averello/swift-mirror
1
0
Fork 0
mirror of https://github.com/apple/swift.git synced 2025-12-21 12:14:44 +01:00
Code Issues Packages Projects Releases Wiki Activity

Fix undefined behavior in SmallString.withUTF8

Browse Source 
withUTF8 currently vends a typed UInt8 pointer to the underlying
SmallString. That pointer type differs from SmallString's
representation. It should simply vend a raw pointer, which would be
both type safe and convenient for UTF8 data. However, since this
method is already @inlinable, I added calls to bindMemory to prevent
the optimizer from reasoning about access to the typed pointer that we
vend.

rdar://67983613 (Undefinied behavior in SmallString.withUTF8 is miscompiled)

Additional commentary:

SmallString creates a situation where there are two types, the
in-memory type, (UInt64, UInt64), vs. the element type,
UInt8. `UnsafePointer<T>` specifies the in-memory type of the pointee,
because that's how C works. If you want to specify an element type,
not the in-memory type, then you need to use something other than
UnsafePointer to view the memory. A trivial `BufferView<UInt8>` would
be fine, although, frankly, I think UnsafeRawPointer is a perfectly
good type on its own for UTF8 bytes.

Unfortunately, a lot of the UTF8 helper code is ABI-exposed, so to
work around this, we need to insert calls to bindMemory at strategic
points to avoid undefined behavior. This is high-risk and can
negatively affect performance. So far, I was able to resolve the
regressions in our microbenchmarks just by tweaking the inliner.
This commit is contained in:
Andrew Trick
2020-08-29 01:31:24 -07:00
parent 2544806bba
commit 5eafc20cdd
3 changed files with 31 additions and 15 deletions
Download Patch File Download Diff File Expand all files Collapse all files

34
stdlib/public/core/SmallString.swift
View File

@@ -197,11 +197,18 @@ extension _SmallString {
internal func withUTF8<Result>(
_ f: (UnsafeBufferPointer<UInt8>) throws -> Result
) rethrows -> Result {
let count = self.count
var raw = self.zeroTerminatedRawCodeUnits
return try Swift.withUnsafeBytes(of: &raw) { rawBufPtr in
let ptr = rawBufPtr.baseAddress._unsafelyUnwrappedUnchecked
.assumingMemoryBound(to: UInt8.self)
return try f(UnsafeBufferPointer(start: ptr, count: self.count))
return try Swift.withUnsafeBytes(of: &raw) {
let rawPtr = $0.baseAddress._unsafelyUnwrappedUnchecked
// Rebind the underlying (UInt64, UInt64) tuple to UInt8 for the
// duration of the closure. Accessing self after this rebind is undefined.
let ptr = rawPtr.bindMemory(to: UInt8.self, capacity: count)
defer {
// Restore the memory type of self._storage
_ = rawPtr.bindMemory(to: RawBitPattern.self, capacity: 1)
}
return try f(UnsafeBufferPointer(start: ptr, count: count))
}
}
@@ -209,14 +216,11 @@ extension _SmallString {
// new count.
@inline(__always)
internal mutating func withMutableCapacity(
_ f: (UnsafeMutableBufferPointer<UInt8>) throws -> Int
_ f: (UnsafeMutableRawBufferPointer) throws -> Int
) rethrows {
let len = try withUnsafeMutableBytes(of: &self._storage) {
(rawBufPtr: UnsafeMutableRawBufferPointer) -> Int in
let ptr = rawBufPtr.baseAddress._unsafelyUnwrappedUnchecked
.assumingMemoryBound(to: UInt8.self)
return try f(UnsafeMutableBufferPointer(
start: ptr, count: _SmallString.capacity))
return try f(rawBufPtr)
}
if len == 0 {
self = _SmallString()
@@ -273,7 +277,17 @@ extension _SmallString {
) rethrows {
self.init()
try self.withMutableCapacity {
return try initializer($0)
let capacity = $0.count
let rawPtr = $0.baseAddress._unsafelyUnwrappedUnchecked
// Rebind the underlying (UInt64, UInt64) tuple to UInt8 for the
// duration of the closure. Accessing self after this rebind is undefined.
let ptr = rawPtr.bindMemory(to: UInt8.self, capacity: capacity)
defer {
// Restore the memory type of self._storage
_ = rawPtr.bindMemory(to: RawBitPattern.self, capacity: 1)
}
return try initializer(
UnsafeMutableBufferPointer<UInt8>(start: ptr, count: capacity))
}
self._invariantCheck()
}