mailing list of musl libc
 help / color / mirror / code / Atom feed
From: Waldek Kozaczuk <jwkozaczuk@gmail.com>
To: Rich Felker <dalias@libc.org>
Cc: musl@lists.openwall.com
Subject: Re: [musl] netlink.c: missing handling of EAGAIN and EWOULDBLOCK
Date: Mon, 13 Jun 2022 14:38:45 -0400	[thread overview]
Message-ID: <CAL9cFfMX1a5WhVE6RF2m2W5HYjn_biRNDuC-gRdz__2R2CS41w@mail.gmail.com> (raw)
In-Reply-To: <20220613170849.GG7074@brightrain.aerifal.cx>

[-- Attachment #1: Type: text/plain, Size: 2292 bytes --]

Hi,

I appreciate your reply.

Unfortunately, I am not an expert on netlink but another thing I have found
is that for comparison Golang uses a 0 flags argument when reading from
netlink socket -
https://github.com/golang/go/blob/master/src/syscall/netlink_linux.go#L79.

On a side note, I could not replicate the EAGAIN errors in Linux so you may
be right that Linux has some special handling of recv() for netlink sockets
which OSv might be missing. But I could not find anything in the
official netlink docs - https://man7.org/linux/man-pages/man7/netlink.7.html
and https://man7.org/linux/man-pages/man7/rtnetlink.7.html. And if that is
undocumented, shall we rely on it implicitly?

Waldek

On Mon, Jun 13, 2022 at 1:08 PM Rich Felker <dalias@libc.org> wrote:

> On Mon, Jun 13, 2022 at 11:41:57AM -0400, Waldek Kozaczuk wrote:
> > Hi,
> >
> > Very recently we implemented minimal rnetlink support on OSv side which
> > allowed us to finally switch to the netlink-based implementation of
> > getifaddrs() and if_nameindex().
> >
> > However, I noticed that the function __netlink_enumerate() in
> > https://github.com/ifduyue/musl/blob/master/src/network/netlink.c uses
> > MSG_DONTWAIT flag when calling recv() which may fail with EAGAIN or
> > EWOULDBLOCK and there is no error/retry handling for that. I actually saw
> > both functions fail occasionally on OSv.
> >
> > One way to fix is to add missing error handling. But another simpler
> > solution is to stop using MSG_DONTWAIT altogether and force recv() to
> > block. In other words, the line:
> >
> > r = recv(fd, u.buf, sizeof(u.buf), MSG_DONTWAIT);
> >
> > should change to:
> >
> > r = recv(fd, u.buf, sizeof(u.buf), 0);
> >
> > For time being we are applying a header trick on OSv side to re-define
> > MSG_DONTWAIT as 0 when compiling those specific musl sources.
>
> Thanks! I'll try to track this down. One concern is that I'm not sure
> how MSG_DONTWAIT is supposed to interact with "short reads" -- is it
> needed (for netlink) to prevent blocking when some data has been read
> but there is still buffer space for more?
>
> On a related issue, I'm pretty sure the netlink API doesn't allow for
> partial reads with some data remaining buffered on the kernel side,
> but we should probably verify that too.
>
> Rich
>

[-- Attachment #2: Type: text/html, Size: 3219 bytes --]

  reply	other threads:[~2022-06-13 18:46 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-06-13 15:41 Waldek Kozaczuk
2022-06-13 17:08 ` Rich Felker
2022-06-13 18:38   ` Waldek Kozaczuk [this message]
2022-06-14  5:19   ` Timo Teras

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=CAL9cFfMX1a5WhVE6RF2m2W5HYjn_biRNDuC-gRdz__2R2CS41w@mail.gmail.com \
    --to=jwkozaczuk@gmail.com \
    --cc=dalias@libc.org \
    --cc=musl@lists.openwall.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
Code repositories for project(s) associated with this public inbox

	https://git.vuxu.org/mirror/musl/

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).