Add "request read" event

[umanwizard]

If a page is not mapped in a process, bpf_probe_read_user will not be able to read it (even if the page is resident, so reading would have only caused a minor fault).

In this case, instead of giving up, we can ask the agent to fault it into the process by reading /proc//mem at the specified offset.

To test, compile the following program with -O0 and attempt to profile it:

#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

void x(uint64_t orig_page) {
        uint64_t this_page = ((uint64_t)(&orig_page) >> 12);

        if (this_page != orig_page) {
                pid_t pid = fork();
                if (pid)
                        printf("Forked child pid: %d\n", pid);
                else
                        for (;;)
                                ;
        } else
                x(orig_page);
}

int main(int argc, char *argv[]) {
        x((uint64_t)(&argc) >> 12);
}

This program does the following:

1. Recurses until the stack crosses a page boundary
2. Forks -- the new process will initially not have any pages mapped in its address space
3. Loops forever -- thus it will never go back to the old stack frame and read it.

This program consistently produces 100% PreviousRipZero errors before this commit, but with this commit, it eventually starts bieng profiled successfully.

[kakkoyun]

This looks promising indeed 🎉 🌮