libloading/os/unix/

mod.rs

use util::{ensure_compatible_types, cstr_cow_from_bytes};

use std::ffi::{CStr, OsStr};
use std::{fmt, marker, mem, ptr};
use std::os::raw;
use std::os::unix::ffi::OsStrExt;

// dl* family of functions did not have enough thought put into it.
//
// Whole error handling scheme is done via setting and querying some global state, therefore it is
// not safe to use dynamic library loading in MT-capable environment at all. Only in POSIX 2008+TC1
// a thread-local state was allowed for `dlerror`, making the dl* family of functions MT-safe.
//
// In practice (as of 2020-04-01) most of the widely used targets use a thread-local for error
// state and have been doing so for a long time. Regardless the comments in this function shall
// remain as a documentation for the future generations.
fn with_dlerror<T, F>(wrap: fn(crate::error::DlDescription) -> crate::Error, closure: F)
-> Result<T, Option<crate::Error>>
where F: FnOnce() -> Option<T> {
    // We used to guard all uses of dl* functions with our own mutex. This made them safe to use in
    // MT programs provided the only way a program used dl* was via this library. However, it also
    // had a number of downsides or cases where it failed to handle the problems. For instance,
    // if any other library called `dlerror` internally concurrently with `libloading` things would
    // still go awry.
    //
    // On platforms where `dlerror` is still MT-unsafe, `dlsym` (`Library::get`) can spuriously
    // succeed and return a null pointer for a symbol when the actual symbol look-up operation
    // fails. Instances where the actual symbol _could_ be `NULL` are platform specific. For
    // instance on GNU glibc based-systems (an excerpt from dlsym(3)):
    //
    // > The value of a symbol returned by dlsym() will never be NULL if the shared object is the
    // > result of normal compilation,  since  a  global  symbol is never placed at the NULL
    // > address. There are nevertheless cases where a lookup using dlsym() may return NULL as the
    // > value of a symbol. For example, the symbol value may be  the  result of a GNU indirect
    // > function (IFUNC) resolver function that returns NULL as the resolved value.

    // While we could could call `dlerror` here to clear the previous error value, only the `dlsym`
    // call depends on it being cleared beforehand and only in some cases too. We will instead
    // clear the error inside the dlsym binding instead.
    //
    // In all the other cases, clearing the error here will only be hiding misuse of these bindings
    // or a bug in implementation of dl* family of functions.
    closure().ok_or_else(|| unsafe {
        // This code will only get executed if the `closure` returns `None`.
        let error = dlerror();
        if error.is_null() {
            // In non-dlsym case this may happen when there’re bugs in our bindings or there’s
            // non-libloading user of libdl; possibly in another thread.
            None
        } else {
            // You can’t even rely on error string being static here; call to subsequent dlerror
            // may invalidate or overwrite the error message. Why couldn’t they simply give up the
            // ownership over the message?
            // TODO: should do locale-aware conversion here. OTOH Rust doesn’t seem to work well in
            // any system that uses non-utf8 locale, so I doubt there’s a problem here.
            let message = CStr::from_ptr(error).into();
            Some(wrap(crate::error::DlDescription(message)))
            // Since we do a copy of the error string above, maybe we should call dlerror again to
            // let libdl know it may free its copy of the string now?
        }
    })
}

/// A platform-specific equivalent of the cross-platform `Library`.
pub struct Library {
    handle: *mut raw::c_void
}

unsafe impl Send for Library {}

// That being said... this section in the volume 2 of POSIX.1-2008 states:
//
// > All functions defined by this volume of POSIX.1-2008 shall be thread-safe, except that the
// > following functions need not be thread-safe.
//
// With notable absence of any dl* function other than dlerror in the list. By “this volume”
// I suppose they refer precisely to the “volume 2”. dl* family of functions are specified
// by this same volume, so the conclusion is indeed that dl* functions are required by POSIX
// to be thread-safe. Great!
//
// See for more details:
//
//  * https://github.com/nagisa/rust_libloading/pull/17
//  * http://pubs.opengroup.org/onlinepubs/9699919799/functions/V2_chap02.html#tag_15_09_01
unsafe impl Sync for Library {}

impl Library {
    /// Find and load a shared library (module).
    ///
    /// Locations where library is searched for is platform specific and can’t be adjusted
    /// portably.
    ///
    /// Corresponds to `dlopen(filename, RTLD_NOW)`.
    #[inline]
    pub fn new<P: AsRef<OsStr>>(filename: P) -> Result<Library, crate::Error> {
        Library::open(Some(filename), RTLD_NOW)
    }

    /// Load the dynamic libraries linked into main program.
    ///
    /// This allows retrieving symbols from any **dynamic** library linked into the program,
    /// without specifying the exact library.
    ///
    /// Corresponds to `dlopen(NULL, RTLD_NOW)`.
    #[inline]
    pub fn this() -> Library {
        Library::open(None::<&OsStr>, RTLD_NOW).expect("this should never fail")
    }

    /// Find and load a shared library (module).
    ///
    /// Locations where library is searched for is platform specific and can’t be adjusted
    /// portably.
    ///
    /// If the `filename` is None, null pointer is passed to `dlopen`.
    ///
    /// Corresponds to `dlopen(filename, flags)`.
    pub fn open<P>(filename: Option<P>, flags: raw::c_int) -> Result<Library, crate::Error>
    where P: AsRef<OsStr> {
        let filename = match filename {
            None => None,
            Some(ref f) => Some(cstr_cow_from_bytes(f.as_ref().as_bytes())?),
        };
        with_dlerror(|desc| crate::Error::DlOpen { desc }, move || {
            let result = unsafe {
                let r = dlopen(match filename {
                    None => ptr::null(),
                    Some(ref f) => f.as_ptr()
                }, flags);
                // ensure filename lives until dlopen completes
                drop(filename);
                r
            };
            if result.is_null() {
                None
            } else {
                Some(Library {
                    handle: result
                })
            }
        }).map_err(|e| e.unwrap_or(crate::Error::DlOpenUnknown))
    }

    unsafe fn get_impl<T, F>(&self, symbol: &[u8], on_null: F) -> Result<Symbol<T>, crate::Error>
    where F: FnOnce() -> Result<Symbol<T>, crate::Error>
    {
        ensure_compatible_types::<T, *mut raw::c_void>()?;
        let symbol = cstr_cow_from_bytes(symbol)?;
        // `dlsym` may return nullptr in two cases: when a symbol genuinely points to a null
        // pointer or the symbol cannot be found. In order to detect this case a double dlerror
        // pattern must be used, which is, sadly, a little bit racy.
        //
        // We try to leave as little space as possible for this to occur, but we can’t exactly
        // fully prevent it.
        match with_dlerror(|desc| crate::Error::DlSym { desc }, || {
            dlerror();
            let symbol = dlsym(self.handle, symbol.as_ptr());
            if symbol.is_null() {
                None
            } else {
                Some(Symbol {
                    pointer: symbol,
                    pd: marker::PhantomData
                })
            }
        }) {
            Err(None) => on_null(),
            Err(Some(e)) => Err(e),
            Ok(x) => Ok(x)
        }

    }

    /// Get a pointer to function or static variable by symbol name.
    ///
    /// The `symbol` may not contain any null bytes, with an exception of last byte. A null
    /// terminated `symbol` may avoid a string allocation in some cases.
    ///
    /// Symbol is interpreted as-is; no mangling is done. This means that symbols like `x::y` are
    /// most likely invalid.
    ///
    /// ## Unsafety
    ///
    /// This function does not validate the type `T`. It is up to the user of this function to
    /// ensure that the loaded symbol is in fact a `T`. Using a value with a wrong type has no
    /// definied behaviour.
    ///
    ///
    ///
    /// ## Platform-specific behaviour
    ///
    /// OS X uses some sort of lazy initialization scheme, which makes loading TLS variables
    /// impossible. Using a TLS variable loaded this way on OS X is undefined behaviour.
    ///
    /// On POSIX implementations where the `dlerror` function is not confirmed to be MT-safe (such
    /// as FreeBSD), this function will unconditionally return an error the underlying `dlsym` call
    /// returns a null pointer. There are rare situations where `dlsym` returns a genuine null
    /// pointer without it being an error. If loading a null pointer is something you care about,
    /// consider using the [`Library::get_singlethreaded`] call.
    #[inline(always)]
    pub unsafe fn get<T>(&self, symbol: &[u8]) -> Result<Symbol<T>, crate::Error> {
        #[cfg(mtsafe_dlerror)]
        { return self.get_singlethreaded(symbol); }
        #[cfg(not(mtsafe_dlerror))]
        {
            return self.get_impl(symbol, || Err(crate::Error::DlSymUnknown));
        }
    }

    /// Get a pointer to function or static variable by symbol name.
    ///
    /// The `symbol` may not contain any null bytes, with an exception of last byte. A null
    /// terminated `symbol` may avoid a string allocation in some cases.
    ///
    /// Symbol is interpreted as-is; no mangling is done. This means that symbols like `x::y` are
    /// most likely invalid.
    ///
    /// ## Unsafety
    ///
    /// This function does not validate the type `T`. It is up to the user of this function to
    /// ensure that the loaded symbol is in fact a `T`. Using a value with a wrong type has no
    /// definied behaviour.
    ///
    /// It is up to the user of this library to ensure that no other calls to an MT-unsafe
    /// implementation of `dlerror` occur while this function is executing. Failing that the
    /// results of this function are not defined.
    ///
    /// ## Platform-specific behaviour
    ///
    /// OS X uses some sort of lazy initialization scheme, which makes loading TLS variables
    /// impossible. Using a TLS variable loaded this way on OS X is undefined behaviour.
    #[inline(always)]
    pub unsafe fn get_singlethreaded<T>(&self, symbol: &[u8]) -> Result<Symbol<T>, crate::Error> {
        self.get_impl(symbol, || Ok(Symbol {
            pointer: ptr::null_mut(),
            pd: marker::PhantomData
        }))
    }

    /// Convert the `Library` to a raw handle.
    ///
    /// The handle returned by this function shall be usable with APIs which accept handles
    /// as returned by `dlopen`.
    pub fn into_raw(self) -> *mut raw::c_void {
        let handle = self.handle;
        mem::forget(self);
        handle
    }

    /// Convert a raw handle returned by `dlopen`-family of calls to a `Library`.
    ///
    /// ## Unsafety
    ///
    /// The pointer shall be a result of a successful call of the `dlopen`-family of functions or a
    /// pointer previously returned by `Library::into_raw` call. It must be valid to call `dlclose`
    /// with this pointer as an argument.
    pub unsafe fn from_raw(handle: *mut raw::c_void) -> Library {
        Library {
            handle: handle
        }
    }

    /// Unload the library.
    ///
    /// This method might be a no-op, depending on the flags with which the `Library` was opened,
    /// what library was opened or other platform specifics.
    ///
    /// You only need to call this if you are interested in handling any errors that may arise when
    /// library is unloaded. Otherwise the implementation of `Drop` for `Library` will close the
    /// library and ignore the errors were they arise.
    pub fn close(self) -> Result<(), crate::Error> {
        let result = with_dlerror(|desc| crate::Error::DlClose { desc }, || {
            if unsafe { dlclose(self.handle) } == 0 {
                Some(())
            } else {
                None
            }
        }).map_err(|e| e.unwrap_or(crate::Error::DlCloseUnknown));
        std::mem::forget(self);
        result
    }
}

impl Drop for Library {
    fn drop(&mut self) {
        unsafe {
            dlclose(self.handle);
        }
    }
}

impl fmt::Debug for Library {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.write_str(&format!("Library@{:p}", self.handle))
    }
}

/// Symbol from a library.
///
/// A major difference compared to the cross-platform `Symbol` is that this does not ensure the
/// `Symbol` does not outlive `Library` it comes from.
pub struct Symbol<T> {
    pointer: *mut raw::c_void,
    pd: marker::PhantomData<T>
}

impl<T> Symbol<T> {
    /// Convert the loaded Symbol into a raw pointer.
    pub fn into_raw(self) -> *mut raw::c_void {
        let pointer = self.pointer;
        mem::forget(self);
        pointer
    }
}

impl<T> Symbol<Option<T>> {
    /// Lift Option out of the symbol.
    pub fn lift_option(self) -> Option<Symbol<T>> {
        if self.pointer.is_null() {
            None
        } else {
            Some(Symbol {
                pointer: self.pointer,
                pd: marker::PhantomData,
            })
        }
    }
}

unsafe impl<T: Send> Send for Symbol<T> {}
unsafe impl<T: Sync> Sync for Symbol<T> {}

impl<T> Clone for Symbol<T> {
    fn clone(&self) -> Symbol<T> {
        Symbol { ..*self }
    }
}

impl<T> ::std::ops::Deref for Symbol<T> {
    type Target = T;
    fn deref(&self) -> &T {
        unsafe {
            // Additional reference level for a dereference on `deref` return value.
            mem::transmute(&self.pointer)
        }
    }
}

impl<T> fmt::Debug for Symbol<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        unsafe {
            let mut info = mem::MaybeUninit::<DlInfo>::uninit();
            if dladdr(self.pointer, info.as_mut_ptr()) != 0 {
                let info = info.assume_init();
                if info.dli_sname.is_null() {
                    f.write_str(&format!("Symbol@{:p} from {:?}",
                                         self.pointer,
                                         CStr::from_ptr(info.dli_fname)))
                } else {
                    f.write_str(&format!("Symbol {:?}@{:p} from {:?}",
                                         CStr::from_ptr(info.dli_sname), self.pointer,
                                         CStr::from_ptr(info.dli_fname)))
                }
            } else {
                f.write_str(&format!("Symbol@{:p}", self.pointer))
            }
        }
    }
}

// Platform specific things

extern {
    fn dlopen(filename: *const raw::c_char, flags: raw::c_int) -> *mut raw::c_void;
    fn dlclose(handle: *mut raw::c_void) -> raw::c_int;
    fn dlsym(handle: *mut raw::c_void, symbol: *const raw::c_char) -> *mut raw::c_void;
    fn dlerror() -> *mut raw::c_char;
    fn dladdr(addr: *mut raw::c_void, info: *mut DlInfo) -> raw::c_int;
}

#[cfg(not(target_os="android"))]
const RTLD_NOW: raw::c_int = 2;
#[cfg(target_os="android")]
const RTLD_NOW: raw::c_int = 0;

#[repr(C)]
struct DlInfo {
  dli_fname: *const raw::c_char,
  dli_fbase: *mut raw::c_void,
  dli_sname: *const raw::c_char,
  dli_saddr: *mut raw::c_void
}

#[cfg(test)]
mod tests {
    #[test]
    fn this() {
        super::Library::this();
    }
}