Re: [PATCH 1/2] perf probe: Fix module probe issue if no dwarf support

[Masami Hiramatsu]

On Tue, 26 Apr 2016 19:55:40 +0530
Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx> wrote:

> Perf is not able to register probe in kernel module when dwarf supprt
> is not there(and so it goes for symtab). Perf passes full path of
> module where only module name is required which is causing the problem.
> This patch fixes this issue.
> 
> Before applying patch:
> 
>   $ dpkg -s libdw-dev
>   dpkg-query: package 'libdw-dev' is not installed and no information is...
> 
>   $ sudo ./perf probe -m /linux/samples/kprobes/kprobe_example.ko kprobe_init
>   Added new event:
>     probe:kprobe_init (on kprobe_init in /linux/samples/kprobes/kprobe_example.ko)
> 
>   You can now use it in all perf tools, such as:
> 
>   perf record -e probe:kprobe_init -aR sleep 1
> 
>   $ sudo cat /sys/kernel/debug/tracing/kprobe_events
>   p:probe/kprobe_init /linux/samples/kprobes/kprobe_example.ko:kprobe_init
> 
>   $ sudo ./perf record -a -e probe:kprobe_init
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 0.105 MB perf.data ]
> 
>   $ sudo ./perf script 	# No output here 
> 
> After applying patch:
> 
>   $ sudo ./perf probe -m /linux/samples/kprobes/kprobe_example.ko kprobe_init
>   Added new event:
>     probe:kprobe_init    (on kprobe_init in kprobe_example)
> 
>   You can now use it in all perf tools, such as:
> 
>   perf record -e probe:kprobe_init -aR sleep 1
> 
>   $ sudo cat /sys/kernel/debug/tracing/kprobe_events
>   p:probe/kprobe_init kprobe_example:kprobe_init
> 
>   $ sudo ./perf record -a -e probe:kprobe_init
>   [ perf record: Woken up 1 times to write data ]
>   [ perf record: Captured and wrote 0.105 MB perf.data (2 samples) ]
> 
>   $ sudo ./perf script
>   insmod 13990 [002]  5961.216833: probe:kprobe_init: ...
>   insmod 13995 [002]  5962.889384: probe:kprobe_init: ...
> 

Looks good to me!
Thanks for updating.

Acked-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>

Thank you!

> Signed-off-by: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxxxxxxx>
> ---
>  tools/perf/util/probe-event.c | 76 +++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 73 insertions(+), 3 deletions(-)
> 
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index 8319fbb..d58de20 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -265,6 +265,65 @@ static bool kprobe_warn_out_range(const char *symbol, unsigned long address)
>  	return true;
>  }
>  
> +/*
> + * NOTE:
> + * '.gnu.linkonce.this_module' section of kernel module elf directly
> + * maps to 'struct module' from linux/module.h. This section contains
> + * actual module name which will be used by kernel after loading it.
> + * But, we cannot use 'struct module' here since linux/module.h is not
> + * exposed to user-space. Offset of 'name' has remained same from long
> + * time, so hardcoding it here.
> + */
> +#ifdef __LP64__
> +#define MOD_NAME_OFFSET 24
> +#else
> +#define MOD_NAME_OFFSET 12
> +#endif
> +
> +/*
> + * @module can be module name of module file path. In case of path,
> + * inspect elf and find out what is actual module name.
> + * Caller has to free mod_name after using it.
> + */
> +static char *find_module_name(const char *module)
> +{
> +	int fd;
> +	Elf *elf;
> +	GElf_Ehdr ehdr;
> +	GElf_Shdr shdr;
> +	Elf_Data *data;
> +	Elf_Scn *sec;
> +	char *mod_name = NULL;
> +
> +	fd = open(module, O_RDONLY);
> +	if (fd < 0)
> +		return NULL;
> +
> +	elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
> +	if (elf == NULL)
> +		goto elf_err;
> +
> +	if (gelf_getehdr(elf, &ehdr) == NULL)
> +		goto ret_err;
> +
> +	sec = elf_section_by_name(elf, &ehdr, &shdr,
> +			".gnu.linkonce.this_module", NULL);
> +	if (!sec)
> +		goto ret_err;
> +
> +	data = elf_getdata(sec, NULL);
> +	if (!data || !data->d_buf)
> +		goto ret_err;
> +
> +	mod_name = strdup((char *)data->d_buf + MOD_NAME_OFFSET);
> +
> +ret_err:
> +	elf_end(elf);
> +elf_err:
> +	close(fd);
> +	return mod_name;
> +}
> +
>  #ifdef HAVE_DWARF_SUPPORT
>  
>  static int kernel_get_module_dso(const char *module, struct dso **pdso)
> @@ -2516,6 +2575,7 @@ static int find_probe_trace_events_from_map(struct perf_probe_event *pev,
>  	struct probe_trace_point *tp;
>  	int num_matched_functions;
>  	int ret, i, j, skipped = 0;
> +	char *mod_name;
>  
>  	map = get_target_map(pev->target, pev->uprobes);
>  	if (!map) {
> @@ -2600,9 +2660,19 @@ static int find_probe_trace_events_from_map(struct perf_probe_event *pev,
>  		tp->realname = strdup_or_goto(sym->name, nomem_out);
>  
>  		tp->retprobe = pp->retprobe;
> -		if (pev->target)
> -			tev->point.module = strdup_or_goto(pev->target,
> -							   nomem_out);
> +		if (pev->target) {
> +			if (pev->uprobes) {
> +				tev->point.module = strdup_or_goto(pev->target,
> +								   nomem_out);
> +			} else {
> +				mod_name = find_module_name(pev->target);
> +				tev->point.module =
> +					strdup(mod_name ? mod_name : pev->target);
> +				free(mod_name);
> +				if (!tev->point.module)
> +					goto nomem_out;
> +			}
> +		}
>  		tev->uprobes = pev->uprobes;
>  		tev->nargs = pev->nargs;
>  		if (tev->nargs) {
> -- 
> 1.9.1
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>