splice(2) returns "Invalid argument" when reading from ftrace raw files

I wanted to try splice syscall, which trace-cmd uses to zero copy raw files of off ftrace.

Here is the partial list of ftrace raw files splice fails to read:

/sys/kernel/debug/tracing/per_cpu/cpo0/trace_pipe_raw

/sys/kernel/debug/tracing/per_cpu/cpo0/snapshot_raw

/sys/kernel/debug/tracing/per_cpu/cpo1/trace_pipe_raw

/sys/kernel/debug/tracing/per_cpu/cpo1/snapshot_raw

And here are some other files (that splice handles just fine):

/sys/kernel/debug/tracing/per_cpu/cpo0/trace_pipe

/sys/kernel/debug/tracing/per_cpu/cpo0/snapshot

/sys/kernel/debug/tracing/per_cpu/cpo1/trace_pipe

/sys/kernel/debug/tracing/per_cpu/cpo1/snapshot

What works:

• using the read() system call works great to read from raw ftrace files.
• using the cat() system call will display raw ftrace files.
• using the trace-cmd tool which is a CLI front end for ftrace.

This is my code:

static void unit_test_x(void){      int buffer_pipe[2];    pipe(buffer_pipe);    std::string source_path = "/sys/kernel/debug/tracing/per_cpu/cpu1/trace_pipe_raw";    int trace_fd = open(source_path.c_str(), O_RDONLY);    std::string destination_path = "foo";    int dest_fd = open (destination_path.c_str(), O_WRONLY | O_CREAT | O_TRUNC | O_LARGEFILE, 0644);    int actually_read = splice(trace_fd,                               NULL,                               buffer_pipe[1],                               NULL,                               1000,                               SPLICE_F_MORE | SPLICE_F_MOVE );    if (0 > actually_read )    {           printf("Oh dear, something went wrong %s\n", s  trerror(errno));        throw std::runtime_error("writing from source to pipe failed");    }    actually_read = splice(buffer_pipe[0],                           NULL,                           dest_fd,                           NULL,                           actually_read,                           SPLICE_F_MORE | SPLICE_F_MOVE);}

note:

All accsess to /sys/kernel/debug/tracing is done with sudo