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annotate rust/hg-core/src/dirstate_tree/owning.rs @ 48999:cfd270d83169 stable

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rust: explain why the current `OwningDirstateMap` is unsound See inline comments. Differential Revision: https://phab.mercurial-scm.org/D12428
author Rapha?l Gom?s <rgomes@octobus.net>
date Tue, 05 Apr 2022 10:55:27 +0200
parents d1210d56008b
children dd6b67d5c256
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1 use super::dirstate_map::DirstateMap;
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2 use stable_deref_trait::StableDeref;
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3 use std::ops::Deref;
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4
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5 /*
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6 // /!\ This is unsound and can cause use after free. It will be fixed in the
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7 // next patch
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8
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9 // If we change `value` from its current use of `HgPathBuf` to `&HgPath`,
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10 // nothing here tells that `value` will outlive `OwningDirstateMap`
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11 pub fn copy_map_insert<'a,'owned>(
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12 &'owned mut self,
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13 key: &HgPath,
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14 value: &'a HgPath,
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15 ) -> Result<Option<HgPathBuf>, DirstateV2ParseError> {
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16 // `'local` is smaller than `'a` here
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17 let map: &'local mut DirstateMap<'local> = self.get_map_mut();
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18 let node: &'local mut Node<'local> = DirstateMap::get_or_insert_node(
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19 map.on_disk,
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20 &mut map.unreachable_bytes,
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21 &mut map.root,
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22 &key,
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23 WithBasename::to_cow_owned,
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24 |_ancestor| {},
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25 )?;
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26 if node.copy_source.is_none() {
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27 map.nodes_with_copy_source_count += 1
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28 }
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29 Ok(node.copy_source.replace(value.into()).map(Cow::into_owned))
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30 // and right here ----------^^^^^^^^^^^^
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31 // we are storing `&'a HgPath` in `Node<'local>` which is possible
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32 // because to the compiler, `'a` is longer than ``local`.
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33 // It is wrong because nothing proves that `&'a HgPath` will outlive `self`.
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34 }
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35
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36 // All of this is caused by the wrong cast of the DirstateMap pointer that
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37 // fakes the lifetime of `DirstateMap` and ensures the compiler that it lives
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38 // as long as `on_disk`, which is only true for its immutable data.
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39 // This will be fixed in the next commit.
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40 */
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41
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42 /// Keep a `DirstateMap<'on_disk>` next to the `on_disk` buffer that it
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43 /// borrows.
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44 ///
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45 /// This is similar to [`OwningRef`] which is more limited because it
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46 /// represents exactly one `&T` reference next to the value it borrows, as
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47 /// opposed to a struct that may contain an arbitrary number of references in
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48 /// arbitrarily-nested data structures.
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49 ///
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50 /// [`OwningRef`]: https://docs.rs/owning_ref/0.4.1/owning_ref/struct.OwningRef.html
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51 pub struct OwningDirstateMap {
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52 /// Owned handle to a bytes buffer with a stable address.
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53 ///
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54 /// See <https://docs.rs/owning_ref/0.4.1/owning_ref/trait.StableAddress.html>.
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55 on_disk: Box<dyn Deref<Target = [u8]> + Send>,
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56
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57 /// Pointer for `Box<DirstateMap<'on_disk>>`, typed-erased because the
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58 /// language cannot represent a lifetime referencing a sibling field.
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59 /// This is not quite a self-referencial struct (moving this struct is not
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60 /// a problem as it doesn’t change the address of the bytes buffer owned
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61 /// by `on_disk`) but touches similar borrow-checker limitations.
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62 ptr: *mut (),
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63 }
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64
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65 impl OwningDirstateMap {
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66 pub fn new_empty<OnDisk>(on_disk: OnDisk) -> Self
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67 where
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68 OnDisk: Deref<Target = [u8]> + StableDeref + Send + 'static,
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69 {
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70 let on_disk = Box::new(on_disk);
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71 let bytes: &'_ [u8] = &on_disk;
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72 let map = DirstateMap::empty(bytes);
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73
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74 // Like in `bytes` above, this `'_` lifetime parameter borrows from
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75 // the bytes buffer owned by `on_disk`.
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76 let ptr: *mut DirstateMap<'_> = Box::into_raw(Box::new(map));
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77
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78 // Erase the pointed type entirely in order to erase the lifetime.
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79 let ptr: *mut () = ptr.cast();
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80
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81 Self { on_disk, ptr }
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82 }
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83
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84 pub fn get_pair_mut<'a>(
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85 &'a mut self,
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86 ) -> (&'a [u8], &'a mut DirstateMap<'a>) {
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87 // SAFETY: We cast the type-erased pointer back to the same type it had
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88 // in `new`, except with a different lifetime parameter. This time we
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89 // connect the lifetime to that of `self`. This cast is valid because
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90 // `self` owns the same `on_disk` whose buffer `DirstateMap`
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91 // references. That buffer has a stable memory address because our
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92 // `Self::new_empty` counstructor requires `StableDeref`.
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93 let ptr: *mut DirstateMap<'a> = self.ptr.cast();
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94 // SAFETY: we dereference that pointer, connecting the lifetime of the
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95 // new `&mut` to that of `self`. This is valid because the
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96 // raw pointer is to a boxed value, and `self` owns that box.
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97 (&self.on_disk, unsafe { &mut *ptr })
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98 }
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99
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100 pub fn get_map_mut<'a>(&'a mut self) -> &'a mut DirstateMap<'a> {
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101 self.get_pair_mut().1
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102 }
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103
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104 pub fn get_map<'a>(&'a self) -> &'a DirstateMap<'a> {
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105 // SAFETY: same reasoning as in `get_pair_mut` above.
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106 let ptr: *mut DirstateMap<'a> = self.ptr.cast();
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107 unsafe { &*ptr }
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108 }
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109
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110 pub fn on_disk<'a>(&'a self) -> &'a [u8] {
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111 &self.on_disk
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112 }
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113 }
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114
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115 impl Drop for OwningDirstateMap {
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116 fn drop(&mut self) {
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117 // Silence a "field is never read" warning, and demonstrate that this
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118 // value is still alive.
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119 let _: &Box<dyn Deref<Target = [u8]> + Send> = &self.on_disk;
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120 // SAFETY: this cast is the same as in `get_mut`, and is valid for the
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121 // same reason. `self.on_disk` still exists at this point, drop glue
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122 // will drop it implicitly after this `drop` method returns.
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123 let ptr: *mut DirstateMap<'_> = self.ptr.cast();
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124 // SAFETY: `Box::from_raw` takes ownership of the box away from `self`.
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125 // This is fine because drop glue does nothing for `*mut ()` and we’re
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126 // in `drop`, so `get` and `get_mut` cannot be called again.
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127 unsafe { drop(Box::from_raw(ptr)) }
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128 }
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129 }
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130
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131 fn _static_assert_is_send<T: Send>() {}
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132
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133 fn _static_assert_fields_are_send() {
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134 _static_assert_is_send::<Box<DirstateMap<'_>>>();
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135 }
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136
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137 // SAFETY: we don’t get this impl implicitly because `*mut (): !Send` because
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138 // thread-safety of raw pointers is unknown in the general case. However this
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139 // particular raw pointer represents a `Box<DirstateMap<'on_disk>>` that we
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140 // own. Since that `Box` is `Send` as shown in above, it is sound to mark
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141 // this struct as `Send` too.
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142 unsafe impl Send for OwningDirstateMap {}