Re: [PATCH v3] tracing: expose current->comm to [ku]probe events

[Masami Hiramatsu <mhiramat@kernel.org>]

On Wed,  8 Jun 2016 18:38:02 -0700
Omar Sandoval <osandov@osandov.com> wrote:

> From: Omar Sandoval <osandov@fb.com>
> 
> ftrace is very quick to give up on saving the task command line (see
> `trace_save_cmdline()`). The workaround for events which really care
> about the command line is to explicitly assign it as part of the entry.
> However, this doesn't work for kprobe events, as there's no
> straightforward way to get access to current->comm. Add a kprobe/uprobe
> event variable $comm which provides exactly that.
> 
> Signed-off-by: Omar Sandoval <osandov@fb.com>

Looks good to me:)

Acked-by: Masami Hiramatsu <mhiramat@kernel.org>

> ---
> Alexei pointed out to me that this same functionality can be acheived
> with BPF and the bpf_get_current_comm() helper, so strictly speaking,
> this patch isn't necessary. It's still convenient for ad-hoc stuff,
> though, so I figured I'd throw this up and see what everyone thought.

Anyway, I'd like to pull this patch since this still be useful
in handy way to get comm on ftrace :)

Thank you!

> 
>  Documentation/trace/kprobetrace.txt  |  3 +++
>  Documentation/trace/uprobetracer.txt |  3 +++
>  kernel/trace/trace_kprobe.c          |  1 +
>  kernel/trace/trace_probe.c           | 33 +++++++++++++++++++++++++++++++++
>  kernel/trace/trace_probe.h           | 10 ++++++++++
>  5 files changed, 50 insertions(+)
> 
> diff --git a/Documentation/trace/kprobetrace.txt b/Documentation/trace/kprobetrace.txt
> index d68ea5fc812b..ea52ec1f8484 100644
> --- a/Documentation/trace/kprobetrace.txt
> +++ b/Documentation/trace/kprobetrace.txt
> @@ -40,6 +40,7 @@ Synopsis of kprobe_events
>    $stackN	: Fetch Nth entry of stack (N >= 0)
>    $stack	: Fetch stack address.
>    $retval	: Fetch return value.(*)
> +  $comm		: Fetch current task comm.
>    +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
>    NAME=FETCHARG : Set NAME as the argument name of FETCHARG.
>    FETCHARG:TYPE : Set TYPE as the type of FETCHARG. Currently, basic types
> @@ -62,6 +63,8 @@ offset, and container-size (usually 32). The syntax is;
>  
>   b<bit-width>@<bit-offset>/<container-size>
>  
> +For $comm, the default type is "string"; any other type is invalid.
> +
>  
>  Per-Probe Event Filtering
>  -------------------------
> diff --git a/Documentation/trace/uprobetracer.txt b/Documentation/trace/uprobetracer.txt
> index f1cf9a34ad9d..72d1cd4f7bf3 100644
> --- a/Documentation/trace/uprobetracer.txt
> +++ b/Documentation/trace/uprobetracer.txt
> @@ -36,6 +36,7 @@ Synopsis of uprobe_tracer
>     $stackN	: Fetch Nth entry of stack (N >= 0)
>     $stack	: Fetch stack address.
>     $retval	: Fetch return value.(*)
> +   $comm	: Fetch current task comm.
>     +|-offs(FETCHARG) : Fetch memory at FETCHARG +|- offs address.(**)
>     NAME=FETCHARG     : Set NAME as the argument name of FETCHARG.
>     FETCHARG:TYPE     : Set TYPE as the type of FETCHARG. Currently, basic types
> @@ -57,6 +58,8 @@ offset, and container-size (usually 32). The syntax is;
>  
>   b<bit-width>@<bit-offset>/<container-size>
>  
> +For $comm, the default type is "string"; any other type is invalid.
> +
>  
>  Event Profiling
>  ---------------
> diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> index 5546eec0505f..9aedb0b06683 100644
> --- a/kernel/trace/trace_kprobe.c
> +++ b/kernel/trace/trace_kprobe.c
> @@ -587,6 +587,7 @@ static int create_trace_kprobe(int argc, char **argv)
>  	 *  $retval	: fetch return value
>  	 *  $stack	: fetch stack address
>  	 *  $stackN	: fetch Nth of stack (N:0-)
> +	 *  $comm       : fetch current task comm
>  	 *  @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
> diff --git a/kernel/trace/trace_probe.c b/kernel/trace/trace_probe.c
> index 1d372fa6fefb..74e80a582c28 100644
> --- a/kernel/trace/trace_probe.c
> +++ b/kernel/trace/trace_probe.c
> @@ -218,6 +218,28 @@ free_bitfield_fetch_param(struct bitfield_fetch_param *data)
>  	kfree(data);
>  }
>  
> +void FETCH_FUNC_NAME(comm, string)(struct pt_regs *regs,
> +					  void *data, void *dest)
> +{
> +	int maxlen = get_rloc_len(*(u32 *)dest);
> +	u8 *dst = get_rloc_data(dest);
> +	long ret;
> +
> +	if (!maxlen)
> +		return;
> +
> +	ret = strlcpy(dst, current->comm, maxlen);
> +	*(u32 *)dest = make_data_rloc(ret, get_rloc_offs(*(u32 *)dest));
> +}
> +NOKPROBE_SYMBOL(FETCH_FUNC_NAME(comm, string));
> +
> +void FETCH_FUNC_NAME(comm, string_size)(struct pt_regs *regs,
> +					       void *data, void *dest)
> +{
> +	*(u32 *)dest = strlen(current->comm) + 1;
> +}
> +NOKPROBE_SYMBOL(FETCH_FUNC_NAME(comm, string_size));
> +
>  static const struct fetch_type *find_fetch_type(const char *type,
>  						const struct fetch_type *ftbl)
>  {
> @@ -348,6 +370,11 @@ static int parse_probe_vars(char *arg, const struct fetch_type *t,
>  			}
>  		} else
>  			ret = -EINVAL;
> +	} else if (strcmp(arg, "comm") == 0) {
> +		if (strcmp(t->name, "string") != 0 &&
> +		    strcmp(t->name, "string_size") != 0)
> +			return -EINVAL;
> +		f->fn = t->fetch[FETCH_MTD_comm];
>  	} else
>  		ret = -EINVAL;
>  
> @@ -522,6 +549,12 @@ int traceprobe_parse_probe_arg(char *arg, ssize_t *size,
>  		arg[t - parg->comm] = '\0';
>  		t++;
>  	}
> +	/*
> +	 * The default type of $comm should be "string", and it can't be
> +	 * dereferenced.
> +	 */
> +	if (!t && strcmp(arg, "$comm") == 0)
> +		t = "string";
>  	parg->type = find_fetch_type(t, ftbl);
>  	if (!parg->type) {
>  		pr_info("Unsupported type: %s\n", t);
> diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
> index f6398db09114..45400ca5ded1 100644
> --- a/kernel/trace/trace_probe.h
> +++ b/kernel/trace/trace_probe.h
> @@ -102,6 +102,7 @@ enum {
>  	FETCH_MTD_reg = 0,
>  	FETCH_MTD_stack,
>  	FETCH_MTD_retval,
> +	FETCH_MTD_comm,
>  	FETCH_MTD_memory,
>  	FETCH_MTD_symbol,
>  	FETCH_MTD_deref,
> @@ -183,6 +184,14 @@ DECLARE_BASIC_FETCH_FUNCS(bitfield);
>  #define fetch_bitfield_string			NULL
>  #define fetch_bitfield_string_size		NULL
>  
> +/* comm only makes sense as a string */
> +#define fetch_comm_u8		NULL
> +#define fetch_comm_u16		NULL
> +#define fetch_comm_u32		NULL
> +#define fetch_comm_u64		NULL
> +DECLARE_FETCH_FUNC(comm, string);
> +DECLARE_FETCH_FUNC(comm, string_size);
> +
>  /*
>   * Define macro for basic types - we don't need to define s* types, because
>   * we have to care only about bitwidth at recording time.
> @@ -213,6 +222,7 @@ DEFINE_FETCH_##method(u64)
>  ASSIGN_FETCH_FUNC(reg, ftype),				\
>  ASSIGN_FETCH_FUNC(stack, ftype),			\
>  ASSIGN_FETCH_FUNC(retval, ftype),			\
> +ASSIGN_FETCH_FUNC(comm, ftype),				\
>  ASSIGN_FETCH_FUNC(memory, ftype),			\
>  ASSIGN_FETCH_FUNC(symbol, ftype),			\
>  ASSIGN_FETCH_FUNC(deref, ftype),			\
> -- 
> 2.8.3
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>