Re: [musl] graceful nscd fallback on kernels without AF_UNIX support

[Rich Felker <dalias@libc.org>]

On Sat, Apr 03, 2021 at 12:58:42PM +0200, Joakim Sindholt wrote:
> I was reminded of this coincidentally on IRC and apparently it was never
> applied, presumably because I never actually sent a proper patch.
> 
> 5 years ago someone came in asking about musl on a system without
> AF_UNIX support and this patch is what we came up with. It does require
> access to /dev/null which might itself be a problem now that musl is
> shedding filesystem requirements.

It's not really a problem, but it's also not necessary. fmemopen of an
empty buffer would also work, but might be undesirable for pulling in
more unnecessary code. So this is probably best as you wrote it:

> >From 9984ac4ad320844a91bbcdbc9f5fa07d3154621e Mon Sep 17 00:00:00 2001
> From: Joakim Sindholt <opensource@zhasha.com>
> Date: Sat, 3 Apr 2021 12:50:18 +0200
> Subject: [PATCH] nscd: fall back gracefully on kernels without AF_UNIX support
> 
> ---
>  src/passwd/nscd_query.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/src/passwd/nscd_query.c b/src/passwd/nscd_query.c
> index d38e371b..8641e4f0 100644
> --- a/src/passwd/nscd_query.c
> +++ b/src/passwd/nscd_query.c
> @@ -40,7 +40,13 @@ retry:
>  	buf[0] = NSCDVERSION;
>  
>  	fd = socket(PF_UNIX, SOCK_STREAM | SOCK_CLOEXEC, 0);
> -	if (fd < 0) return NULL;
> +	if (fd < 0) {
> +		if (errno == EACCES || errno == EAFNOSUPPORT || errno == EPROTONOSUPPORT) {
> +			errno = errno_save;
> +			return fopen("/dev/null", "re");
> +		}
> +		return 0;
> +	}

What is the EACCESS case for here? EPROTONOSUPPORT is probably not
needed either (is it possible to have AF_UNIX supported but stream
mode support disabled?). Not a big deal but I'd rather not violate
YAGNI adding cases that don't have a legitimate reason to be possible.

Rich