On Sat, Dec 3, 2022 at 9:34 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Fri, Dec 2, 2022 at 8:29 AM Daniel T. Lee <danieltimlee@xxxxxxxxx> wrote:
> >
> > Currently, there is a problem with tracex2, as it doesn't print the
> > histogram properly and the results are misleading. (all results report
> > as 0)
> >
> > The problem is caused by a change in arguments of the function to which
> > the kprobe connects. This tracex2 bpf program uses kprobe (attached
> > to __x64_sys_write) to figure out the size of the write system call. In
> > order to achieve this, the third argument 'count' must be intact.
> >
> > The following is a prototype of the sys_write variant. (checked with
> > pfunct)
> >
> > ~/git/linux$ pfunct -P fs/read_write.o | grep sys_write
> > ssize_t ksys_write(unsigned int fd, const char * buf, size_t count);
> > long int __x64_sys_write(const struct pt_regs * regs);
> > ... cross compile with s390x ...
> > long int __s390_sys_write(struct pt_regs * regs);
> >
> > Since the __x64_sys_write (or s390x also) doesn't have the proper
> > argument, changing the kprobe event to ksys_write will fix the problem.
> >
> > Signed-off-by: Daniel T. Lee <danieltimlee@xxxxxxxxx>
> > ---
> > samples/bpf/tracex2_kern.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/samples/bpf/tracex2_kern.c b/samples/bpf/tracex2_kern.c
> > index 93e0b7680b4f..fc65c589e87f 100644
> > --- a/samples/bpf/tracex2_kern.c
> > +++ b/samples/bpf/tracex2_kern.c
> > @@ -78,7 +78,7 @@ struct {
> > __uint(max_entries, 1024);
> > } my_hist_map SEC(".maps");
> >
> > -SEC("kprobe/" SYSCALL(sys_write))
> > +SEC("kprobe/ksys_write")
> > int bpf_prog3(struct pt_regs *ctx)
> > {
> > long write_size = PT_REGS_PARM3(ctx);
>
>
> use
>
> SEC("ksyscall/write")
> int BPF_KSYSCALL(bpf_prog3, unsigned int fd, const char *buf, size_t count)
>
> instead?
>
> And maybe let's update other samples to use SEC("ksyscall") and
> BPF_KSYSCALL() macro as well?
>
>
Thanks for the review!
I'll check with the new BPF_KSYSCALL and try to fix others as well!
> > --
> > 2.34.1
> >
--
Best,
Daniel T. Lee
