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From: Rich Felker <dalias@libc.org>
To: Sascha Braun <sascha.braun.lpz@googlemail.com>
Cc: musl@lists.openwall.com
Subject: Re: [musl] Problably Issue in name_from_dns // __res_msend_rc (lookup_name.c)
Date: Thu, 2 Jun 2022 10:25:05 -0400	[thread overview]
Message-ID: <20220602142504.GY7074@brightrain.aerifal.cx> (raw)
In-Reply-To: <CALATqHkTXq7ODHXiPGeGvzka-4pbmDZ=-2WEDXhCLmC52YGw5A@mail.gmail.com>

On Wed, Jun 01, 2022 at 11:52:46PM +0200, Sascha Braun wrote:
> I see - getaddrinfo always returns -2, EAI_NONAME in case of these rare
> errors.
> 
> The errno value (EAGAIN) came from my syscall, recvfrom, which does not
> have data in these cases (all packets indicated by 'poll' were read
> previously by recvfrom calls). (Btw., I did try to return 0 for these cases
> in my recvfrom implementation, did not help either.)

It's normal to see these EAGAINs. They are not errors. Returning 0
would be wrong; a zero-length read means EOF not EAGAIN.

I don't see an immediate cause visible for why you're getting
EAI_NONAME. Note that your debug output does not seem to show the
*length* recvfrom returned, so is it possible you're just showing old
data that was already in the buffer, when recvfrom really
malfunctioned and returned a short read? At the res_msend loop layer
musl would possibly accept this (like you seem to be seeing) but then
find no results later.

Rich



> 
> Thanks
> 
> Am Mi., 1. Juni 2022 um 23:30 Uhr schrieb Rich Felker <dalias@libc.org>:
> 
> > On Wed, Jun 01, 2022 at 10:35:52PM +0200, Sascha Braun wrote:
> > > Hi Rich,
> > >
> > > Thanks for your time. The EADDRINUSE is in reality an EAGAIN, this is a
> > > glitch in the Emscripten/WASI modification for musl (there are different
> > > values for errnos per implementation):
> > > __WASI_ERRNO_ADDRINUSE == -3 == EAI_AGAIN.
> > >
> > > My question currently breaks down to: Is it common for this
> > implementation
> > > of getaddrinfo to return EAGAIN sometimes (I experience 1:100), even if
> > > answers from DNS servers came in?
> > > If not, I need to investigate much further. I really could not find
> > issues
> > > with my sockets implementation, although your thought is of course very
> > > reasonable.
> >
> > OK, looking at your code again, part of your problem is that you're
> > misusing perror. You need to save the return value of getaddrinfo.
> > Unless it's EAI_SYSTEM, errno is meaningless and you need to use
> > gai_strerror to get a string for the error code (the value that was
> > returned) rather than inspecting errno.
> >
> > If getaddrinfo actually returned EAI_AGAIN, this probably means you
> > got an inconclusive result from one of the nameservers, probably
> > ServFail. However if your debug output is packet dumps, I'm not seeing
> > a ServFail there, and I'm not convinced it actually returned EAI_AGAIN
> > since you're not saving the value to check it. The value that happens
> > to be in errno is NOT the error getaddrinfo returned. Can you check
> > what it's actually returning?
> >
> >
> > > Am Mi., 1. Juni 2022 um 16:14 Uhr schrieb Rich Felker <dalias@libc.org>:
> > >
> > > > On Tue, May 31, 2022 at 12:24:19PM +0200, Sascha Braun wrote:
> > > > > Hi everyone,
> > > > >
> > > > > I'm implementing a socket protocol with similar works to Emscripten
> > - ok:
> > > > >
> > > > > Here's what I noticed
> > > > >
> > > > > - resolv.conf using multiple DNS servers
> > > > > options timeout:22 attempts:5
> > > > > nameserver 8.8.4.4
> > > > > nameserver 208.67.222.222
> > > > > nameserver 9.9.9.9
> > > > > nameserver 1.1.1.1
> > > > >
> > > > > - getaddrinfo with no hints, so that IPV6 and IPV4 is resolved
> > > > > void socketstest1_client_dnsonly(void) {
> > > > >
> > > > > struct addrinfo hints, * res;
> > > > > memset(&hints, 0, sizeof(hints));
> > > > > printf("begin lookup1...\n");
> > > > > if (getaddrinfo("www.web.de", "80", &hints, &res) != 0) {
> > > > > perror("getaddrinfo1");
> > > > > }
> > > > > printf("begin lookup2...\n");
> > > > > if (getaddrinfo("www.google.de", "80", &hints, &res) != 0) {
> > > > > perror("getaddrinfo2");
> > > > > }
> > > > > printf("begin lookup3...\n");
> > > > > if (getaddrinfo("www.google.com", "80", &hints, &res) != 0) {
> > > > > perror("getaddrinfo3");
> > > > > }
> > > > >
> > > > > }
> > > > >
> > > > > When repeating socketstest1_client_dnsonly often, it appears that
> > IPV6
> > > > > answers for IPV4 requests to another server [or vice-versa]
> > responses can
> > > > > get mixed up and getaddrinfo reports sometimes only an error where
> > none
> > > > is.
> > > > >
> > > > > Below is a console dump of my test where you see the critical
> > situation
> > > > > (end)
> > > > > 8.8.4.4:53 was queried, 9.9.9.9:53 responded for a (different IP
> > Proto),
> > > > > resulting in error getaddrinfo1: Address in use
> > > > >
> > > > > This happens in about 1/100 tests. When you specify IPV4 or IPV6 in
> > > > hints,
> > > > > this issue does NOT show up.
> > > >
> > > > Can you provide more information on how to interpret the console dump,
> > > > and in particular, which of your syscalls is returning the EADDRINUSE
> > > > error? I'm pretty sure this is just a bug in your socket
> > > > stack/emulation. It looks like it's coming from recvfrom, and
> > > > EADDRINUSE is not a valid error for recvfrom to produce.
> > > >
> > > > Rich
> > > >
> >

  reply	other threads:[~2022-06-02 14:25 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-05-31 10:24 Sascha Braun
2022-06-01 14:14 ` Rich Felker
2022-06-01 20:35   ` Sascha Braun
2022-06-01 21:30     ` Rich Felker
2022-06-01 21:52       ` Sascha Braun
2022-06-02 14:25         ` Rich Felker [this message]
2022-06-02 22:28           ` Sascha Braun
2022-06-03 12:27             ` Rich Felker
2022-06-03 12:34               ` Rich Felker
2022-06-03 13:44                 ` Sascha Braun
2022-06-05 18:14                 ` Sascha Braun
2022-06-05 18:47                   ` Rich Felker

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