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?
> --
> 2.34.1
>
