Re: [musl] Re: musl getaddr info breakage on older kernels

[Rich Felker <dalias@aerifal.cx>]

On Sun, Feb 06, 2022 at 08:29:16PM -0500, Satadru Pramanik wrote:
> Here are illustrative logs of output and strace logs.
> 
> Note that while the musl toolchain is built in a container on a much more
> powerful machine, this "musl_getaddrinfo_test" app is built locally on the
> machine with the 3.8 kernel.
> 
> I ran the following to get the output on the smaller i686 machine
> immediately after the app is built.
> Apologies for the ruby code wrapping the shell commands.
> 
>     @musl_ver = `#{CREW_MUSL_PREFIX}/lib/libc.so 2>&1 >/dev/null | head -2
> | tail -1 | awk '{print $2}'`.chomp
>     puts 'Testing the musl resolver to see if it can resolve google.com:
> '.lightblue
>     system "./musl_getaddrinfo_test google.com set_ai_family 2>&1 |tee -a
> /tmp/musl_#{@musl_ver}_getaddrinfo_test_google.com_set_ai_family.txt "
>     system "./musl_getaddrinfo_test google.com 2>&1 |tee -a
> /tmp/musl_#{@musl_ver}_getaddrinfo_test_google.com.txt"
>     system "strace -o
> /tmp/musl_#{@musl_ver}_getaddrinfo_test_google.com_set_ai_family_STRACE.txt
> ../musl_getaddrinfo_test google.com set_ai_family"
>     system "strace -o
> /tmp/musl_#{@musl_ver}_getaddrinfo_test_google.com_STRACE.txt
> ../musl_getaddrinfo_test google.com"
> 
> And here is the output for each run before running again via strace. Note
> how IPv6 addresses show up sporadically, and for 1.2.2 nothing at all shows
> up, but everything works fine according to the strace logs. (Strace is
> built against glibc 2.23.)
> 
> ==>
> musl_1.2.0-git-17-g33338ebc_getaddrinfo_test_google.com_set_ai_family.txt
> <==
> AF_INET: 142.251.40.110
> 
> ==> musl_1.2.0-git-17-g33338ebc_getaddrinfo_test_google.com.txt <==
> AF_INET: 142.251.40.110
> 
> ==>
> musl_1.2.0-git-39-g5cf1ac24_getaddrinfo_test_google.com_set_ai_family.txt
> <==
> AF_INET: 142.251.40.142
> 
> ==> musl_1.2.0-git-39-g5cf1ac24_getaddrinfo_test_google.com.txt <==
> getaddrinfo: Try again
> 
> ==>
> musl_1.2.0-git-40-g1b4e84c5_getaddrinfo_test_google.com_set_ai_family.txt
> <==
> AF_INET: 142.251.40.206
> 
> ==> musl_1.2.0-git-40-g1b4e84c5_getaddrinfo_test_google.com.txt <==
> AF_INET6: 2607:f8b0:4006:81f::200e
> AF_INET: 142.251.40.206
> 
> ==>
> musl_1.2.0-git-6-g2f2348c9_getaddrinfo_test_google.com_set_ai_family.txt <==
> AF_INET: 142.250.65.206
> 
> ==> musl_1.2.0-git-6-g2f2348c9_getaddrinfo_test_google.com.txt <==
> AF_INET: 142.250.65.206
> 
> ==> musl_1.2.1_getaddrinfo_test_google.com_set_ai_family.txt <==
> AF_INET: 142.251.40.110
> 
> ==> musl_1.2.1_getaddrinfo_test_google.com.txt <==
> getaddrinfo: Try again
> 
> ==> musl_1.2.2_getaddrinfo_test_google.com_set_ai_family.txt <==
> getaddrinfo: Try again
> 
> ==> musl_1.2.2_getaddrinfo_test_google.com.txt <==
> getaddrinfo: Try again
> 
> Regards,

OK, I don't see anything in the strace suggesting a cause. The kernel
version (or whether a container was used) present on the system where
you built musl or the test programs should make no difference
whatsoever; musl has no build dependencies on the host kernel or
kernel headers or anything like that (and doesn't even need to be
built on a Linux host).

A couple questions:

Are the test programs on the i686 machine running under Docker or any
other container environment?

Can you tcpdump the traffic between the test program and the dnsmasq
during a failing run, with verbose display of the packet contents
(-vvv or something like that)?

I don't see any plausible explanation for the result varying between
runs and with timing like this unless dnsmasq is doing something
odd/wrong. I thought it might be related to something blocking time64
syscalls but that doesn't seem to be the case -- according to the
strace logs they're getting ENOSYS as expected with fallback to the
legacy 32-bit clock_gettime etc. which is fine.