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Rollup merge of rust-lang#87868 - Kixiron:packing-on-the-pounds, r=eddyb
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Added -Z randomize-layout flag

An implementation of rust-lang#77316, it currently randomly shuffles the fields of `repr(rust)` types based on their `DefPathHash`
r? ``@eddyb``
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@Manishearth
Manishearth authored Oct 1, 2021
2 parents ed93759 + 09f1542 commit 37df275
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Showing 6 changed files with 93 additions and 19 deletions.
21 changes: 16 additions & 5 deletions Cargo.lock
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Original file line number Diff line number Diff line change
Expand Up @@ -1660,7 +1660,7 @@ checksum = "3ca8957e71f04a205cb162508f9326aea04676c8dfd0711220190d6b83664f3f"
dependencies = [
"bitmaps",
"rand_core 0.5.1",
"rand_xoshiro",
"rand_xoshiro 0.4.0",
"sized-chunks",
"typenum",
"version_check",
Expand Down Expand Up @@ -2256,7 +2256,7 @@ dependencies = [
"libc",
"log",
"measureme",
"rand 0.8.3",
"rand 0.8.4",
"rustc-workspace-hack",
"rustc_version",
"shell-escape",
Expand Down Expand Up @@ -2852,9 +2852,9 @@ dependencies = [

[[package]]
name = "rand"
version = "0.8.3"
version = "0.8.4"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "0ef9e7e66b4468674bfcb0c81af8b7fa0bb154fa9f28eb840da5c447baeb8d7e"
checksum = "2e7573632e6454cf6b99d7aac4ccca54be06da05aca2ef7423d22d27d4d4bcd8"
dependencies = [
"libc",
"rand_chacha 0.3.0",
Expand Down Expand Up @@ -2945,6 +2945,15 @@ dependencies = [
"rand_core 0.5.1",
]

[[package]]
name = "rand_xoshiro"
version = "0.6.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
checksum = "6f97cdb2a36ed4183de61b2f824cc45c9f1037f28afe0a322e9fff4c108b5aaa"
dependencies = [
"rand_core 0.6.2",
]

[[package]]
name = "rayon"
version = "1.3.1"
Expand Down Expand Up @@ -4087,6 +4096,8 @@ dependencies = [
"either",
"gsgdt",
"polonius-engine",
"rand 0.8.4",
"rand_xoshiro 0.6.0",
"rustc-rayon-core",
"rustc_apfloat",
"rustc_arena",
Expand Down Expand Up @@ -5097,7 +5108,7 @@ checksum = "dac1c663cfc93810f88aed9b8941d48cabf856a1b111c29a40439018d870eb22"
dependencies = [
"cfg-if 1.0.0",
"libc",
"rand 0.8.3",
"rand 0.8.4",
"redox_syscall",
"remove_dir_all",
"winapi",
Expand Down
2 changes: 2 additions & 0 deletions compiler/rustc_middle/Cargo.toml
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Expand Up @@ -32,3 +32,5 @@ chalk-ir = "0.55.0"
smallvec = { version = "1.6.1", features = ["union", "may_dangle"] }
rustc_session = { path = "../rustc_session" }
rustc_type_ir = { path = "../rustc_type_ir" }
rand = "0.8.4"
rand_xoshiro = "0.6.0"
48 changes: 35 additions & 13 deletions compiler/rustc_middle/src/ty/layout.rs
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Expand Up @@ -24,6 +24,9 @@ use std::iter;
use std::num::NonZeroUsize;
use std::ops::Bound;

use rand::{seq::SliceRandom, SeedableRng};
use rand_xoshiro::Xoshiro128StarStar;

pub fn provide(providers: &mut ty::query::Providers) {
*providers =
ty::query::Providers { layout_of, fn_abi_of_fn_ptr, fn_abi_of_instance, ..*providers };
Expand Down Expand Up @@ -324,6 +327,10 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {

let mut inverse_memory_index: Vec<u32> = (0..fields.len() as u32).collect();

// `ReprOptions.layout_seed` is a deterministic seed that we can use to
// randomize field ordering with
let mut rng = Xoshiro128StarStar::seed_from_u64(repr.field_shuffle_seed);

let optimize = !repr.inhibit_struct_field_reordering_opt();
if optimize {
let end =
Expand All @@ -332,20 +339,35 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
let field_align = |f: &TyAndLayout<'_>| {
if let Some(pack) = pack { f.align.abi.min(pack) } else { f.align.abi }
};
match kind {
StructKind::AlwaysSized | StructKind::MaybeUnsized => {
optimizing.sort_by_key(|&x| {
// Place ZSTs first to avoid "interesting offsets",
// especially with only one or two non-ZST fields.
let f = &fields[x as usize];
(!f.is_zst(), cmp::Reverse(field_align(f)))
});
}
StructKind::Prefixed(..) => {
// Sort in ascending alignment so that the layout stay optimal
// regardless of the prefix
optimizing.sort_by_key(|&x| field_align(&fields[x as usize]));

// If `-Z randomize-layout` was enabled for the type definition we can shuffle
// the field ordering to try and catch some code making assumptions about layouts
// we don't guarantee
if repr.can_randomize_type_layout() {
// Shuffle the ordering of the fields
optimizing.shuffle(&mut rng);

// Otherwise we just leave things alone and actually optimize the type's fields
} else {
match kind {
StructKind::AlwaysSized | StructKind::MaybeUnsized => {
optimizing.sort_by_key(|&x| {
// Place ZSTs first to avoid "interesting offsets",
// especially with only one or two non-ZST fields.
let f = &fields[x as usize];
(!f.is_zst(), cmp::Reverse(field_align(f)))
});
}

StructKind::Prefixed(..) => {
// Sort in ascending alignment so that the layout stays optimal
// regardless of the prefix
optimizing.sort_by_key(|&x| field_align(&fields[x as usize]));
}
}

// FIXME(Kixiron): We can always shuffle fields within a given alignment class
// regardless of the status of `-Z randomize-layout`
}
}

Expand Down
38 changes: 37 additions & 1 deletion compiler/rustc_middle/src/ty/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1491,6 +1491,9 @@ bitflags! {
const IS_LINEAR = 1 << 3;
// If true, don't expose any niche to type's context.
const HIDE_NICHE = 1 << 4;
// If true, the type's layout can be randomized using
// the seed stored in `ReprOptions.layout_seed`
const RANDOMIZE_LAYOUT = 1 << 5;
// Any of these flags being set prevent field reordering optimisation.
const IS_UNOPTIMISABLE = ReprFlags::IS_C.bits |
ReprFlags::IS_SIMD.bits |
Expand All @@ -1505,6 +1508,14 @@ pub struct ReprOptions {
pub align: Option<Align>,
pub pack: Option<Align>,
pub flags: ReprFlags,
/// The seed to be used for randomizing a type's layout
///
/// Note: This could technically be a `[u8; 16]` (a `u128`) which would
/// be the "most accurate" hash as it'd encompass the item and crate
/// hash without loss, but it does pay the price of being larger.
/// Everything's a tradeoff, a `u64` seed should be sufficient for our
/// purposes (primarily `-Z randomize-layout`)
pub field_shuffle_seed: u64,
}

impl ReprOptions {
Expand All @@ -1513,6 +1524,11 @@ impl ReprOptions {
let mut size = None;
let mut max_align: Option<Align> = None;
let mut min_pack: Option<Align> = None;

// Generate a deterministically-derived seed from the item's path hash
// to allow for cross-crate compilation to actually work
let field_shuffle_seed = tcx.def_path_hash(did).0.to_smaller_hash();

for attr in tcx.get_attrs(did).iter() {
for r in attr::find_repr_attrs(&tcx.sess, attr) {
flags.insert(match r {
Expand Down Expand Up @@ -1541,33 +1557,45 @@ impl ReprOptions {
}
}

// If `-Z randomize-layout` was enabled for the type definition then we can
// consider performing layout randomization
if tcx.sess.opts.debugging_opts.randomize_layout {
flags.insert(ReprFlags::RANDOMIZE_LAYOUT);
}

// This is here instead of layout because the choice must make it into metadata.
if !tcx.consider_optimizing(|| format!("Reorder fields of {:?}", tcx.def_path_str(did))) {
flags.insert(ReprFlags::IS_LINEAR);
}
ReprOptions { int: size, align: max_align, pack: min_pack, flags }

Self { int: size, align: max_align, pack: min_pack, flags, field_shuffle_seed }
}

#[inline]
pub fn simd(&self) -> bool {
self.flags.contains(ReprFlags::IS_SIMD)
}

#[inline]
pub fn c(&self) -> bool {
self.flags.contains(ReprFlags::IS_C)
}

#[inline]
pub fn packed(&self) -> bool {
self.pack.is_some()
}

#[inline]
pub fn transparent(&self) -> bool {
self.flags.contains(ReprFlags::IS_TRANSPARENT)
}

#[inline]
pub fn linear(&self) -> bool {
self.flags.contains(ReprFlags::IS_LINEAR)
}

#[inline]
pub fn hide_niche(&self) -> bool {
self.flags.contains(ReprFlags::HIDE_NICHE)
Expand All @@ -1594,9 +1622,17 @@ impl ReprOptions {
return true;
}
}

self.flags.intersects(ReprFlags::IS_UNOPTIMISABLE) || self.int.is_some()
}

/// Returns `true` if this type is valid for reordering and `-Z randomize-layout`
/// was enabled for its declaration crate
pub fn can_randomize_type_layout(&self) -> bool {
!self.inhibit_struct_field_reordering_opt()
&& self.flags.contains(ReprFlags::RANDOMIZE_LAYOUT)
}

/// Returns `true` if this `#[repr()]` should inhibit union ABI optimisations.
pub fn inhibit_union_abi_opt(&self) -> bool {
self.c()
Expand Down
2 changes: 2 additions & 0 deletions compiler/rustc_session/src/options.rs
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Expand Up @@ -1246,6 +1246,8 @@ options! {
"enable queries of the dependency graph for regression testing (default: no)"),
query_stats: bool = (false, parse_bool, [UNTRACKED],
"print some statistics about the query system (default: no)"),
randomize_layout: bool = (false, parse_bool, [TRACKED],
"randomize the layout of types (default: no)"),
relax_elf_relocations: Option<bool> = (None, parse_opt_bool, [TRACKED],
"whether ELF relocations can be relaxed"),
relro_level: Option<RelroLevel> = (None, parse_relro_level, [TRACKED],
Expand Down
1 change: 1 addition & 0 deletions src/tools/tidy/src/deps.rs
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Expand Up @@ -167,6 +167,7 @@ const PERMITTED_DEPENDENCIES: &[&str] = &[
"rand_hc",
"rand_pcg",
"rand_xorshift",
"rand_xoshiro",
"redox_syscall",
"regex",
"regex-automata",
Expand Down

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