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Reimplement how unwind information is stored (#3180)

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* Reimplement how unwind information is stored

This commit is a major refactoring of how unwind information is stored
after compilation of a function has finished. Previously we would store
the raw `UnwindInfo` as a result of compilation and this would get
serialized/deserialized alongside the rest of the ELF object that
compilation creates. Whenever functions were registered with
`CodeMemory` this would also result in registering unwinding information
dynamically at runtime, which in the case of Unix, for example, would
dynamically created FDE/CIE entries on-the-fly.

Eventually I'd like to support compiling Wasmtime without Cranelift, but
this means that `UnwindInfo` wouldn't be easily available to decode into
and create unwinding information from. To solve this I've changed the
ELF object created to have the unwinding information encoded into it
ahead-of-time so loading code into memory no longer needs to create
unwinding tables. This change has two different implementations for
Windows/Unix:

* On Windows the implementation was much easier. The unwinding
  information on Windows is already stored after the function itself in
  the text section. This was actually slightly duplicated in object
  building and in code memory allocation. Now the object building
  continues to do the same, recording unwinding information after
  functions, and code memory no longer manually tracks this.
  Additionally Wasmtime will emit a special custom section in the object
  file with unwinding information which is the list of
  `RUNTIME_FUNCTION` structures that `RtlAddFunctionTable` expects. This
  means that the object file has all the information precompiled into it
  and registration at runtime is simply passing a few pointers around to
  the runtime.

* Unix was a little bit more difficult than Windows. Today a `.eh_frame`
  section is created on-the-fly with offsets in FDEs specified as the
  absolute address that functions are loaded at. This absolute
  address hindered the ability to precompile the FDE into the object
  file itself. I've switched how addresses are encoded, though, to using
  `DW_EH_PE_pcrel` which means that FDE addresses are now specified
  relative to the FDE itself. This means that we can maintain a fixed
  offset between the `.eh_frame` loaded in memory and the beginning of
  code memory. When doing so this enables precompiling the `.eh_frame`
  section into the object file and at runtime when loading an object no
  further construction of unwinding information is needed.

The overall result of this commit is that unwinding information is no
longer stored in its cranelift-data-structure form on disk. This means
that this unwinding information format is only present during
compilation, which will make it that much easier to compile out
cranelift in the future.

This commit also significantly refactors `CodeMemory` since the way
unwinding information is handled is not much different from before.
Previously `CodeMemory` was suitable for incrementally adding more and
more functions to it, but nowadays a `CodeMemory` either lives per
module (in which case all functions are known up front) or it's created
once-per-`Func::new` with two trampolines. In both cases we know all
functions up front so the functionality of incrementally adding more and
more segments is no longer needed. This commit removes the ability to
add a function-at-a-time in `CodeMemory` and instead it can now only
load objects in their entirety. A small helper function is added to
build a small object file for trampolines in `Func::new` to handle
allocation there.

Finally, this commit also folds the `wasmtime-obj` crate directly into
the `wasmtime-cranelift` crate and its builder structure to be more
amenable to this strategy of managing unwinding tables.

It is not intentional to have any real functional change as a result of
this commit. This might accelerate loading a module from cache slightly
since less work is needed to manage the unwinding information, but
that's just a side benefit from the main goal of this commit which is to
remove the dependence on cranelift unwinding information being available
at runtime.

* Remove isa reexport from wasmtime-environ

* Trim down reexports of `cranelift-codegen`

Remove everything non-essential so that only the bits which will need to
be refactored out of cranelift remain.

* Fix debug tests

* Review comments
This commit is contained in:
Alex Crichton
2021-08-17 17:14:18 -05:00
committed by GitHub
parent 9311c38f7e
commit e8aa7bb53b
41 changed files with 992 additions and 1474 deletions
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294
crates/jit/src/code_memory.rs
View File

@@ -1,44 +1,38 @@
//! Memory management for executable code.
use crate::object::{
utils::{try_parse_func_name, try_parse_trampoline_name},
ObjectUnwindInfo,
};
use crate::unwind::UnwindRegistry;
use crate::Compiler;
use crate::unwind::UnwindRegistration;
use anyhow::{Context, Result};
use object::read::{File as ObjectFile, Object, ObjectSection, ObjectSymbol};
use region;
use std::collections::BTreeMap;
use std::mem::ManuallyDrop;
use std::{cmp, mem};
use wasmtime_environ::{
isa::unwind::UnwindInfo,
wasm::{FuncIndex, SignatureIndex},
CompiledFunction,
};
use wasmtime_environ::obj::{try_parse_func_name, try_parse_trampoline_name};
use wasmtime_environ::wasm::{FuncIndex, SignatureIndex};
use wasmtime_runtime::{Mmap, VMFunctionBody};
struct CodeMemoryEntry {
mmap: ManuallyDrop<Mmap>,
registry: ManuallyDrop<UnwindRegistry>,
len: usize,
unwind_registration: ManuallyDrop<Option<UnwindRegistration>>,
text_len: usize,
unwind_info_len: usize,
}
impl CodeMemoryEntry {
fn with_capacity(cap: usize) -> Result<Self> {
let mmap = ManuallyDrop::new(Mmap::with_at_least(cap)?);
let registry = ManuallyDrop::new(UnwindRegistry::new(mmap.as_ptr() as usize));
fn new(text_len: usize, unwind_info_len: usize) -> Result<Self> {
let mmap = ManuallyDrop::new(Mmap::with_at_least(text_len + unwind_info_len)?);
Ok(Self {
mmap,
registry,
len: 0,
unwind_registration: ManuallyDrop::new(None),
text_len,
unwind_info_len,
})
}
// Note that this intentionally excludes any unwinding information, if
// present, since consumers largely are only interested in code memory
// itself.
fn range(&self) -> (usize, usize) {
let start = self.mmap.as_ptr() as usize;
let end = start + self.len;
let end = start + self.text_len;
(start, end)
}
}
@@ -47,23 +41,20 @@ impl Drop for CodeMemoryEntry {
fn drop(&mut self) {
unsafe {
// The registry needs to be dropped before the mmap
ManuallyDrop::drop(&mut self.registry);
ManuallyDrop::drop(&mut self.unwind_registration);
ManuallyDrop::drop(&mut self.mmap);
}
}
}
pub(crate) struct CodeMemoryObjectAllocation<'a> {
buf: &'a mut [u8],
pub struct CodeMemoryObjectAllocation<'a, 'b> {
pub code_range: &'a mut [u8],
funcs: BTreeMap<FuncIndex, (usize, usize)>,
trampolines: BTreeMap<SignatureIndex, (usize, usize)>,
pub obj: ObjectFile<'b>,
}
impl<'a> CodeMemoryObjectAllocation<'a> {
pub fn code_range(self) -> &'a mut [u8] {
self.buf
}
impl<'a> CodeMemoryObjectAllocation<'a, '_> {
pub fn funcs_len(&self) -> usize {
self.funcs.len()
}
@@ -73,7 +64,7 @@ impl<'a> CodeMemoryObjectAllocation<'a> {
}
pub fn funcs(&'a self) -> impl Iterator<Item = (FuncIndex, &'a mut [VMFunctionBody])> + 'a {
let buf = self.buf as *const _ as *mut [u8];
let buf = self.code_range as *const _ as *mut [u8];
self.funcs.iter().map(move |(i, (start, len))| {
(*i, unsafe {
CodeMemory::view_as_mut_vmfunc_slice(&mut (*buf)[*start..*start + *len])
@@ -84,7 +75,7 @@ impl<'a> CodeMemoryObjectAllocation<'a> {
pub fn trampolines(
&'a self,
) -> impl Iterator<Item = (SignatureIndex, &'a mut [VMFunctionBody])> + 'a {
let buf = self.buf as *const _ as *mut [u8];
let buf = self.code_range as *const _ as *mut [u8];
self.trampolines.iter().map(move |(i, (start, len))| {
(*i, unsafe {
CodeMemory::view_as_mut_vmfunc_slice(&mut (*buf)[*start..*start + *len])
@@ -95,7 +86,6 @@ impl<'a> CodeMemoryObjectAllocation<'a> {
/// Memory manager for executable code.
pub struct CodeMemory {
current: Option<CodeMemoryEntry>,
entries: Vec<CodeMemoryEntry>,
published: usize,
}
@@ -109,140 +99,49 @@ impl CodeMemory {
/// Create a new `CodeMemory` instance.
pub fn new() -> Self {
Self {
current: None,
entries: Vec::new(),
published: 0,
}
}
/// Allocate a continuous memory block for a single compiled function.
/// TODO: Reorganize the code that calls this to emit code directly into the
/// mmap region rather than into a Vec that we need to copy in.
pub fn allocate_for_function<'a>(
&mut self,
func: &'a CompiledFunction,
) -> Result<&mut [VMFunctionBody]> {
let size = Self::function_allocation_size(func);
let (buf, registry, start) = self.allocate(size)?;
let (_, _, vmfunc) = Self::copy_function(func, start as u32, buf, registry);
Ok(vmfunc)
}
/// Make all allocated memory executable.
pub fn publish(&mut self, compiler: &Compiler) {
self.push_current(0)
.expect("failed to push current memory map");
pub fn publish(&mut self) {
for entry in &mut self.entries[self.published..] {
assert!(!entry.mmap.is_empty());
for CodeMemoryEntry {
mmap: m,
registry: r,
..
} in &mut self.entries[self.published..]
{
// Remove write access to the pages due to the relocation fixups.
r.publish(compiler)
.expect("failed to publish function unwind registry");
if !m.is_empty() {
unsafe {
region::protect(m.as_mut_ptr(), m.len(), region::Protection::READ_EXECUTE)
}
unsafe {
// Switch the executable portion from read/write to
// read/execute, notably not using read/write/execute to prevent
// modifications.
region::protect(
entry.mmap.as_mut_ptr(),
entry.text_len,
region::Protection::READ_EXECUTE,
)
.expect("unable to make memory readonly and executable");
if entry.unwind_info_len == 0 {
continue;
}
// With all our memory setup use the platform-specific
// `UnwindRegistration` implementation to inform the general
// runtime that there's unwinding information available for all
// our just-published JIT functions.
*entry.unwind_registration = Some(
UnwindRegistration::new(
entry.mmap.as_mut_ptr(),
entry.mmap.as_mut_ptr().add(entry.text_len),
entry.unwind_info_len,
)
.expect("failed to create unwind info registration"),
);
}
}
self.published = self.entries.len();
}
/// Allocate `size` bytes of memory which can be made executable later by
/// calling `publish()`. Note that we allocate the memory as writeable so
/// that it can be written to and patched, though we make it readonly before
/// actually executing from it.
///
/// A few values are returned:
///
/// * A mutable slice which references the allocated memory
/// * A function table instance where unwind information is registered
/// * The offset within the current mmap that the slice starts at
///
/// TODO: Add an alignment flag.
fn allocate(&mut self, size: usize) -> Result<(&mut [u8], &mut UnwindRegistry, usize)> {
assert!(size > 0);
if match &self.current {
Some(e) => e.mmap.len() - e.len < size,
None => true,
} {
self.push_current(cmp::max(0x10000, size))?;
}
let e = self.current.as_mut().unwrap();
let old_position = e.len;
e.len += size;
Ok((
&mut e.mmap.as_mut_slice()[old_position..e.len],
&mut e.registry,
old_position,
))
}
/// Calculates the allocation size of the given compiled function.
fn function_allocation_size(func: &CompiledFunction) -> usize {
match &func.unwind_info {
Some(UnwindInfo::WindowsX64(info)) => {
// Windows unwind information is required to be emitted into code memory
// This is because it must be a positive relative offset from the start of the memory
// Account for necessary unwind information alignment padding (32-bit alignment)
((func.body.len() + 3) & !3) + info.emit_size()
}
_ => func.body.len(),
}
}
/// Copies the data of the compiled function to the given buffer.
///
/// This will also add the function to the current unwind registry.
fn copy_function<'a>(
func: &CompiledFunction,
func_start: u32,
buf: &'a mut [u8],
registry: &mut UnwindRegistry,
) -> (u32, &'a mut [u8], &'a mut [VMFunctionBody]) {
let func_len = func.body.len();
let mut func_end = func_start + (func_len as u32);
let (body, mut remainder) = buf.split_at_mut(func_len);
body.copy_from_slice(&func.body);
let vmfunc = Self::view_as_mut_vmfunc_slice(body);
if let Some(UnwindInfo::WindowsX64(info)) = &func.unwind_info {
// Windows unwind information is written following the function body
// Keep unwind information 32-bit aligned (round up to the nearest 4 byte boundary)
let unwind_start = (func_end + 3) & !3;
let unwind_size = info.emit_size();
let padding = (unwind_start - func_end) as usize;
let (slice, r) = remainder.split_at_mut(padding + unwind_size);
info.emit(&mut slice[padding..]);
func_end = unwind_start + (unwind_size as u32);
remainder = r;
}
if let Some(info) = &func.unwind_info {
registry
.register(func_start, func_len as u32, info)
.expect("failed to register unwind information");
}
(func_end, remainder, vmfunc)
}
/// Convert mut a slice from u8 to VMFunctionBody.
fn view_as_mut_vmfunc_slice(slice: &mut [u8]) -> &mut [VMFunctionBody] {
let byte_ptr: *mut [u8] = slice;
@@ -250,24 +149,6 @@ impl CodeMemory {
unsafe { &mut *body_ptr }
}
/// Pushes the current entry and allocates a new one with the given size.
fn push_current(&mut self, new_size: usize) -> Result<()> {
let previous = mem::replace(
&mut self.current,
if new_size == 0 {
None
} else {
Some(CodeMemoryEntry::with_capacity(cmp::max(0x10000, new_size))?)
},
);
if let Some(e) = previous {
self.entries.push(e);
}
Ok(())
}
/// Returns all published segment ranges.
pub fn published_ranges<'a>(&'a self) -> impl Iterator<Item = (usize, usize)> + 'a {
self.entries[..self.published]
@@ -277,29 +158,52 @@ impl CodeMemory {
/// Allocates and copies the ELF image code section into CodeMemory.
/// Returns references to functions and trampolines defined there.
pub(crate) fn allocate_for_object<'a>(
pub fn allocate_for_object<'a, 'b>(
&'a mut self,
obj: &ObjectFile,
unwind_info: &[ObjectUnwindInfo],
) -> Result<CodeMemoryObjectAllocation<'a>> {
obj: &'b [u8],
) -> Result<CodeMemoryObjectAllocation<'a, 'b>> {
let obj = ObjectFile::parse(obj)
.with_context(|| "failed to parse internal ELF compilation artifact")?;
let text_section = obj.section_by_name(".text").unwrap();
let text_section_size = text_section.size() as usize;
if text_section.size() == 0 {
if text_section_size == 0 {
// No code in the image.
return Ok(CodeMemoryObjectAllocation {
buf: &mut [],
code_range: &mut [],
funcs: BTreeMap::new(),
trampolines: BTreeMap::new(),
obj,
});
}
// Allocate chunk memory that spans entire code section.
let (buf, registry, start) = self.allocate(text_section.size() as usize)?;
buf.copy_from_slice(
// Find the platform-specific unwind section, if present, which contains
// unwinding tables that will be used to load unwinding information
// dynamically at runtime.
let unwind_section = obj.section_by_name(UnwindRegistration::section_name());
let unwind_section_size = unwind_section
.as_ref()
.map(|s| s.size() as usize)
.unwrap_or(0);
// Allocate memory for the text section and unwinding information if it
// is present. Then we can copy in all of the code and unwinding memory
// over.
let entry = CodeMemoryEntry::new(text_section_size, unwind_section_size)?;
self.entries.push(entry);
let entry = self.entries.last_mut().unwrap();
entry.mmap.as_mut_slice()[..text_section_size].copy_from_slice(
text_section
.data()
.with_context(|| "cannot read section data")?,
.with_context(|| "cannot read text section data")?,
);
if let Some(section) = unwind_section {
entry.mmap.as_mut_slice()[text_section_size..][..unwind_section_size].copy_from_slice(
section
.data()
.with_context(|| "cannot read unwind section data")?,
);
}
// Track locations of all defined functions and trampolines.
let mut funcs = BTreeMap::new();
@@ -310,43 +214,21 @@ impl CodeMemory {
if let Some(index) = try_parse_func_name(name) {
let is_import = sym.section_index().is_none();
if !is_import {
funcs.insert(
index,
(start + sym.address() as usize, sym.size() as usize),
);
funcs.insert(index, (sym.address() as usize, sym.size() as usize));
}
} else if let Some(index) = try_parse_trampoline_name(name) {
trampolines
.insert(index, (start + sym.address() as usize, sym.size() as usize));
trampolines.insert(index, (sym.address() as usize, sym.size() as usize));
}
}
Err(_) => (),
}
}
// Register all unwind entries for functions and trampolines.
// TODO will `u32` type for start/len be enough for large code base.
for i in unwind_info {
match i {
ObjectUnwindInfo::Func(func_index, info) => {
let (start, len) = funcs.get(&func_index).unwrap();
registry
.register(*start as u32, *len as u32, &info)
.expect("failed to register unwind information");
}
ObjectUnwindInfo::Trampoline(trampoline_index, info) => {
let (start, len) = trampolines.get(&trampoline_index).unwrap();
registry
.register(*start as u32, *len as u32, &info)
.expect("failed to register unwind information");
}
}
}
Ok(CodeMemoryObjectAllocation {
buf: &mut buf[..text_section.size() as usize],
code_range: &mut entry.mmap.as_mut_slice()[..text_section_size],
funcs,
trampolines,
obj,
})
}
}