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Fix all problems encounted with Miri -Ztag-raw-pointers #277

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merged 2 commits into from Feb 7, 2022

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saethlin
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@saethlin saethlin commented Feb 1, 2022

I poked at this crate with -Zmiri-tag-raw-pointers before, and I was unable to fix what I found (I just added a test case that ruled out one of my wrong ideas #271). I tried again just now and I guess I just understand better this time.

This PR fixes 3 separate pointer invalidation problems, which are detected by running MIRIFLAGS=-Zmiri-tag-raw-pointers cargo miri test.

Depending on how you squint, 2 or 3 of these are rust-lang/unsafe-code-guidelines#133. The last one is probably still present even with late invalidation, because set_len does a write through a &mut.

It's unclear to me if any of these things that Miri complains about are potentially a miscompilation in rustc due to the use of LLVM noalias. But perhaps given how subtle this codebase is overall, it would be best to run the tools on their pickiest settings, even if there are a few things more like a false positive than a real problem.

Miri does not check all of Stacked Borrows (the prototype aliasing model
for Rust) without -Zmiri-tag-raw-pointers. This enables the check in CI,
and makes a few adjustments to fix places where pointers were
invalidated by construction or use of a mutable reference.
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@mbrubeck mbrubeck left a comment

Thank you!

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mbrubeck commented Feb 1, 2022

@bors-servo r+

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bors-servo commented Feb 1, 2022

📌 Commit b257aad has been approved by mbrubeck

bors-servo added a commit that referenced this issue Feb 1, 2022
Fix all problems encounted with Miri -Ztag-raw-pointers

I poked at this crate with `-Zmiri-tag-raw-pointers` before, and I was unable to fix what I found (I just added a test case that ruled out one of my wrong ideas #271). I tried again just now and I guess I just understand better this time.

This PR fixes 3 separate pointer invalidation problems, which are detected by running `MIRIFLAGS=-Zmiri-tag-raw-pointers cargo miri test`.

Depending on how you squint, 2 or 3 of these are rust-lang/unsafe-code-guidelines#133. The last one is _probably_ still present even with late invalidation, because `set_len` does a write through a `&mut`.

It's unclear to me if any of these things that Miri complains about are potentially a miscompilation in rustc due to the use of LLVM `noalias`. But perhaps given how subtle this codebase is overall, it would be best to run the tools on their pickiest settings, even if there are a few things more like a false positive than a real problem.
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bors-servo commented Feb 1, 2022

Testing commit b257aad with merge deb82ed...

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bors-servo commented Feb 1, 2022

💔 Test failed - checks-github

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mbrubeck commented Feb 1, 2022

(We need to wait for a nightly with working MIRI, or fix our CI script to find a working nightly build automatically.)

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saethlin commented Feb 1, 2022

To get Miri building again, it needs to support readdir64. I've been meaning to improve Miri's libc support, but don't hold your breath. I definitely do not mind waiting for a nightly where Miri builds.

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saethlin commented Feb 7, 2022

@mbrubeck Miri is on recent nightlies, I pushed an empty commit (please squash it away) to make CI re-run 🥳

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mbrubeck commented Feb 7, 2022

@bors-servo r+

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bors-servo commented Feb 7, 2022

📌 Commit ecd69b9 has been approved by mbrubeck

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bors-servo commented Feb 7, 2022

Testing commit ecd69b9 with merge 8363ea8...

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bors-servo commented Feb 7, 2022

☀️ Test successful - checks-github
Approved by: mbrubeck
Pushing 8363ea8 to master...

@bors-servo bors-servo merged commit 8363ea8 into servo:master Feb 7, 2022
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3 participants