Re: [musl] runtime error: initgroups(www-data, 33) failed (5: I/O error)

[Rich Felker <dalias@libc.org>]

On Sat, Oct 03, 2020 at 09:06:50AM +0800, Static Php wrote:
> I has this runtime error on Ubuntu 18.04.5 LTS, CPU is AMD EPYC Processor.
> 
> Kernel: 5.4.0-49-generic #53~18.04.1-Ubuntu (other kernel also has this
> problem)
> 
> more details: https://github.com/richfelker/musl-cross-make/issues/107
> 
> strace:
> 
> execve("./a.out", ["./a.out"], 0x7fff12cd26d0 /* 20 vars */) = 0
> >
> > arch_prctl(ARCH_SET_FS, 0x7ff53bb3b618) = 0
> >
> > set_tid_address(0x7ff53bb3bbe8)         = 40778
> >
> > socket(AF_UNIX, SOCK_STREAM|SOCK_CLOEXEC, 0) = 3
> >
> > brk(NULL)                               = 0x555556f23000
> >
> > brk(0x555556f25000)                     = 0x555556f25000
> >
> > mmap(0x555556f23000, 4096, PROT_NONE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS,
> > -1, 0) = 0x555556f23000
> >
> > mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
> > 0x7ff53bb39000
> >
> > connect(3, {sa_family=AF_UNIX, sun_path="/var/run/nscd/socket"}, 24) = 0
> >
> > sendmsg(3, {msg_name=NULL, msg_namelen=0,
> > msg_iov=[{iov_base="\2\0\0\0\17\0\0\0\t\0\0\0", iov_len=12},
> > {iov_base="www-data\0", iov_len=9}], msg_iovlen=2, msg_controllen=0,
> > msg_flags=0}, MSG_NOSIGNAL) = 21
> >
> > readv(3, [{iov_base="\2\0\0\0\1\0\0\0\0\0\0", iov_len=11}, {iov_base="\0",
> > iov_len=1024}], 2) = 12
> >
> > mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
> > 0x7ff53bb38000
> >
> > close(3)                                = 0
> >
> > munmap(0x7ff53bb39000, 4096)            = 0
> >
> > munmap(0x7ff53bb38000, 4096)            = 0
> >
> > ioctl(1, TIOCGWINSZ, {ws_row=59, ws_col=225, ws_xpixel=1575,
> > ws_ypixel=826}) = 0
> >
> > writev(1, [{iov_base="err=-1, errno=5", iov_len=15}, {iov_base="\n",
> > iov_len=1}], 2err=-1, errno=5
> >
> > ) = 16
> >
> > exit_group(0)                           = ?
> >
> > +++ exited with 0 +++
> >

Ah, this looks like a bug in musl causing a zero-groups response from
nscd to be interpreted as an error rather than success with no
members:

	if (!fread(nscdbuf, sizeof(*nscdbuf)*resp[INITGRNGRPS], 1, f)) {
		if (!ferror(f)) errno = EIO;
		goto cleanup;
	}

The problem is that this code was written assuming the fread call
returns 1 on success, but fread has a stupid corner case (which we
used to get wrong) where a zero-length read is required by the
standard to return 0 even though logically it should return nmemb.

You can work around the problem by adding www-data to a useless dummy
group. I'll prepare a patch for musl, though, and post it here as a
follow-up soon.

Thanks for the report!

Rich