Re: [PATCH] tracing: expose current->comm to kprobe events
From: Omar Sandoval
Date: Thu Jun 02 2016 - 14:21:21 EST
Hi,
On Fri, Jun 03, 2016 at 12:40:02AM +0900, Namhyung Kim wrote:
> Hello,
>
> On Wed, Jun 1, 2016 at 3:17 PM, Omar Sandoval <osandov@xxxxxxxxxxx> wrote:
> > From: Omar Sandoval <osandov@xxxxxx>
> >
> > 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 event
> > variable $comm which provides exactly that.
> >
> > Signed-off-by: Omar Sandoval <osandov@xxxxxx>
> > ---
> > This is a problem I ran into while trying to debug an unrelated issue. I
> > was considering trying to make ftrace try harder to save the command
> > line, but that would most likely be at the expense at most events which
> > don't care about the task comm. This is much less work.
> >
> > Documentation/trace/kprobetrace.txt | 2 ++
> > kernel/trace/trace_kprobe.c | 13 +++++++++++++
> > kernel/trace/trace_probe.c | 5 +++++
> > kernel/trace/trace_probe.h | 2 ++
> > kernel/trace/trace_uprobe.c | 7 +++++++
> > 5 files changed, 29 insertions(+)
> >
> > diff --git a/Documentation/trace/kprobetrace.txt b/Documentation/trace/kprobetrace.txt
> > index d68ea5fc812b..90914cbc1e51 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
> > @@ -48,6 +49,7 @@ Synopsis of kprobe_events
> >
> > (*) only for return probe.
> > (**) this is useful for fetching a field of data structures.
> > + (***) you probably want this as a string, i.e., $comm:string
> >
> > Types
> > -----
> > diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
> > index 5546eec0505f..e3a087552d49 100644
> > --- a/kernel/trace/trace_kprobe.c
> > +++ b/kernel/trace/trace_kprobe.c
> > @@ -214,6 +214,18 @@ static void FETCH_FUNC_NAME(memory, string_size)(struct pt_regs *regs,
> > }
> > NOKPROBE_SYMBOL(FETCH_FUNC_NAME(memory, string_size));
> >
> > +#define DEFINE_FETCH_comm(type) \
> > +void FETCH_FUNC_NAME(comm, type)(struct pt_regs *regs, \
> > + void *data, void *dest) \
> > +{ \
> > + fetch_memory_##type(regs, current->comm, dest); \
> > +} \
> > +NOKPROBE_SYMBOL(FETCH_FUNC_NAME(comm, type));
> > +
> > +DEFINE_BASIC_FETCH_FUNCS(comm)
> > +DEFINE_FETCH_comm(string)
> > +DEFINE_FETCH_comm(string_size)
>
> I think you can define default type fetch functions (such as u8, ...)
> to NULL to make sure it's used as string type only.
Yeah, my original thinking was to not make $comm a special case, but it
really is kind of useless to get it as anything other than a string.
I'll also try to make the type default to string just for $comm.
> > +
> > #define DEFINE_FETCH_symbol(type) \
> > void FETCH_FUNC_NAME(symbol, type)(struct pt_regs *regs, void *data, void *dest)\
> > { \
> > @@ -587,6 +599,7 @@ 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..918775fcacd3 100644
> > --- a/kernel/trace/trace_probe.c
> > +++ b/kernel/trace/trace_probe.c
> > @@ -348,6 +348,11 @@ static int parse_probe_vars(char *arg, const struct fetch_type *t,
> > }
> > } else
> > ret = -EINVAL;
> > + } else if (strcmp(arg, "comm") == 0) {
> > + if (is_kprobe)
> > + f->fn = t->fetch[FETCH_MTD_comm];
> > + else
> > + ret = -EINVAL;
>
> I think it should work with uprobes too. Why not enabling it to uprobes?
You're right, it should work for uprobes, too, I'll add that.
Thanks for taking a look!
> Thanks,
> Namhyung
>
>
> > } else
> > ret = -EINVAL;
> >
> > diff --git a/kernel/trace/trace_probe.h b/kernel/trace/trace_probe.h
> > index f6398db09114..3f1a987e6dc1 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,
> > @@ -213,6 +214,7 @@ 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), \
> > diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> > index c53485441c88..f7eedb191d1f 100644
> > --- a/kernel/trace/trace_uprobe.c
> > +++ b/kernel/trace/trace_uprobe.c
> > @@ -172,6 +172,13 @@ static void FETCH_FUNC_NAME(memory, string_size)(struct pt_regs *regs,
> > *(u32 *)dest = len;
> > }
> >
> > +#define fetch_comm_u8 NULL
> > +#define fetch_comm_u16 NULL
> > +#define fetch_comm_u32 NULL
> > +#define fetch_comm_u64 NULL
> > +#define fetch_comm_string NULL
> > +#define fetch_comm_string_size NULL
> > +
> > static unsigned long translate_user_vaddr(void *file_offset)
> > {
> > unsigned long base_addr;
> > --
> > 2.8.3
> >
--
Omar