From ec4b957deb82172a1a2eca371e67f945a9cf64cc Mon Sep 17 00:00:00 2001 From: Dan Gohman Date: Thu, 9 Feb 2023 16:41:59 -0800 Subject: [PATCH] Fix `use-libc-auxv` to use a weak dependency on `getauxval`. (#535) * Fix `use-libc-auxv` to use a weak dependency on `getauxval`. glibc <= 2.15 lacks `getauxval`, so avoid a strong dependency on it in the `use-libc-auxv` implementation. --- src/backend/linux_raw/mod.rs | 5 + src/backend/linux_raw/param/libc_auxv.rs | 37 ++-- src/backend/linux_raw/weak.rs | 228 +++++++++++++++++++++++ 3 files changed, 256 insertions(+), 14 deletions(-) create mode 100644 src/backend/linux_raw/weak.rs diff --git a/src/backend/linux_raw/mod.rs b/src/backend/linux_raw/mod.rs index e7e073e32..1b91fc3ab 100644 --- a/src/backend/linux_raw/mod.rs +++ b/src/backend/linux_raw/mod.rs @@ -14,6 +14,11 @@ //! such as which pointers are array slices, out parameters, or in-out //! parameters, which integers are owned or borrowed file descriptors, etc. +// Weak symbols used by the use-libc-auxv feature for glibc 2.15 support. +#[cfg(feature = "use-libc-auxv")] +#[macro_use] +mod weak; + #[macro_use] mod arch; mod conv; diff --git a/src/backend/linux_raw/param/libc_auxv.rs b/src/backend/linux_raw/param/libc_auxv.rs index 1597fd727..467a2800c 100644 --- a/src/backend/linux_raw/param/libc_auxv.rs +++ b/src/backend/linux_raw/param/libc_auxv.rs @@ -5,14 +5,15 @@ //! This uses raw pointers to locate and read the kernel-provided auxv array. #![allow(unsafe_code)] -#[cfg(any(feature = "param", feature = "runtime"))] -use super::super::c; use super::super::elf::*; #[cfg(feature = "param")] use crate::ffi::CStr; #[cfg(feature = "runtime")] use core::slice; +// `getauxval` wasn't supported in glibc until 2.16. +weak!(fn getauxval(libc::c_ulong) -> *mut libc::c_void); + #[cfg(feature = "param")] #[inline] pub(crate) fn page_size() -> usize { @@ -22,35 +23,39 @@ pub(crate) fn page_size() -> usize { #[cfg(feature = "param")] #[inline] pub(crate) fn clock_ticks_per_second() -> u64 { - unsafe { libc::getauxval(libc::AT_CLKTCK) as u64 } + unsafe { libc::sysconf(libc::_SC_CLK_TCK) as u64 } } #[cfg(feature = "param")] #[inline] pub(crate) fn linux_hwcap() -> (usize, usize) { - unsafe { - ( - libc::getauxval(libc::AT_HWCAP) as usize, - libc::getauxval(libc::AT_HWCAP2) as usize, - ) + if let Some(libc_getauxval) = getauxval.get() { + unsafe { + let hwcap = libc_getauxval(libc::AT_HWCAP) as usize; + let hwcap2 = libc_getauxval(libc::AT_HWCAP2) as usize; + (hwcap, hwcap2) + } + } else { + (0, 0) } } #[cfg(feature = "param")] #[inline] pub(crate) fn linux_execfn() -> &'static CStr { - unsafe { - let execfn = libc::getauxval(libc::AT_EXECFN) as *const c::c_char; - CStr::from_ptr(execfn.cast()) + if let Some(libc_getauxval) = getauxval.get() { + unsafe { CStr::from_ptr(libc_getauxval(libc::AT_EXECFN).cast()) } + } else { + cstr!("") } } #[cfg(feature = "runtime")] #[inline] -pub(crate) fn exe_phdrs() -> (*const c::c_void, usize) { +pub(crate) fn exe_phdrs() -> (*const libc::c_void, usize) { unsafe { ( - libc::getauxval(libc::AT_PHDR) as *const c::c_void, + libc::getauxval(libc::AT_PHDR) as *const libc::c_void, libc::getauxval(libc::AT_PHNUM) as usize, ) } @@ -70,5 +75,9 @@ pub(in super::super) fn exe_phdrs_slice() -> &'static [Elf_Phdr] { /// so if we don't see it, this function returns a null pointer. #[inline] pub(in super::super) fn sysinfo_ehdr() -> *const Elf_Ehdr { - unsafe { libc::getauxval(linux_raw_sys::general::AT_SYSINFO_EHDR.into()) as *const Elf_Ehdr } + if let Some(libc_getauxval) = getauxval.get() { + unsafe { libc_getauxval(linux_raw_sys::general::AT_SYSINFO_EHDR.into()) as *const Elf_Ehdr } + } else { + core::ptr::null() + } } diff --git a/src/backend/linux_raw/weak.rs b/src/backend/linux_raw/weak.rs new file mode 100644 index 000000000..265d8b701 --- /dev/null +++ b/src/backend/linux_raw/weak.rs @@ -0,0 +1,228 @@ +// Implementation derived from `weak` in Rust's +// library/std/src/sys/unix/weak.rs at revision +// fd0cb0cdc21dd9c06025277d772108f8d42cb25f. + +#![allow(unsafe_code)] + +//! Support for "weak linkage" to symbols on Unix +//! +//! Some I/O operations we do in libstd require newer versions of OSes but we +//! need to maintain binary compatibility with older releases for now. In order +//! to use the new functionality when available we use this module for +//! detection. +//! +//! One option to use here is weak linkage, but that is unfortunately only +//! really workable on Linux. Hence, use dlsym to get the symbol value at +//! runtime. This is also done for compatibility with older versions of glibc, +//! and to avoid creating dependencies on `GLIBC_PRIVATE` symbols. It assumes +//! that we've been dynamically linked to the library the symbol comes from, +//! but that is currently always the case for things like libpthread/libc. +//! +//! A long time ago this used weak linkage for the `__pthread_get_minstack` +//! symbol, but that caused Debian to detect an unnecessarily strict versioned +//! dependency on libc6 (#23628). + +// There are a variety of `#[cfg]`s controlling which targets are involved in +// each instance of `weak!` and `syscall!`. Rather than trying to unify all of +// that, we'll just allow that some unix targets don't use this module at all. +#![allow(dead_code, unused_macros)] +#![allow(clippy::doc_markdown)] + +use crate::ffi::CStr; +use core::ffi::c_void; +use core::ptr::null_mut; +use core::sync::atomic::{self, AtomicPtr, Ordering}; +use core::{marker, mem}; + +const NULL: *mut c_void = null_mut(); +const INVALID: *mut c_void = 1 as *mut c_void; + +macro_rules! weak { + ($vis:vis fn $name:ident($($t:ty),*) -> $ret:ty) => ( + #[allow(non_upper_case_globals)] + $vis static $name: $crate::backend::weak::Weak $ret> = + $crate::backend::weak::Weak::new(concat!(stringify!($name), '\0')); + ) +} + +pub(crate) struct Weak { + name: &'static str, + addr: AtomicPtr, + _marker: marker::PhantomData, +} + +impl Weak { + pub(crate) const fn new(name: &'static str) -> Self { + Self { + name, + addr: AtomicPtr::new(INVALID), + _marker: marker::PhantomData, + } + } + + pub(crate) fn get(&self) -> Option { + assert_eq!(mem::size_of::(), mem::size_of::()); + unsafe { + // Relaxed is fine here because we fence before reading through the + // pointer (see the comment below). + match self.addr.load(Ordering::Relaxed) { + INVALID => self.initialize(), + NULL => None, + addr => { + let func = mem::transmute_copy::<*mut c_void, F>(&addr); + // The caller is presumably going to read through this value + // (by calling the function we've dlsymed). This means we'd + // need to have loaded it with at least C11's consume + // ordering in order to be guaranteed that the data we read + // from the pointer isn't from before the pointer was + // stored. Rust has no equivalent to memory_order_consume, + // so we use an acquire fence (sorry, ARM). + // + // Now, in practice this likely isn't needed even on CPUs + // where relaxed and consume mean different things. The + // symbols we're loading are probably present (or not) at + // init, and even if they aren't the runtime dynamic loader + // is extremely likely have sufficient barriers internally + // (possibly implicitly, for example the ones provided by + // invoking `mprotect`). + // + // That said, none of that's *guaranteed*, and so we fence. + atomic::fence(Ordering::Acquire); + Some(func) + } + } + } + } + + // Cold because it should only happen during first-time initialization. + #[cold] + unsafe fn initialize(&self) -> Option { + let val = fetch(self.name); + // This synchronizes with the acquire fence in `get`. + self.addr.store(val, Ordering::Release); + + match val { + NULL => None, + addr => Some(mem::transmute_copy::<*mut c_void, F>(&addr)), + } + } +} + +unsafe fn fetch(name: &str) -> *mut c_void { + let name = match CStr::from_bytes_with_nul(name.as_bytes()) { + Ok(c_str) => c_str, + Err(..) => return null_mut(), + }; + libc::dlsym(libc::RTLD_DEFAULT, name.as_ptr().cast()) +} + +#[cfg(not(any(target_os = "android", target_os = "linux")))] +macro_rules! syscall { + (fn $name:ident($($arg_name:ident: $t:ty),*) via $_sys_name:ident -> $ret:ty) => ( + unsafe fn $name($($arg_name: $t),*) -> $ret { + weak! { fn $name($($t),*) -> $ret } + + if let Some(fun) = $name.get() { + fun($($arg_name),*) + } else { + libc_errno::set_errno(libc_errno::Errno(libc::ENOSYS)); + -1 + } + } + ) +} + +#[cfg(any(target_os = "android", target_os = "linux"))] +macro_rules! syscall { + (fn $name:ident($($arg_name:ident: $t:ty),*) via $sys_name:ident -> $ret:ty) => ( + unsafe fn $name($($arg_name:$t),*) -> $ret { + // This looks like a hack, but concat_idents only accepts idents + // (not paths). + use libc::*; + + trait AsSyscallArg { + type SyscallArgType; + fn into_syscall_arg(self) -> Self::SyscallArgType; + } + + // Pass pointer types as pointers, to preserve provenance. + impl AsSyscallArg for *mut T { + type SyscallArgType = *mut T; + fn into_syscall_arg(self) -> Self::SyscallArgType { self } + } + impl AsSyscallArg for *const T { + type SyscallArgType = *const T; + fn into_syscall_arg(self) -> Self::SyscallArgType { self } + } + + // Pass `BorrowedFd` values as the integer value. + impl AsSyscallArg for $crate::fd::BorrowedFd<'_> { + type SyscallArgType = c::c_long; + fn into_syscall_arg(self) -> Self::SyscallArgType { + $crate::fd::AsRawFd::as_raw_fd(&self) as _ + } + } + + // Coerce integer values into `c_long`. + impl AsSyscallArg for i32 { + type SyscallArgType = c::c_long; + fn into_syscall_arg(self) -> Self::SyscallArgType { self as _ } + } + impl AsSyscallArg for u32 { + type SyscallArgType = c::c_long; + fn into_syscall_arg(self) -> Self::SyscallArgType { self as _ } + } + impl AsSyscallArg for usize { + type SyscallArgType = c::c_long; + fn into_syscall_arg(self) -> Self::SyscallArgType { self as _ } + } + + // `concat_idents is unstable, so we take an extra `sys_name` + // parameter and have our users do the concat for us for now. + /* + syscall( + concat_idents!(SYS_, $name), + $($arg_name.into_syscall_arg()),* + ) as $ret + */ + + syscall($sys_name, $($arg_name.into_syscall_arg()),*) as $ret + } + ) +} + +macro_rules! weakcall { + ($vis:vis fn $name:ident($($arg_name:ident: $t:ty),*) -> $ret:ty) => ( + $vis unsafe fn $name($($arg_name: $t),*) -> $ret { + weak! { fn $name($($t),*) -> $ret } + + // Use a weak symbol from libc when possible, allowing `LD_PRELOAD` + // interposition, but if it's not found just fail. + if let Some(fun) = $name.get() { + fun($($arg_name),*) + } else { + libc_errno::set_errno(libc_errno::Errno(libc::ENOSYS)); + -1 + } + } + ) +} + +/// A combination of `weakcall` and `syscall`. Use the libc function if it's +/// available, and fall back to `libc::syscall` otherwise. +macro_rules! weak_or_syscall { + ($vis:vis fn $name:ident($($arg_name:ident: $t:ty),*) via $sys_name:ident -> $ret:ty) => ( + $vis unsafe fn $name($($arg_name: $t),*) -> $ret { + weak! { fn $name($($t),*) -> $ret } + + // Use a weak symbol from libc when possible, allowing `LD_PRELOAD` + // interposition, but if it's not found just fail. + if let Some(fun) = $name.get() { + fun($($arg_name),*) + } else { + syscall! { fn $name($($arg_name: $t),*) via $sys_name -> $ret } + $name($($arg_name),*) + } + } + ) +}