Major cmsg cleanup by asomers · Pull Request #1020 · nix-rust/nix

asomers · 2019-01-30T20:54:54Z

This PR fixes multiple bugs in the cmsg code.
Fixes #994
Fixes #999
Fixes #1013

asomers · 2019-01-30T20:55:58Z

Ralith · 2019-01-30T21:02:18Z

-    /// [`unix(7)`](http://man7.org/linux/man-pages/man7/unix.7.html) man page.
-    // FIXME: When `#[repr(transparent)]` is stable, use it on `UnixCredentials`
-    // and put that in here instead of a raw ucred.
+pub enum ControlMessageOwned {


Per my previous comment, I think this is the wrong way to go, as it incurs avoidable heap allocation and copying.

I chose to go this path because it's closer to backwards-compatible. I tried the 100% backwards-compatible method first, but the code got very ugly very quick, and it involved even more copying.

It seems to be that you've settled on the worst of both worlds: both a breaking change and inefficient.

Can you suggest something more efficient that causes no more pain for existing users? This solution, while not exactly backwards compatible, only requires a simple search and replace.

I don't think the current specific level of breakage makes sense as a line in the sand that cannot be crossed by a single step. That said, you could compromise by only using the flyweight pattern where heap allocation is otherwise necessary, i.e. in ScmRights, though this would make "Owned" no longer an accurate descriptor.

But what's to say that ScmRights's payload is aligned on all platforms that Nix will ever support? I don't think that's guaranteed, especially if a user ever passes an unaligned msg_control field to recvmsg.

That's why I said "flyweight pattern" and not "a slice", yes. Refer to the comment I linked for an example implementation.

Ralith · 2019-01-30T21:06:31Z

+/// let _ = cmsg_space!(RawFd, TimeVal);
+/// ```
+// Unfortunately, CMSG_SPACE isn't a const_fn, or else we could return a
+// stack-allocated array.


Could this be corrected? This seems like exactly the special case that const fns can already handle gracefully, and is the intended use of the C macros.

~~The libc people are resistant to that kind of change. rust-lang/libc#1213~~ (actually that's a separate issue). The cmsg functions can't be made const because that requires rustc 1.31.0. libc guarantees support for 1.13.0.

Ralith · 2019-01-30T21:06:52Z

+/// // Create a buffer big enough for a `ControlMessageOwned::ScmRights` message
+/// // and a `ControlMessageOwned::ScmTimestamp` message
+/// let _ = cmsg_space!(RawFd, TimeVal);
+/// ```


I love this interface!

Ralith · 2019-01-30T21:08:21Z

-    // The number of bytes received.
-    pub bytes: usize,
-    cmsg_buffer: &'a [u8],
+    cmsghdr: *const cmsghdr,


Why not a real reference?

Because there are only three things we're going to do with that member. Pass it to CMSG_NXTHDR and CMSG_DATA, which expect pointere, and do some raw pointer arithmetic in decode_from. So no sense converting it to a reference when we'd just have to convert it back again.

References can convert to pointers implicitly, and the PhantomData would be rendered unnecessary.

As a matter of fact, this turned out to be pretty easy. I'll do it.

Ralith · 2019-01-30T21:15:26Z

Thanks for your work on the libc macros, by the way!

asomers · 2019-01-31T03:58:55Z

The QEMU failures are due to a bug in QEMU itself. For once I'm sure of it. The emulated recvmsg is writing beyond the end of the supplied msg_control buffer. I think it's probably due to the linked qemu bug, which is already fixed. I'll disable the test. Once japaric/cross updates its image to use qemu 2.12.0, then we can reenable it.
https://bugs.launchpad.net/qemu/+bug/1701808
japaric/rust-cross#49

asomers · 2019-01-31T04:19:15Z

Sigh. Travis got confused somehow and thinks I'm missing a merge commit. I guess I may as well rebase.

asomers · 2019-02-12T15:26:31Z

I'm going to go ahead and merge this. @Ralith if you're still concerned, then please submit a separate PR to reduce data copies.

asomers · 2019-02-12T20:44:30Z

bors r+

bors · 2019-02-12T22:04:49Z

Merge conflict (retrying...)

bors · 2019-02-12T22:05:01Z

Merge conflict

Our hand-rolled logic had subtle alignment bugs that caused test_scm_rights to fail on OpenBSD (and probably could cause problems on other platforms too). Using cmsg(3) is much cleaner, shorter, and more portable. No user-visible changes.

There were two problems: 1) It would always return Ok, even on error 2) It could panic if there was an error, because sockaddr_storage_to_addr would be called on uninitialized memory.

CmsgSpace had three problems: 1) It would oversize buffers that expect multiple control messages 2) It didn't use the libc CMSG_SPACE(3) macro, so it might actually undersize a buffer for a single control message. 3) It could do bad things on drop, if you instantiate it with a type that implements Drop (which none of the currently supported ControlMessage types do). Fixes nix-rust#994

On some platforms the alignment of cmsg_data could be less than the alignment of the messages that it contains. That led to unaligned reads on those platforms. This change fixes the issue by always copying the message contents into aligned objects. The change is not 100% backwards compatible when using recvmsg. Users may have to replace code like this: ```rust if let ControlMessage::ScmRights(&fds) = cmsg { ``` with this: ```rust if let ControlMessageOwned::ScmRights(fds) = cmsg { ``` Fixes nix-rust#999

This was an oversight from PR nix-rust#1002

asomers · 2019-02-14T18:13:20Z

bors r+

1020: Major cmsg cleanup r=asomers a=asomers This PR fixes multiple bugs in the cmsg code. Fixes #994 Fixes #999 Fixes #1013 Co-authored-by: Alan Somers <asomers@gmail.com>

Ralith · 2019-02-14T18:27:55Z

@Ralith if you're still concerned, then please submit a separate PR to reduce data copies

I don't actually use Nix at present, so I'm just going to stick with my current implementation that doesn't have these issues.

bors · 2019-02-14T18:55:35Z

Build succeeded

continuous-integration/travis-ci/push
FreeBSD 11.2

It was leftover from internal churn during PR nix-rust#1020.

asomers requested a review from Susurrus January 30, 2019 20:55

Ralith reviewed Jan 30, 2019

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asomers force-pushed the cmsg branch 2 times, most recently from 55b67b0 to 55e28c7 Compare January 31, 2019 23:28

asomers force-pushed the cmsg branch 2 times, most recently from 2125d63 to f11fe7f Compare February 12, 2019 19:58

asomers added 6 commits February 14, 2019 10:15

Fix error handling of RecvMsg

368b9fe

There were two problems: 1) It would always return Ok, even on error 2) It could panic if there was an error, because sockaddr_storage_to_addr would be called on uninitialized memory.

Add CHANGELOG entry.

2966dcd

Enable IPv4PacketInfo and Ipv6PacketInfo on more OSes.

b5c4c7a

This was an oversight from PR nix-rust#1002

asomers force-pushed the cmsg branch from f11fe7f to b5c4c7a Compare February 14, 2019 17:17

bors bot added a commit that referenced this pull request Feb 14, 2019

Merge #1020

a2fa282

1020: Major cmsg cleanup r=asomers a=asomers This PR fixes multiple bugs in the cmsg code. Fixes #994 Fixes #999 Fixes #1013 Co-authored-by: Alan Somers <asomers@gmail.com>

bors bot merged commit b5c4c7a into nix-rust:master Feb 14, 2019

asomers deleted the cmsg branch August 19, 2019 22:37

asomers added a commit to asomers/nix that referenced this pull request Aug 19, 2019

Remove an obsolete comment

de80078

It was leftover from internal churn during PR nix-rust#1020.

asomers added a commit to asomers/nix that referenced this pull request Aug 19, 2019

Remove an obsolete comment

4dca1b7

It was leftover from internal churn during PR nix-rust#1020.

asomers added a commit to asomers/nix that referenced this pull request Aug 29, 2019

Remove an obsolete comment

64e9e8f

It was leftover from internal churn during PR nix-rust#1020.

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Merge conflict (retrying...)

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