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Prevent String::retain from creating non-utf8 strings when abusing panic #78499

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Oct 30, 2020
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Prevent String::retain from creating non-utf8 strings when abusing panic #78499

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e83666f
Prevent String::retain from creating non-utf8 strings when abusing panic
SkiFire13 Oct 28, 2020
1f6f917
Added test for issue #78498
SkiFire13 Oct 29, 2020
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10 changes: 6 additions & 4 deletions library/alloc/src/string.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1235,6 +1235,10 @@ impl String {
let mut del_bytes = 0;
let mut idx = 0;

unsafe {
self.vec.set_len(0);
}

while idx < len {
let ch = unsafe { self.get_unchecked(idx..len).chars().next().unwrap() };
Comment on lines +1238 to 1243
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@RalfJung this was brought to my attention by @DJMcNab (the author of retain_more), as somewhat suspicious - it's calling get_unchecked on a &str obtained via auto-deref which is zero-length now. So we more or less expect this needs to go through a raw pointer directly or something similar.

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@SkiFire13 SkiFire13 Feb 26, 2021 •
edited
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Good catch! I guess another fix would be using a DropGuard for setting the length at the end of the loop. This way it's not necessary to set the length to 0 anymore because its drop implementation would called even in case of panic.

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I've opened #82554, let me know what you think about it

let ch_len = ch.len_utf8();
Expand All @@ -1255,10 +1259,8 @@ impl String {
idx += ch_len;
}

if del_bytes > 0 {
unsafe {
self.vec.set_len(len - del_bytes);
}
unsafe {
self.vec.set_len(len - del_bytes);
}
}

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15 changes: 15 additions & 0 deletions library/alloc/tests/string.rs
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Original file line number Diff line number Diff line change
@@ -1,6 +1,7 @@
use std::borrow::Cow;
use std::collections::TryReserveError::*;
use std::ops::Bound::*;
use std::panic;

pub trait IntoCow<'a, B: ?Sized>
where
Expand Down Expand Up @@ -378,6 +379,20 @@ fn test_retain() {

s.retain(|_| false);
assert_eq!(s, "");

let mut s = String::from("0è0");
let _ = panic::catch_unwind(panic::AssertUnwindSafe(|| {
let mut count = 0;
s.retain(|_| {
count += 1;
match count {
1 => false,
2 => true,
_ => panic!(),
}
});
}));
assert!(std::str::from_utf8(s.as_bytes()).is_ok());
}

#[test]
Expand Down