Re: [PATCH 09/18] tracing: Add indexing of arguments for function based events

[Namhyung Kim]

On Fri, Feb 02, 2018 at 06:05:07PM -0500, Steven Rostedt wrote:
> From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> 
> Currently reading of 8 byte words can only happen 8 bytes aligned from the
> argument. But there may be cases that they are 4 bytes aligned. To make the
> capturing of arguments more flexible, add a plus '+' operator that can index
> the variable at arbitrary indexes to get any location.
> 
>  u64 arg+4[3]
> 
> Will get an 8 byte word at index 28 (3 * 8 + 4)
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
> ---
>  Documentation/trace/function-based-events.rst | 24 +++++++++++++++++++++++-
>  kernel/trace/trace_event_ftrace.c             | 18 ++++++++++++++++++
>  2 files changed, 41 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/trace/function-based-events.rst b/Documentation/trace/function-based-events.rst
> index 72e3e7730d63..bdb28f433bfb 100644
> --- a/Documentation/trace/function-based-events.rst
> +++ b/Documentation/trace/function-based-events.rst
> @@ -100,10 +100,12 @@ as follows:
>           'x8' | 'x16' | 'x32' | 'x64' |
>           'char' | 'short' | 'int' | 'long' | 'size_t'
>  
> - FIELD := <name> | <name> INDEX
> + FIELD := <name> | <name> INDEX | <name> OFFSET | <name> OFFSET INDEX
>  
>   INDEX := '[' <number> ']'
>  
> + OFFSET := '+' <number>
> +
>   Where <name> is a unique string starting with an alphabetic character
>   and consists only of letters and numbers and underscores.
>  
> @@ -221,3 +223,23 @@ format:
>  print fmt: "%pS->%pS(skb=%u)", REC->__ip, REC->__parent_ip, REC->skb
>  
>  It is now printed with a "%u".
> +
> +
> +Offsets
> +=======
> +
> +After the name of the variable, brackets '[' number ']' will index the value of
> +the argument by the number given times the size of the field.
> +
> + int field[5] will dereference the value of the argument 20 bytes away (4 * 5)
> +  as sizeof(int) is 4.
> +
> +If there's a case where the type is of 8 bytes in size but is not 8 bytes
> +alligned in the structure, an offset may be required.
> +
> +  For example: x64 param+4[2]
> +
> +The above will take the parameter value, add it by 4, then index it by two
> +8 byte words. It's the same in C as: (u64 *)((void *)param + 4)[2]
> +
> + Note: "int skb[32]" is the same as "int skb+4[31]".
> diff --git a/kernel/trace/trace_event_ftrace.c b/kernel/trace/trace_event_ftrace.c
> index 9548b93eb8cd..4c23fa18453d 100644
> --- a/kernel/trace/trace_event_ftrace.c
> +++ b/kernel/trace/trace_event_ftrace.c
> @@ -19,6 +19,7 @@ struct func_arg {
>  	char				*type;
>  	char				*name;
>  	long				indirect;
> +	long				index;
>  	short				offset;
>  	short				size;
>  	s8				arg;
> @@ -62,6 +63,7 @@ enum func_states {
>  	FUNC_STATE_INDIRECT,
>  	FUNC_STATE_UNSIGNED,
>  	FUNC_STATE_PIPE,
> +	FUNC_STATE_PLUS,
>  	FUNC_STATE_TYPE,
>  	FUNC_STATE_VAR,
>  	FUNC_STATE_COMMA,
> @@ -182,6 +184,7 @@ static char *next_token(char **ptr, char *last)
>  		    *str == ']' ||
>  		    *str == ',' ||
>  		    *str == '|' ||
> +		    *str == '+' ||
>  		    *str == ')')
>  			break;
>  	}
> @@ -323,6 +326,15 @@ process_event(struct func_event *fevent, const char *token, enum func_states sta
>  		}
>  		break;
>  
> +	case FUNC_STATE_PLUS:
> +		if (WARN_ON(!fevent->last_arg))
> +			break;
> +		ret = kstrtol(token, 0, &val);
> +		if (ret)
> +			break;
> +		fevent->last_arg->index += val;
> +		return FUNC_STATE_VAR;
> +
>  	case FUNC_STATE_VAR:
>  		switch (token[0]) {
>  		case ')':
> @@ -331,6 +343,8 @@ process_event(struct func_event *fevent, const char *token, enum func_states sta
>  			return FUNC_STATE_COMMA;
>  		case '|':
>  			return FUNC_STATE_PIPE;
> +		case '+':
> +			return FUNC_STATE_PLUS;
>  		case '[':
>  			return FUNC_STATE_BRACKET;
>  		}
> @@ -347,6 +361,8 @@ static long long get_arg(struct func_arg *arg, unsigned long val)
>  	char buf[8];
>  	int ret;
>  
> +	val += arg->index;
> +
>  	if (!arg->indirect)
>  		return val;

So this also works without the indirect, and just add the immediate to
the value.

Thanks,
Namhyung


>  
> @@ -779,6 +795,8 @@ static int func_event_seq_show(struct seq_file *m, void *v)
>  		last_arg = arg->arg;
>  		comma = true;
>  		seq_printf(m, "%s %s", arg->type, arg->name);
> +		if (arg->index)
> +			seq_printf(m, "+%ld", arg->index);
>  		if (arg->indirect && arg->size)
>  			seq_printf(m, "[%ld]",
>  				   (arg->indirect ^ INDIRECT_FLAG) / arg->size);
> -- 
> 2.15.1
> 
>