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Date: Mon, 3 Aug 2020 23:02:01 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] extend gethostid beyond a stub

On Mon, Aug 03, 2020 at 05:55:29PM -0300, √Črico Rolim wrote:
> From: √Črico Rolim <erico.erc@...il.com>
> 
> Implement part of the glibc behavior, where the 32-bit identifier stored
> in /etc/hostid, if the file exists, is returned. If this file doesn't
> contain at least 32 bits or can't be opened for some reason, return 0.
> ---
>  src/misc/gethostid.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/src/misc/gethostid.c b/src/misc/gethostid.c
> index 25bb35db..e2e98b99 100644
> --- a/src/misc/gethostid.c
> +++ b/src/misc/gethostid.c
> @@ -1,6 +1,19 @@
>  #include <unistd.h>
> +#include <stdio.h>
>  
>  long gethostid()
>  {
> -	return 0;
> +	FILE *f;
> +	unsigned char hostid[4];
> +	long rv = 0;
> +
> +	f = fopen("/etc/hostid", "reb");
> +	if (f) {
> +		if (fread(hostid, 1, 4, f) == 4) {
> +			rv = (hostid[3] << 24) | (hostid[2] << 16) | (hostid[1] << 8) | hostid[0];
> +		}
> +		fclose(f);
> +	}
> +
> +	return rv;
>  }
> -- 
> 2.28.0

I somewhat dislike the use of stdio here, but this is something of a
junk function that's not really worth writing read() retry loop, etc.

hostid[3]<<24 is UB due to integer overflow (the promoted type is int,
a signed type). This could be fixed via promotion to unsigned before
the shift, but rather than constructing the value manually like this
I'd probably lean towards reading it into a uint32_t object x then
returning ntohl(x).

It's unfortunate that fopen can fail for spurious reasons like ENOMEM
or EMFILE/ENFILE, and that gethostid has no way of reporting this
error rather than returning the wrong id, but this seems like a
fundamental design bug in the interface and not something we can fix,
at least not while using the existing design with data in a file. I
think it could be avoided by using readlink() and storing the id in
the contents of a symlink, which should have no spurious failure
modes, but I'm not really keen on inventing a new convention for this
fundamentally-broken function.

So overall this looks pretty good. I'll revisit it after release and
see if anyone else has thoughts on it in the mean time.

Rich

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