Set the length of a socket address when calling `recvmsg` on Linux #2041

JarredAllen · 2023-05-20T00:12:09Z

Background

I think I've found a bug with recvmsg on Linux where it doesn't set the length of a received socket address. I found this while working with unix datagram sockets with path struct sockaddr_un addresses, but I think this applies to any variable-length socket address type.

At present, the sun_len field of UnixAddr on Linux shows up as 0 whenever I call recvmsg. I think it's reading uninitialized memory (afaict the recvmsg function never initializes it before we call MaybeUninit::assume_init), though it's possible that it's being zero-initialized somewhere that I missed. Either way, this isn't the correct behavior, which should set it to the length of the struct sockaddr_un contents (or whatever other type of socket).

Changes

I changed the recvmsg function to construct the returned socket address from the struct sockaddr and length returned by the libc recvmsg call (using the from_raw function the trait provides). Since the trait is sealed so downstream crates can't implement it, I believe this shouldn't be a breaking change.

Validation

I've tested that my code (which failed due to this bug) now works. I also added a new test case which tests that we construct UnixAddrs correctly in the recvmsg function, which passes locally for me and fails if I cherry-pick it onto the current master.

I've also checked that cargo test and cargo clippy both still pass on aarch64-apple-darwin and on aarch64-unknown-linux-musl targets (the two targets I develop for/have access to). Hopefully your CI will confirm that everything else still works.

asomers

It's unfortunate that this change results in an extra data copy, in S::from_raw. Instead, what about adding a SockaddrLike::set_len method? recvmsg would call it after libc::recv_msg. For most socket types, it would be a noop, but for UnixAddr on Linux it would set the sun_len field.

asomers · 2023-05-20T14:51:06Z

test/sys/test_socket.rs

+        print!("Couldn't send ({e:?}), so skipping test");
+        return;


Suggested change

print!("Couldn't send ({e:?}), so skipping test");

return;

skip!("Couldn't send ({e:?}), so skipping test");

Done in 72a2f56

asomers · 2023-05-20T14:56:41Z

src/sys/socket/mod.rs

@@ -2048,20 +2047,23 @@ pub fn recvmsg<'a, 'outer, 'inner, S>(fd: RawFd, iov: &'outer mut [IoSliceMut<'i
    where S: SockaddrLike + 'a,
    'inner: 'outer
 {
-    let mut address = mem::MaybeUninit::uninit();
+    let mut address: libc::sockaddr_storage = unsafe { mem::MaybeUninit::zeroed().assume_init() };


Why use sockaddr_storage instead of S?

It didn't pass tests when I used S, I think because () is shorter than the actual socket address type so it ran out of room (though I didn't check for the cause, so I might be wrong about that). Though the point is moot as I put it back to how it was before for implementing your suggestion about the different approach.

JarredAllen · 2023-05-22T18:54:40Z

Instead, what about adding a SockaddrLike::set_len method? recvmsg would call it after libc::recv_msg.

I implemented that change, though I wasn't sure if you wanted it to be publicly-visible or not. I can hide it from the docs and mark it an internal implementation detail, if you prefer.

Also, I made it return an error if you try to set the length on any socket address type whose length is static (anything other that UnixAddr or SockaddrStorage containing a UnixAddr internally), but I could make it validate the length or some other behavior instead, if you'd prefer that.

asomers

I like this version a lot better. Just a few inline comments, and:

Please do add #[doc(hidden)] to set_length, as you suggested, and
Please add a CHANGELOG entry.

asomers · 2023-07-15T22:24:07Z

src/sys/socket/addr.rs

+
+    /// Set the length of this socket address
+    ///
+    /// This method may only be called on socket addresses whose lenghts are dynamic, and it


Suggested change

/// This method may only be called on socket addresses whose lenghts are dynamic, and it

/// This method may only be called on socket addresses whose lengths are dynamic, and it

asomers · 2023-07-15T22:29:45Z

src/sys/socket/addr.rs

+                cfg_if! {
+                    if #[cfg(any(target_os = "android",
+                                 target_os = "fuchsia",
+                                 target_os = "illumos",
+                                 target_os = "linux",
+                                 target_os = "redox",
+                        ))] {
+                        addr.sun_len = new_length as u8;
+                    } else {
+                        addr.sun.sun_len = new_length as u8;
+                    }
+                }
+                Ok(())


Suggested change

cfg_if! {

if #[cfg(any(target_os = "android",

target_os = "fuchsia",

target_os = "illumos",

target_os = "linux",

target_os = "redox",

))] {

addr.sun_len = new_length as u8;

} else {

addr.sun.sun_len = new_length as u8;

}

}

Ok(())

addr.set_length(new_length)

asomers · 2023-07-15T22:30:39Z

src/sys/socket/addr.rs

+    ///
+    /// `new_length` must be a valid length for this type of address. Specifically, reads of that
+    /// length from `self` must be valid.
+    unsafe fn set_length(&mut self, new_length: usize) -> std::result::Result<(), SocketAddressLengthNotDynamic>;


If you add a default implementation here, then you can avoid a lot of boilerplate elsewhere.

Suggested change

unsafe fn set_length(&mut self, new_length: usize) -> std::result::Result<(), SocketAddressLengthNotDynamic>;

unsafe fn set_length(&mut self, new_length: usize) -> std::result::Result<(), SocketAddressLengthNotDynamic> {

Err(SocketAddressLengthNotDynamic)

}

asomers · 2023-07-15T22:32:54Z

src/sys/socket/mod.rs

@@ -2048,7 +2053,7 @@ pub fn recvmsg<'a, 'outer, 'inner, S>(fd: RawFd, iov: &'outer mut [IoSliceMut<'i
    where S: SockaddrLike + 'a,
    'inner: 'outer
 {
-    let mut address = mem::MaybeUninit::uninit();
+    let mut address = mem::MaybeUninit::zeroed();


Why do you think it's necessary to zero the address here?

I originally thought it might be necessary when I first started working on this (since the length was being left uninitialized initially), but looking back at it now I think everything is initialized now so I put it back to uninit()

JarredAllen · 2023-07-17T18:38:49Z

I like this version a lot better. Just a few inline comments, and:
* Please do add `#[doc(hidden)]` to `set_length`, as you suggested, and

* Please add a CHANGELOG entry.

I've done those changes, please let me know if there's anything else you want to see.

src/sys/socket/addr.rs

Co-authored-by: Alan Somers <asomers@gmail.com>

asomers · 2023-07-17T21:54:23Z

The linter isn't happy with that change.

JarredAllen · 2023-07-17T21:58:44Z

The linter isn't happy with that change.

Fixed! I had to make an allowance because the variable was only used on some platforms and was left unused on others.

asomers

bors r+

bors · 2023-07-17T22:35:51Z

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2041: Set the length of a socket address when calling `recvmsg` on Linux r=asomers a=JarredAllen # Background I think I've found a bug with `recvmsg` on Linux where it doesn't set the length of a received socket address. I found this while working with unix datagram sockets with path `struct sockaddr_un` addresses, but I think this applies to any variable-length socket address type. At present, the `sun_len` field of `UnixAddr` on Linux shows up as 0 whenever I call `recvmsg`. I think it's reading uninitialized memory (afaict the `recvmsg` function never initializes it before we call `MaybeUninit::assume_init`), though it's possible that it's being zero-initialized somewhere that I missed. Either way, this isn't the correct behavior, which should set it to the length of the `struct sockaddr_un` contents (or whatever other type of socket). # Changes I changed the `recvmsg` function to construct the returned socket address from the `struct sockaddr` and length returned by the libc `recvmsg` call (using the `from_raw` function the trait provides). Since the trait is sealed so downstream crates can't implement it, I believe this shouldn't be a breaking change. # Validation I've tested that my code (which failed due to this bug) now works. I also added a new test case which tests that we construct `UnixAddr`s correctly in the `recvmsg` function, which passes locally for me and fails if I cherry-pick it onto the current `master`. I've also checked that `cargo test` and `cargo clippy` both still pass on `aarch64-apple-darwin` and on `aarch64-unknown-linux-musl` targets (the two targets I develop for/have access to). Hopefully your CI will confirm that everything else still works. Co-authored-by: Jarred Allen <jarred@moveparallel.com> Co-authored-by: Jarred Allen <jarredallen73@gmail.com>

JarredAllen added 2 commits May 19, 2023 16:40

Set the length of a sockaddr received on Linux

905c23e

Add test coverage for bug with unset socket address length

96fa5a8

asomers requested changes May 20, 2023

View reviewed changes

JarredAllen added 2 commits May 22, 2023 11:45

PR suggestion: Set the length later instead of using from_raw

c80828e

PR suggestions: use skip macro

72a2f56

JarredAllen added 2 commits May 22, 2023 12:43

Fixup accidentally-introduced changes

e5bd9ba

Remove redundant unsafe block

ff0fb3d

JarredAllen requested a review from asomers May 22, 2023 19:48

Merge branch 'master' into set-sockaddr-length-linux

3174d85

asomers requested changes Jul 15, 2023

View reviewed changes

JarredAllen added 2 commits July 17, 2023 11:33

More PR feedback

012e788

Merge branch 'master' into set-sockaddr-length-linux

23125cb

JarredAllen requested a review from asomers July 17, 2023 18:38

asomers requested changes Jul 17, 2023

View reviewed changes

src/sys/socket/addr.rs Outdated Show resolved Hide resolved

Correctly let the kernel set the length on BSDs

2378f12

Co-authored-by: Alan Somers <asomers@gmail.com>

JarredAllen requested a review from asomers July 17, 2023 21:45

Fix lint

2d60044

asomers approved these changes Jul 17, 2023

View reviewed changes

bors bot merged commit c375a10 into nix-rust:master Jul 17, 2023

pacak mentioned this pull request Sep 22, 2024

Re-use MultiHeader<()> in subsequent recvmmsg() calls #2506

Closed

SteveLauC mentioned this pull request Oct 31, 2024

initialize msg_name with null pointer when msg_name is empty #2530

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Set the length of a socket address when calling `recvmsg` on Linux #2041

Set the length of a socket address when calling `recvmsg` on Linux #2041

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bors bot commented Jul 17, 2023

	print!("Couldn't send ({e:?}), so skipping test");
	return;
	skip!("Couldn't send ({e:?}), so skipping test");

	/// This method may only be called on socket addresses whose lenghts are dynamic, and it
	/// This method may only be called on socket addresses whose lengths are dynamic, and it

-    unsafe fn set_length(&mut self, new_length: usize) -> std::result::Result<(), SocketAddressLengthNotDynamic>;
+    unsafe fn set_length(&mut self, new_length: usize) -> std::result::Result<(), SocketAddressLengthNotDynamic> {
+        Err(SocketAddressLengthNotDynamic)
+    }

Set the length of a socket address when calling recvmsg on Linux #2041

Set the length of a socket address when calling recvmsg on Linux #2041

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JarredAllen commented May 20, 2023

Background

Changes

Validation

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JarredAllen commented May 22, 2023 • edited Loading

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JarredAllen commented Jul 17, 2023

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bors bot commented Jul 17, 2023

Set the length of a socket address when calling `recvmsg` on Linux #2041

Set the length of a socket address when calling `recvmsg` on Linux #2041

JarredAllen commented May 22, 2023 •

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