Define `fs::hard_link` to not follow symlinks. #78026

sunfishcode · 2020-10-16T19:09:19Z

POSIX leaves it implementation-defined whether link follows symlinks.
In practice, for example, on Linux it does not and on FreeBSD it does.
So, switch to linkat, so that we can pick a behavior rather than
depending on OS defaults.

Pick the option to not follow symlinks. This is somewhat arbitrary, but
seems the less surprising choice because hard linking is a very
low-level feature which requires the source and destination to be on
the same mounted filesystem, and following a symbolic link could end
up in a different mounted filesystem.

POSIX leaves it implementation-defined whether `link` follows symlinks. In practice, for example, on Linux it does not and on FreeBSD it does. So, switch to `linkat`, so that we can pick a behavior rather than depending on OS defaults. Pick the option to not follow symlinks. This is somewhat arbitrary, but seems the less surprising choice because hard linking is a very low-level feature which requires the source and destination to be on the same mounted filesystem, and following a symbolic link could end up in a different mounted filesystem.

rust-highfive · 2020-10-16T19:09:22Z

r? @withoutboats

(rust_highfive has picked a reviewer for you, use r? to override)

bjorn3 · 2020-10-18T12:09:43Z

What if an OS doesn't have support for this?

sunfishcode · 2020-10-19T15:01:23Z

What if an OS doesn't have support for this?

Good question. linkat is in POSIX.1-2008 and pretty widely supported, but it appears at least VxWorks and Redox don't support it, so I've now added a patch special-casing those two platforms.

steveklabnik · 2020-10-19T15:09:31Z

cc @rust-lang/libs

dtolnay · 2020-10-20T21:20:32Z

@rfcbot fcp merge
@rust-lang/libs:

POSIX leaves it implementation-defined whether link follows symlinks. In practice, for example, on Linux it does not and on FreeBSD it does. So, switch to linkat, so that we can pick a behavior rather than depending on OS defaults.

Pick the option to not follow symlinks. This is somewhat arbitrary, but seems the less surprising choice because hard linking is a very low-level feature which requires the source and destination to be on the same mounted filesystem, and following a symbolic link could end up in a different mounted filesystem.

rfcbot · 2020-10-20T21:20:33Z

Team member @dtolnay has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

sfackler · 2020-10-20T21:30:56Z

I don't think @bjorn3's question was really answered. It seems like it happens to be the case that VxWorks and Redox's link implementations don't follow symlinks, but what happens if we want to support a platform that does not have linkat and has a link that does follow them?

dtolnay · 2020-10-20T21:49:20Z

Would we be comfortable with wording that indicates "symlink is not followed if such a thing is possible on the platform"? I think the concern motivating the PR was that even on platforms which support the behavior we prefer for fs::hard_link, you can't count on that to be the standard library's behavior.

This implies if VxWorks or Redox were to grow support for this operation later, we'd accept a change to the behavior to use it. In other words users would have the new guarantee from this PR on platforms that support it, and would continue to have the old weaker guarantee on other platforms until such time as those support it.

sfackler · 2020-10-20T21:55:20Z

I'd be happy with that language.

Also mention that where possible, `hard_link` does not follow symlinks.

sunfishcode · 2020-10-21T08:50:51Z

I've now updated the comments to take a "if such a thing is possible on the platform" stance.

rfcbot · 2020-10-23T09:28:40Z

🔔 This is now entering its final comment period, as per the review above. 🔔

dtolnay · 2020-10-23T11:55:31Z

@bors r+

bors · 2020-10-23T11:55:33Z

📌 Commit d0178b4 has been approved by dtolnay

…olnay Define `fs::hard_link` to not follow symlinks. POSIX leaves it [implementation-defined] whether `link` follows symlinks. In practice, for example, on Linux it does not and on FreeBSD it does. So, switch to `linkat`, so that we can pick a behavior rather than depending on OS defaults. Pick the option to not follow symlinks. This is somewhat arbitrary, but seems the less surprising choice because hard linking is a very low-level feature which requires the source and destination to be on the same mounted filesystem, and following a symbolic link could end up in a different mounted filesystem. [implementation-defined]: https://pubs.opengroup.org/onlinepubs/9699919799/functions/link.html

Rollup of 12 pull requests Successful merges: - rust-lang#77640 (Refactor IntErrorKind to avoid "underflow" terminology) - rust-lang#78026 (Define `fs::hard_link` to not follow symlinks.) - rust-lang#78114 (Recognize `private_intra_doc_links` as a lint) - rust-lang#78228 (Promote aarch64-unknown-linux-gnu to Tier 1) - rust-lang#78345 (Fix handling of item names for HIR) - rust-lang#78437 (BTreeMap: stop mistaking node for an orderly place) - rust-lang#78476 (fix some incorrect aliasing in the BTree) - rust-lang#78674 (inliner: Use substs_for_mir_body) - rust-lang#78748 (Implement destructuring assignment for tuples) - rust-lang#78868 (Fix tab focus on restyled switches) - rust-lang#78878 (Avoid overlapping cfg attributes when both macOS and aarch64) - rust-lang#78882 (Nicer hunk headers for rust files) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

std: Fix test `symlink_hard_link` on Windows The test was introduced in rust-lang#78026 and fails depending on Windows version and admin rights. Other similar tests check for symlink creation permissions before doing anything, this PR performs the same check for `symlink_hard_link` as well.

Following rust-lang#78026, `std::fs::hard_link` on most platforms does not follow symlinks. Change the WASI implementation to also not follow symlinks.

Make WASI's `hard_link` behavior match other platforms. Following rust-lang#78026, `std::fs::hard_link` on most platforms does not follow symlinks. Change the WASI implementation to also not follow symlinks. r? `@alexcrichton`

Make WASI's `hard_link` behavior match other platforms. Following rust-lang#78026, `std::fs::hard_link` on most platforms does not follow symlinks. Change the WASI implementation to also not follow symlinks. r? ``@alexcrichton``

Make WASI's `hard_link` behavior match other platforms. Following rust-lang#78026, `std::fs::hard_link` on most platforms does not follow symlinks. Change the WASI implementation to also not follow symlinks. r? ```@alexcrichton```

rust-highfive assigned withoutboats Oct 16, 2020

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Oct 16, 2020

sunfishcode mentioned this pull request Oct 16, 2020

Add another test for hardlinks to symlinks. bytecodealliance/cap-std#115

Merged

camelid added A-io Area: `std::io`, `std::fs`, `std::net` and `std::path` T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. labels Oct 17, 2020

sunfishcode added 2 commits October 18, 2020 07:21

Fix a typo in a comment.

23a5c21

Use link on platforms which lack linkat.

ce00b3e

rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Oct 20, 2020

Make it platform-specific whether hard_link follows symlinks.

d0178b4

Also mention that where possible, `hard_link` does not follow symlinks.

rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Oct 23, 2020

dtolnay approved these changes Oct 23, 2020

View reviewed changes

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Oct 23, 2020

jonas-schievink mentioned this pull request Oct 23, 2020

Rollup of 11 pull requests #78282

Closed

m-ou-se mentioned this pull request Nov 7, 2020

Rollup of 12 pull requests #78831

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m-ou-se mentioned this pull request Nov 7, 2020

Rollup of 15 pull requests #78838

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m-ou-se mentioned this pull request Nov 7, 2020

Rollup of 16 pull requests #78846

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Dylan-DPC-zz mentioned this pull request Nov 8, 2020

Rollup of 15 pull requests #78870

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Dylan-DPC-zz mentioned this pull request Nov 8, 2020

Rollup of 12 pull requests #78885

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Dylan-DPC-zz mentioned this pull request Nov 9, 2020

Rollup of 12 pull requests #78889

Merged

bors merged commit 41134be into rust-lang:master Nov 9, 2020

rustbot added this to the 1.49.0 milestone Nov 9, 2020

sunfishcode deleted the symlink-hard-link branch November 9, 2020 03:25

petrochenkov mentioned this pull request Nov 16, 2020

std: Fix test symlink_hard_link on Windows #79105

Merged

pkgw mentioned this pull request Jan 8, 2021

1.49 seems to have bumped required macOS version #80804

Closed

sunfishcode mentioned this pull request Jan 26, 2021

Test updates bytecodealliance/cap-std#144

Merged

sunfishcode mentioned this pull request Feb 11, 2021

Make WASI's hard_link behavior match other platforms. #81984

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inquisitivecrystal mentioned this pull request Jun 28, 2021

Fix linker error #85953

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Define `fs::hard_link` to not follow symlinks. #78026

Define `fs::hard_link` to not follow symlinks. #78026

sunfishcode commented Oct 16, 2020 •

edited

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rust-highfive commented Oct 16, 2020

bjorn3 commented Oct 18, 2020

sunfishcode commented Oct 19, 2020

steveklabnik commented Oct 19, 2020

dtolnay commented Oct 20, 2020

rfcbot commented Oct 20, 2020 •

edited by Amanieu

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dtolnay commented Oct 20, 2020

sfackler commented Oct 20, 2020

sunfishcode commented Oct 21, 2020

rfcbot commented Oct 23, 2020

dtolnay commented Oct 23, 2020

bors commented Oct 23, 2020

Define fs::hard_link to not follow symlinks. #78026

Define fs::hard_link to not follow symlinks. #78026

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sunfishcode commented Oct 16, 2020 • edited Loading

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sunfishcode commented Oct 21, 2020

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dtolnay commented Oct 23, 2020

bors commented Oct 23, 2020

Define `fs::hard_link` to not follow symlinks. #78026

Define `fs::hard_link` to not follow symlinks. #78026

sunfishcode commented Oct 16, 2020 •

edited

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rfcbot commented Oct 20, 2020 •

edited by Amanieu

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