Messages in this thread | | | Date | Mon, 10 Jan 2022 10:58:10 +0800 | From | Pingfan Liu <> | Subject | Re: [PATCH 1/3] tracing/filter: degrade addr in filter_pred_string() from double pointer to pointer |
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On Fri, Jan 07, 2022 at 12:18:42PM -0500, Steven Rostedt wrote: > On Fri, 7 Jan 2022 12:49:49 +0800 > Pingfan Liu <kernelfans@gmail.com> wrote: > > > Since FILTER_PTR_STRING has the type of "char *", it is meaningless to > > convert it to "char **". Hence degrading addr from double pointer to > > single. > > > > Signed-off-by: Pingfan Liu <kernelfans@gmail.com> > > Cc: Steven Rostedt <rostedt@goodmis.org> > > Cc: Ingo Molnar <mingo@redhat.com> > > To: linux-kernel@vger.kernel.org > > --- > > kernel/trace/trace_events_filter.c | 6 +++--- > > 1 file changed, 3 insertions(+), 3 deletions(-) > > > > diff --git a/kernel/trace/trace_events_filter.c b/kernel/trace/trace_events_filter.c > > index c9124038b140..264456e1698f 100644 > > --- a/kernel/trace/trace_events_filter.c > > +++ b/kernel/trace/trace_events_filter.c > > @@ -670,11 +670,11 @@ static int filter_pred_string(struct filter_pred *pred, void *event) > > /* Filter predicate for char * pointers */ > > static int filter_pred_pchar(struct filter_pred *pred, void *event) > > { > > - char **addr = (char **)(event + pred->offset); > > + char *addr = (char *)(event + pred->offset); > > This doesn't look right. The address of the pointer should be in the event. > "event" is an address to the content of the event in the kernel ring buffer. > > event + pred->offset > > Is then the address of position of the event. > > Let's say we have an event record at 0xffff8000, and the pred->offset is at > 0x10. And the pointer to the string (in user space) is at 0x70008000. > > 0xffff8000: <heade> > 0xffff8010: 0x70008000 > > 0x70008000: "user space sting" > > event + pred->offset gives us 0xffff8010 > > If we now have that as char *addr, then addr is 0xffff8010 > > > > int cmp, match; > > - int len = strlen(*addr) + 1; /* including tailing '\0' */ > > + int len = strlen(addr) + 1; /* including tailing '\0' */ > > This would give us the addr = 0xffff8010, which is not where the string > exists. > > How would this work? > No, it can not work.
Thanks,
Pingfan
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