Fix passing multiple file descriptors / control messages via sendmsg #918

jonas-schievink · 2018-07-01T19:03:00Z

Fixes #464
Closes #874 because it's incorporated here
Closes #756 because it adds the test from that issue (with fixes)

jonas-schievink · 2018-07-01T20:31:09Z

Current CI failures:

Only 1 fd was successfully received, the other got lost
- FreeBSD x86_64
- powerpc64-unknown-linux-gnu
- powerpc64le-unknown-linux-gnu
- mips64-unknown-linux-gnuabi64
- mips64el-unknown-linux-gnuabi64
- aarch64-unknown-linux-gnu
EINVAL
- i686-apple-darwin
- x86_64-apple-darwin

jonas-schievink · 2018-07-02T00:04:30Z

Every single CI job that failed passes another test that also passes 2 fds, but only using a single control message. Seems like you're not supposed to use more than 1 SCM_RIGHTS message.

I've deleted the broken test and added a note.

asomers

Looks good so far! Does Nix also have a bug with sending two dissimilar control messages in a single sendmsg call, and if so does this change fix it?

asomers · 2018-07-02T14:25:28Z

Cargo.toml

@@ -50,3 +50,8 @@ harness = false
 [[test]]
 name = "test-ptymaster-drop"
 path = "test/test_ptymaster_drop.rs"
+
+[[test]]


Why did you need to use a separate test program? If this program is meant to be interactive, and provides no additional test coverage over the other unit test that you wrote, then it should go in the examples directory, not the tests directory.

Turns out the test I added in a recent commit essentially does the same thing (pass 2 fds using 1 cmsg), so I'll remove this one.

Adding examples to nix is a very good idea, but I guess it should be done as a more coordinated approach rather than in this PR.

asomers · 2018-07-02T14:31:09Z

src/sys/socket/mod.rs

@@ -301,6 +301,18 @@ unsafe fn copy_bytes<'a, 'b, T: ?Sized>(src: &T, dst: &'a mut &'b mut [u8]) {
    mem::swap(dst, &mut remainder);
 }

+/// Pad byte slice dst with `len` zeroes, and update the slice to point to
+/// the remainder.
+fn pad_bytes(len: usize, dst: &mut &mut [u8]) {


Return arguments is very C-ish. Could you return a reference to the remainder instead? Like this?

fn pad_bytes(len: usize, sl: &'a mut [u8]) -> &'a mut [u8];

asomers · 2018-07-02T14:34:30Z

src/sys/socket/mod.rs

+fn pad_bytes(len: usize, dst: &mut &mut [u8]) {
+    let mut tmpbuf = &mut [][..];
+    mem::swap(&mut tmpbuf, dst);
+    let (padding, mut remainder) = tmpbuf.split_at_mut(len);


Seems like you're padding the left half of the slice, which is not what I though from the function description. Could you please make that more clear?

asomers · 2018-07-02T14:47:02Z

src/sys/socket/mod.rs

@@ -434,6 +446,10 @@ pub enum ControlMessage<'a> {
    ///
    /// See the description in the "Ancillary messages" section of the
    /// [unix(7) man page](http://man7.org/linux/man-pages/man7/unix.7.html).
+    ///
+    /// Do not use more than one `ScmRights` message for a single `sendmsg` call - Depending on the


This is a pretty strong statement. If we're going to lecture our users about portability best-practices, then we should at least describe it as a recommendation, not a requirement.

asomers · 2018-07-02T14:55:00Z

src/sys/socket/mod.rs

-                mem::swap(&mut tmpbuf, buf);
-                let (_padding, mut remainder) = tmpbuf.split_at_mut(padlen);
-                mem::swap(buf, &mut remainder);
+                pad_bytes(padlen, buf);


It looks like the caller doesn't care that encode_into returns the new buffer reference by argument. So we can simplify this function by making buf a &mut [u8], then simplifying pad_bytes into something more like:

for i in 0..padlen { buf[i] = 0; } buf = buf[padlen..]

asomers · 2018-07-02T15:30:45Z

test/test_pass_fds.rs

+                }
+            },
+            Err(_) => {
+                panic!();


These two panic messages are indistinguishable since you don't provide descriptions for them. And if the error case is a simple panic, you may as well just use unwrap instead of if let.

asomers · 2018-07-02T15:32:47Z

test/test_pass_fds.rs

+        let mut buf = [0u8; 8];
+        let iovec = [IoVec::from_mut_slice(&mut buf)];
+        let mut space = CmsgSpace::<[RawFd; 2]>::new();
+        let (paneid, fds) = match recvmsg(receive.as_raw_fd(), &iovec, Some(&mut space), MsgFlags::empty()) {


What is a "paneid" ?

asomers · 2018-07-02T15:34:50Z

test/test_pass_fds.rs

+
+    let slice = [1u8, 2u8, 3u8, 4u8, 5u8, 6u8, 7u8, 8u8];
+    let iov = [IoVec::from_slice(&slice)];
+    let arr = [0, 1]; // Pass stdin and stdout


This should be libc::STDIN_FILENO and libc::STDOUT_FILENO.

asomers · 2018-07-02T15:42:07Z

test/sys/test_socket.rs

+
+    let slice = [1u8, 2u8, 3u8, 4u8, 5u8, 6u8, 7u8, 8u8];
+    let iov = [IoVec::from_slice(&slice)];
+    let fds = [0, 1];    // pass stdin and stdout


should be libc::STDIN_FILENO and libc::STDOUT_FILENO.

jonas-schievink · 2018-07-02T16:34:16Z

Does Nix also have a bug with sending two dissimilar control messages in a single sendmsg call, and if so does this change fix it?

This should fix that, but I haven't added a test since there's no convenient ControlMessage that can be used to test it. I might add support for SCM_CREDENTIALS in a later PR and use that along with SCM_RIGHTS to test this.

asomers · 2018-07-02T16:40:20Z

Why not use ScmRights with ScmTimestamp?

jonas-schievink · 2018-07-02T17:02:17Z

Attempting to send a SCM_TIMESTAMP results in EINVAL on my system (x64 Linux). Even when it's just a single SCM_TIMESTAMP control message, and even from C++, so it's not a bug in nix, you're just not supposed to send those by yourself (the kernel does).

jonas-schievink · 2018-07-04T09:55:58Z

@asomers Anything left to do here?

asomers

Getting close now. I only have cosmetic comments at this point. Also, we'll need a squash before we're done.

asomers · 2018-07-04T17:25:39Z

src/sys/socket/mod.rs

+///
+/// Panics when `dst` is too small for `src` (panics if `mem::size_of_val(src) >= dst.len()`).
+///
+/// Unsafe because it exposes all bytes in `src`, which may be UB if some of them are uninitialized


It's not actually unsafe to read an uninitialized slice, because slices are always supposed to be initialized. The unsafe part is creating the uninitialized slice in the first place. However, it is unsafe to call copy_nonoverlapping without verifying that the slices don't overlap, without verifying that T implements Copy, and without droping the contents of dst. Please update the comment.

The problem is that this is reading the bytes of an arbitrary T, including its padding. If reading the bytes from a T is safe, then so is this function. It doesn't create any uninitialized memory or slices.

AFAIK, LLVM considers reads of padding bytes undef, which would be unsafe, but I don't know if rustc makes it safe in some way (eg. by providing its own padding members that read as junk instead of undef).

So the unsafe part is that it's effectively transmuting the T. Transmutation is unsafe regardless of padding issues.

I don't think transmutation to just bytes is unsafe except when padding is involved (as there's no way to read data that isn't valid as a u8), but I've clarified what's going on here

asomers · 2018-07-04T17:40:25Z

src/sys/socket/mod.rs

+/// Fills `dst` with `len` zero bytes and returns the remainder of the slice.
+///
+/// Panics when `len >= dst.len()`.
+fn pad_bytes(len: usize, dst: &mut [u8]) -> &mut [u8] {


Much nicer than the previous version. I like.

asomers · 2018-07-04T17:44:52Z

test/sys/test_socket.rs

+
+        let mut cmsgs = msg.cmsgs();
+        match cmsgs.next() {
+            Some(ControlMessage::ScmRights(fds)) => assert_eq!(fds.len(), 2, "unexpected fd count (expected 2 fds)"),


80 chars per line, please.

asomers · 2018-07-05T14:01:32Z

bors r+

918: Fix passing multiple file descriptors / control messages via sendmsg r=asomers a=jonas-schievink Fixes #464 Closes #874 because it's incorporated here Closes #756 because it adds the test from that issue (with fixes) Co-authored-by: alecmocatta <[email protected]>

bors · 2018-07-05T15:04:10Z

Build failed

continuous-integration/travis-ci/push

jonas-schievink · 2018-07-05T15:47:40Z

     Running /target/i686-unknown-linux-gnu/release/deps/test_aio_drop-88766c7ced1811bd
running 1 test
test test_drop ... error: An unknown error occurred

That doesn't look good - but also spurious

asomers · 2018-07-05T15:49:04Z

bors retry

918: Fix passing multiple file descriptors / control messages via sendmsg r=asomers a=jonas-schievink Fixes #464 Closes #874 because it's incorporated here Closes #756 because it adds the test from that issue (with fixes) Co-authored-by: alecmocatta <[email protected]>

bors · 2018-07-05T16:17:08Z

Build succeeded

923: Fix control message *decoding* and add support for `ScmCredentials` r=asomers a=jonas-schievink While #918 fixed the *encoding*, the *decoding* done by the `CmsgIterator` still remained broken when multiple messages are received. However, since nix didn't support any control message that could reliably be sent twice in one `sendmsg` call, this couldn't be tested properly. This PR addresses this by adding `SCM_CREDENTIALS` support and testing all of this by passing both an `SCM_CREDENTIALS` and a `SCM_RIGHTS` message at the same time. I've also verified that the resulting encoding is the same as for roughly equivalent C code. Co-authored-by: Jonas Schievink <[email protected]>

asomers requested changes Jul 2, 2018

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asomers requested changes Jul 4, 2018

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Clean up cmsg code and fix passing multiple cmsgs

5d6dc26

asomers approved these changes Jul 5, 2018

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bors bot merged commit 5d6dc26 into nix-rust:master Jul 5, 2018

jonas-schievink deleted the sendmsg-fix branch July 5, 2018 16:17

jonas-schievink mentioned this pull request Jul 6, 2018

Fix control message *decoding* and add support for ScmCredentials #923

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Fix passing multiple file descriptors / control messages via sendmsg #918

Fix passing multiple file descriptors / control messages via sendmsg #918

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Fix passing multiple file descriptors / control messages via sendmsg #918

Fix passing multiple file descriptors / control messages via sendmsg #918

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