Date: Sun, 27 Oct 2019 10:53:56 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH] remaining steps for time64 switchover On Sun, Oct 27, 2019 at 08:32:57AM +0000, Laurent Bercot wrote: > >Or here. So, the story on utmpx: we can either > > > >1. match the current size on 32-bit archs, but move the timeval to > > unused space at the end where a time64 version fits, or > > > >2. match the current size and layout of the 64-bit struct, making it > > possible to share records between 32- and 64-bit processes on the > > same machine. > > > >Keep in mind that this struct is not used anywhere in libc presently, > >but normally it's used as a format for on-disk records. > > > >I'm kinda leaning towards option 2, but being that I don't use (and > >hate) utmp, I'd rather hear opinions from people who do use it. Either > >way time fields in existing data will break, so it's a question of > >whether that one-time breakage is already sufficient to go a bit > >further and get 32/64 compat afterwards. > > I don't use the libc's utmpx, but I maintain utmps, which is a > secure implementation of utmp, including the definition of struct > utmpx. > I haven't been following the time64 thing closely. The current struct > utmpx definition includes a struct timeval. Will it need to change, > or will musl's struct timeval change be enough and naturally propagate > so the struct utmpx will become time64-compatible? It will naturally propagate even if nothing is done, but then you have the worst of both worlds of 1 and 2. You neither maintain the size and layout of other members (which would be useful if you have old data or a mix of old and new binaries using it) nor gain any useful compatibility (between 32- and 64-bit on same system). You just get a new size and layout that doesn't match either. So it's better to make some change, I think. Note that the difference between "do nothing" and option 2 is basically nothing except putting padding around ut_session so that ut_tv will start on a 0 mod 8 boundary. This will happen naturally on most archs but not i386 (and m68k but there's no corresponding 64-bit arch there anyway). The proposal I have in mind is basically: - long ut_session; +#if __BYTE_ORDER == 1234 + int ut_session, __ut_pad2; +#else + int __ut_pad2, ut_session; +#endif doing this for 64-bit too since ut_session is semantically 32-bit (pid_t) and glibc has it as 32-bit on x86_64. (I'd also add explicit padding for ut_type just because m68k is wacky and doesn't align ints even, to fix that while we have the chance.) > On-disk data is not a problem. On the distro that I know uses utmps > (Adélie), the utmp/wtmp records, by design, do not survive a reboot, > so a reboot will fix everything - and will be mandatory anyway on > arches where the musl ABI changes. Reboot is not mandatory; as usual, just atomic replacement of libc.so is. > I'm not aware of any distribution that uses musl, doesn't use utmps, > and still keeps on-disk utmpx records. Thanks. Rich
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