enBoxer/src/gbm_runtime.rs

//! Runtime dlopen wrapper for `libgbm.so.1`.
//!
//! enBoxer reads dmabufs that Hyprland hands us through
//! `zwlr_export_dmabuf_manager_v1`. To turn those GPU-allocated dma-bufs
//! into CPU pixels we need `gbm_create_device` + `gbm_bo_import` +
//! `gbm_bo_map` from libgbm. libgbm is a system library, not a Rust crate,
//! and we don't want a build-time dep on `libgbm-dev`. So we dlopen it
//! at runtime via `libc::dlopen`, look up just the functions we need
//! with `libc::dlsym`, and store their raw addresses as `usize`.
//!
//! If libgbm.so.1 isn't installed on the operator's box the open call
//! fails; callers fall back to the synthetic frame (the round-trip
//! metadata is still useful) and document the libgbm dependency clearly
//! in the docs and CHANGELOG.
//!
//! Why libc + raw usize instead of the `libloading` crate:
//! libloading 0.8's `Symbol<T>::into_raw` returns the `Symbol` wrapper
//! rather than `*mut T`, and threading the lifetimes through `Syms` -> BO
//! -> mapped slices is more trouble than it is worth for the seven
//! symbols we need. A libc + transmute dance is the boring well-trodden
//! path that compiles on every Rust version and every libloading
//! revision without gymnastics.

use std::os::fd::RawFd;
use std::os::raw::{c_char, c_int, c_uint, c_void};
use std::path::{Path, PathBuf};

use thiserror::Error;

#[derive(Debug, Error)]
pub enum GbmError {
    #[error("libgbm.so.1: dlopen failed ({0}); install libgbm or accept the synthetic fallback")]
    LibLoad(String),
    #[error("libgbm.so.1: required symbol missing: {0}")]
    Symbol(&'static str),
    #[error("/dev/dri: no render node found (looked in /dev/dri/)")]
    NoRenderNode,
    #[error("gbm_create_device failed (is the render node accessible?)")]
    CreateDevice,
    #[error("gbm_bo_import failed for fd {0} ({1}x{2} fmt {3:#x})")]
    Import(RawFd, u32, u32, u32),
    #[error("gbm_bo_map failed")]
    Map,
    #[error("io: {0}")]
    Io(#[from] std::io::Error),
}

// GBM_BO_IMPORT_FD from <gbm.h>. Stable ABI.
const GBM_BO_IMPORT_FD: c_uint = 0x5501;
// GBM_BO_USE_LINEAR (1 << 4).
const GBM_BO_USE_LINEAR: c_uint = 1 << 4;
// GBM_BO_TRANSFER_READ (1 << 0).
const GBM_BO_TRANSFER_READ: c_uint = 1 << 0;

type GbmDeviceT = c_void;
type GbmBoT = c_void;

// Mirror of `struct gbm_import_fd_data` from <gbm.h>.
#[repr(C)]
struct GbmImportFdData {
    fd: c_int,
    width: c_uint,
    height: u32,
    stride: c_uint,
    format: c_uint,
}

// Raw function pointers resolved via dlsym, stored as usize.
// At call time we transmute usize -> fn pointer. The pointers stay
// valid for the lifetime of the loaded Library, which `GbmDevice` owns.
#[derive(Debug, Clone, Copy)]
struct Syms {
    create_device: usize,
    destroy_device: usize,
    bo_import: usize,
    #[allow(dead_code)]  // resolved for future stride-overrun sanity
    bo_get_stride: usize,
    bo_destroy: usize,
    bo_map: usize,
    bo_unmap: usize,
}

/// Opaque handle to the dlopen'd libgbm library + the GBM device + the
/// resolved symbol pointers we need. Owns the dlopen handle via libc;
/// closes it on Drop via `dlclose`.
pub struct GbmDevice {
    handle: *mut c_void,
    dev: *mut GbmDeviceT,
    sym: Syms,
}

impl GbmDevice {
    /// dlopen libgbm.so.1 and open the first accessible render node. If
    /// libgbm is missing or no render node exists, returns Err so the
    /// caller can fall back gracefully.
    pub fn open() -> Result<Self, GbmError> {
        // libc::dlopen("libgbm.so.1", libc::RTLD_NOW)
        let path = b"libgbm.so.1\0";
        let handle = unsafe {
            libc::dlopen(path.as_ptr() as *const c_char, libc::RTLD_NOW)
        };
        if handle.is_null() {
            return Err(GbmError::LibLoad(
                std::io::Error::last_os_error().to_string(),
            ));
        }
        let sym = Syms {
            create_device: dlsym_required(handle, b"gbm_create_device\0")?,
            destroy_device: dlsym_required(handle, b"gbm_device_destroy\0")?,
            bo_import: dlsym_required(handle, b"gbm_bo_import\0")?,
            bo_get_stride: dlsym_required(handle, b"gbm_bo_get_stride\0")?,
            bo_destroy: dlsym_required(handle, b"gbm_bo_destroy\0")?,
            bo_map: dlsym_required(handle, b"gbm_bo_map\0")?,
            bo_unmap: dlsym_required(handle, b"gbm_bo_unmap\0")?,
        };
        let fd = open_first_render_node()?;
        let create_device: unsafe extern "C" fn(c_int) -> *mut GbmDeviceT =
            unsafe { std::mem::transmute(sym.create_device) };
        let dev = unsafe { create_device(fd) };
        if dev.is_null() {
            unsafe { libc::dlclose(handle) };
            return Err(GbmError::CreateDevice);
        }
        Ok(Self { handle, dev, sym })
    }

    /// Import a Linux DMA-BUF fd as a linear (CPU-mappable) BO. Width,
    /// height, stride, format must match the producer's view.
    pub fn import_dmabuf(
        &self,
        fd: RawFd,
        width: u32,
        height: u32,
        stride: u32,
        format: u32,
    ) -> Result<GbmBo, GbmError> {
        let duped = unsafe { libc::dup(fd) };
        if duped < 0 {
            return Err(GbmError::Io(std::io::Error::last_os_error()));
        }
        let data = GbmImportFdData {
            fd: duped,
            width,
            height,
            stride,
            format,
        };
        let bo_import: unsafe extern "C" fn(
            *mut GbmDeviceT,
            c_uint,
            *const c_void,
            c_uint,
        ) -> *mut GbmBoT = unsafe { std::mem::transmute(self.sym.bo_import) };
        let bo = unsafe {
            bo_import(
                self.dev,
                GBM_BO_IMPORT_FD,
                (&data as *const GbmImportFdData) as *const c_void,
                GBM_BO_USE_LINEAR,
            )
        };
        if bo.is_null() {
            unsafe { libc::close(duped) };
            return Err(GbmError::Import(fd, width, height, format));
        }
        Ok(GbmBo {
            handle: self.handle,
            inner: bo,
            sym: self.sym,
        })
    }
}

impl Drop for GbmDevice {
    fn drop(&mut self) {
        let destroy_device: unsafe extern "C" fn(*mut GbmDeviceT) =
            unsafe { std::mem::transmute(self.sym.destroy_device) };
        unsafe { destroy_device(self.dev) };
        unsafe { libc::dlclose(self.handle) };
    }
}

/// A BO that has been imported but not yet mapped. Call `map()` to read.
pub struct GbmBo {
    #[allow(dead_code)]
    handle: *mut c_void,
    inner: *mut GbmBoT,
    sym: Syms,
}

/// A mapped (read-only) BO + its stride. Owns the mapping until drop,
/// which calls gbm_bo_unmap.
pub struct MappedBo {
    bo: *mut GbmBoT,
    sym: Syms,
    ptr: *mut c_void,
    map_data: *mut c_void,
    pub stride: u32,
}

impl GbmBo {
    /// Map the BO for reading (CPU side). stride may differ from the
    /// producer's stride; trust this one because GBM aligns as needed.
    pub fn map(&self) -> Result<MappedBo, GbmError> {
        let mut stride: c_uint = 0;
        let mut map_data: *mut c_void = std::ptr::null_mut();
        let mut map_size: usize = 0;
        let bo_map: unsafe extern "C" fn(
            *mut GbmBoT,
            c_uint,
            c_uint,
            c_uint,
            c_uint,
            c_uint,
            *mut c_uint,
            *mut *mut c_void,
            *mut usize,
        ) -> *mut c_void = unsafe { std::mem::transmute(self.sym.bo_map) };
        let ptr = unsafe {
            bo_map(
                self.inner,
                0,
                0,
                u32::MAX,
                u32::MAX,
                GBM_BO_TRANSFER_READ,
                &mut stride,
                &mut map_data,
                &mut map_size,
            )
        };
        if ptr.is_null() {
            return Err(GbmError::Map);
        }
        Ok(MappedBo {
            bo: self.inner,
            sym: self.sym,
            ptr,
            map_data,
            stride,
        })
    }
}

impl Drop for GbmBo {
    fn drop(&mut self) {
        let bo_destroy: unsafe extern "C" fn(*mut GbmBoT) =
            unsafe { std::mem::transmute(self.sym.bo_destroy) };
        unsafe { bo_destroy(self.inner) };
    }
}

impl MappedBo {
    /// Borrow the mapped pixels as a byte slice of (stride * height)
    /// bytes. The caller knows the height from the dmabuf frame event.
    pub fn as_slice(&self, height: u32) -> &[u8] {
        unsafe {
            std::slice::from_raw_parts(
                self.ptr as *const u8,
                (self.stride as usize) * (height as usize),
            )
        }
    }
}

impl Drop for MappedBo {
    fn drop(&mut self) {
        let bo_unmap: unsafe extern "C" fn(*mut GbmBoT, *mut c_void) =
            unsafe { std::mem::transmute(self.sym.bo_unmap) };
        unsafe { bo_unmap(self.bo, self.map_data) };
    }
}

// libc::dlsym helper that returns the raw address as usize. NUL-terminated
// byte slices only.
fn dlsym_required(handle: *mut c_void, name: &[u8]) -> Result<usize, GbmError> {
    let name_str = std::str::from_utf8(name.trim_ascii_end())
        .map_err(|_| GbmError::Symbol("<bad utf8>"))?;
    let sym = unsafe { libc::dlsym(handle, name_str.as_ptr() as *const c_char) };
    if sym.is_null() {
        // Leak a copy: the dlsym lookup runs once per GBM device
        // open and the leak lasts until process exit. Not worth a
        // thread-local String cache for the half-dozen call sites.
        let leaked: &'static str = Box::leak(name_str.to_string().into_boxed_str());
        return Err(GbmError::Symbol(leaked));
    }
    Ok(sym as usize)
}

fn open_first_render_node() -> Result<RawFd, GbmError> {
    let entries = std::fs::read_dir("/dev/dri")
        .map_err(GbmError::Io)?
        .filter_map(Result::ok)
        .map(|e| e.path())
        .filter(|p: &PathBuf| {
            let name = p.file_name().and_then(|n| n.to_str()).unwrap_or("");
            name.starts_with("renderD")
        })
        .collect::<Vec<_>>();
    for path in entries {
        match open_rdwr(&path) {
            Ok(fd) => return Ok(fd),
            Err(_) => continue,
        }
    }
    Err(GbmError::NoRenderNode)
}

fn open_rdwr(path: &Path) -> std::io::Result<RawFd> {
    use std::os::fd::IntoRawFd;
    let f = std::fs::OpenOptions::new()
        .read(true)
        .write(true)
        .open(path)?;
    Ok(f.into_raw_fd())
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn fourcc_constants_match_gbm_h() {
        // The numeric values are part of libgbm's ABI; if upstream
        // renumbers them we want to know.
        assert_eq!(GBM_BO_IMPORT_FD, 0x5501);
        assert_eq!(GBM_BO_USE_LINEAR, 1 << 4);
        assert_eq!(GBM_BO_TRANSFER_READ, 1 << 0);
    }

    #[test]
    fn no_render_node_is_a_clean_error() {
        let e = GbmError::NoRenderNode;
        assert!(e.to_string().contains("render node"));
    }
}