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Message-ID: <CAL9cFfMX1a5WhVE6RF2m2W5HYjn_biRNDuC-gRdz__2R2CS41w@mail.gmail.com>
Date: Mon, 13 Jun 2022 14:38:45 -0400
From: Waldek Kozaczuk <jwkozaczuk@...il.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: netlink.c: missing handling of EAGAIN and EWOULDBLOCK
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@...c.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
>
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