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Message-ID: <20200804030201.GG6949@brightrain.aerifal.cx>
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, Érico Rolim wrote:
> From: Érico 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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