>  	}
>  	kfree(arg->code);
> @@ -525,31 +545,18 @@ void traceprobe_free_probe_arg(struct probe_arg *arg)
>  int traceprobe_update_arg(struct probe_arg *arg)
>  {
>  	struct fetch_insn *code = arg->code;
> -	long offset;
> -	char *tmp;
> -	char c;
> -	int ret = 0;
>  
>  	while (code && code->op != FETCH_OP_END) {
>  		if (code->op == FETCH_NOP_SYMBOL) {
>  			if (code[1].op != FETCH_OP_IMM)
>  				return -EINVAL;
>  
> -			tmp = strpbrk(code->data, "+-");
> -			if (tmp)
> -				c = *tmp;
> -			ret = traceprobe_split_symbol_offset(code->data,
> -							     &offset);
> -			if (ret)
> -				return ret;
> -
>  			code[1].immediate =
> -				(unsigned long)kallsyms_lookup_name(code->data);
> -			if (tmp)
> -				*tmp = c;
> +				(unsigned long)kallsyms_lookup_name(
> +						code->symoffs->symbol);
>  			if (!code[1].immediate)
>  				return -ENOENT;
> -			code[1].immediate += offset;
> +			code[1].immediate += code->symoffs->offset;
>  		}
>  		code++;
>  	}
> diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
> index 974afc1a3e73..942424d05427 100644
> --- a/kernel/trace/trace_probe.h
> +++ b/kernel/trace/trace_probe.h
> @@ -103,6 +103,11 @@ enum fetch_op {
>  	FETCH_NOP_SYMBOL,	/* Unresolved Symbol holder */
>  };
>  
> +struct symbol_offset {
> +	long offset;
> +	char symbol[];
> +};
> +
>  struct fetch_insn {
>  	enum fetch_op op;
>  	union {
> @@ -117,7 +122,7 @@ struct fetch_insn {
>  			unsigned char rshift;
>  		};
>  		unsigned long immediate;
> -		void *data;
> +		struct symbol_offset *symoffs;
>  	};
>  };
>