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Date: Tue, 4 Aug 2020 00:02:58 -0700
From: Andre McCurdy <armccurdy@...il.com>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] extend gethostid beyond a stub

On Mon, Aug 3, 2020 at 8:02 PM Rich Felker <dalias@...c.org> wrote:
>
> 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).

The glibc implementation appears to read and write directly into an
int32_t variable, without any endianness conversion. To be
interoperable with /etc/hostid files created by glibc shouldn't musl
skip the ntohl() and just return 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.

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