Fix extracting pid form bpf_get_current_pid_tgid()

[AliJavidiCS]

Based on Issue #35, pid must be extracted from the return value by using an and operation with 0xFFFFFFFF.

[parttimenerd]

Why? The documentation tells us:

Returns the process ID in the lower 32 bits (kernel's view of the PID, which in user space is usually presented as the thread ID), and the thread group ID in the upper 32 bits (what user space often thinks of as the PID). By directly setting this to a u32, we discard the upper 32 bits.

Therefore the current code seems to be correct.

[AliJavidiCS]

I agree, The documentation states that PID is stored in the lower 32 bits. However the code actually throws away the lower 32 bits as in the line of code below: data.pid = bpf_get_current_pid_tgid() >> 32;

If we insist on shifting the result to get the PID we might use something like this: data.pid = (bpf_get_current_pid_tgid() << 32) >> 32;

Besides, the book refers to the higher 32 bits as PID as I mentioned in #35 and the current code would be correct if that was the case.

[lizrice]

Thanks for this @AliJavidiCS. Per the comments in #35 I think the code is actually correct here, though the footnote needs a better explanation!