Re: [PATCH v5 16/19] tracing: probeevent: Add array type support

[Masami Hiramatsu <mhiramat@kernel.org>]

Hi Ravi,

On Thu, 15 Mar 2018 16:18:40 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Thu, 15 Mar 2018 11:18:19 +0530
> Ravi Bangoria <ravi.bangoria@linux.vnet.ibm.com> wrote:
> 
> > Hi Masami,
> > 
> > On 03/08/2018 02:20 PM, Masami Hiramatsu wrote:
> > > Add array type support for probe events.
> > > This allows user to get arraied types from memory address.
> > > The array type syntax is
> > >
> > > 	TYPE[N]
> > >
> > > Where TYPE is one of types (u8/16/32/64,s8/16/32/64,
> > > x8/16/32/64, symbol, string) and N is a fixed value less
> > > than 64.
> > >
> > > The string array type is a bit different from other types. For
> > > other base types, <base-type>[1] is equal to <base-type>
> > > (e.g. +0(%di):x32[1] is same as +0(%di):x32.) But string[1] is not
> > > equal to string. The string type itself represents "char array",
> > > but string array type represents "char * array". So, for example,
> > > +0(%di):string[1] is equal to +0(+0(%di)):string.
> > 
> > I was trying to test this patch. But I'm not getting proper data.
> > Here is what I'm doing...
> > 
> > $ cat hello.c
> > #include <stdio.h>
> > 
> > void foo1(int a[])
> > {
> >     printf("%d\n", a[2]);
> > }
> > 
> > void foo(int a[])
> > {
> >     printf("%d\n", a[1]);
> >     foo1(a);
> > }
> > 
> > void main()
> > {
> >     int a[3] = {4, 5, 6};
> >     printf("%d\n", a[0]);
> >     foo(a);
> > }
> > 
> > $ gcc -g hello.c -o hello
> > $ sudo ./perf probe -x ~/hello foo1 'a=a:x32[3]'
> > $ sudo cat /sys/kernel/debug/tracing/uprobe_events
> > p:probe_hello/foo1 /home/ravi/hello:0x00000000000005fc a=+96(%gpr31):x32[3]
> > 
> > $ sudo ./perf record -e probe_hello:foo1 ~/hello
> > $ sudo ./perf script
> > hello  6913 [038]  2857.704470: probe_hello:foo1: (100005fc) a={0xd69e4400,0x7fff,0x0}
> > 
> > 
> > I don't see proper values of the 'a'? Anything wrong with my perf commands :) ?
> 
> Ah, I guess since it is a pointer. in main() function, "a" is an array,
> but in foo1(), "a" is a pointer to the array.
> From the viewpoint of C source code, both pointer and array is same
> expression, but actually it is not same from memory point of view.
> perf probe has to be updated to enable it too.
> 
> Could you please try to do as below?
> 
> $ echo "p:probe_hello/foo1 /home/ravi/hello:0x00000000000005fc a=+0(+96(%gpr31)):x32[3]" | sudo tee /sys/kernel/debug/tracing/uprobe_events

Sorry, that is too complicated than enough. you should try to specify a[0] instead of a.

 $ sudo ./perf probe -x ~/hello foo1 'a=a[0]:x32[3]'

:)

Thank you,

-- 
Masami Hiramatsu <mhiramat@kernel.org>