//! # divination
//!
//! the practice of seeking knowledge of the future or the unknown by supernatural means.
//!
//! we ask supernatural means (the dynamic linker) for knowledge of the future
//! (where we will find the dwarves)
//!
//! first, we ask the dynamic linker to give us the `.eh_frame` for the current binary using
//! the GNU extension (`_dl_find_object`)[https://www.gnu.org/software/libc/manual/html_node/Dynamic-Linker-Introspection.html].
//! then, we parse that as beautiful DWARF call frame information, as god (or rather, the x86-64 psABI) intended.

use core::ffi;

use crate::stdext::with_last_os_error_str;

#[allow(non_camel_case_types)]
#[repr(C)]
struct dl_find_object {
    dlfo_flags: ffi::c_ulonglong,
    dlfo_map_start: *const ffi::c_void,
    dlfo_map_end: *const ffi::c_void,
    dlf_link_map: *const ffi::c_void,
    dlfo_eh_frame: *const ffi::c_void,
}

extern "C" {
    fn _dl_find_object(address: *const ffi::c_void, result: *mut dl_find_object) -> ffi::c_int;
}

#[derive(Debug, Clone, Copy)]
pub struct DwarfInfo {
    /// The text segment
    map: *const [u8],
    /// PT_GNU_EH_FRAME
    dwarf: *const u8,
}

pub fn dwarf_info(addr: *const ffi::c_void) -> Option<DwarfInfo> {
    unsafe {
        let mut out = core::mem::zeroed();
        let ret = _dl_find_object(addr, &mut out);
        trace!("dl_find_object returned {ret}");
        if ret != 0 {
            with_last_os_error_str(|err| trace!("dl_find_object error: {err}"));
            return None;
        }
        if out.dlfo_eh_frame.is_null() {
            return None;
        }

        let text_len = out.dlfo_map_end as usize - out.dlfo_map_start as usize;
        trace!(
            "dwarf info; map: ({:p}, {:x}), dwarf: {:p}",
            out.dlfo_map_start,
            text_len,
            out.dlfo_eh_frame
        );

        if !(out.dlfo_map_start..out.dlfo_map_end).contains(&addr) {
            trace!("dl_find_object returned object out of range for addr: {addr:p}");
            return None;
        }

        Some(DwarfInfo {
            map: core::ptr::slice_from_raw_parts(out.dlfo_map_start as _, text_len),
            dwarf: out.dlfo_eh_frame as _,
        })
    }
}