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Date: Fri, 9 Aug 2019 10:48:18 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [RFC] final time64 switch-over patch series
On Fri, Aug 02, 2019 at 05:44:33PM -0400, Rich Felker wrote:
> 2. the rusage, utmpx, and timex structs are not correct with regard to
> ABI or functionality. they need to be fixed before this is safe to
> use.
There's also struct sched_param, which contains gratuitous timespecs
for functionality we don't (and probably won't ever) support. This
will gratuitously change size when we change time_t unless action is
taken to prevent it.
My leaning here is to just remove all the sched_ss_* stuff and replace
it with same-sized reserved space. But nothing in libc accesses (read
or write) anything but a single int (sched_prio) via the sched_param
pointer, so we could just leave it and let the size change; this would
not affect any ABI between libc and the libc-consumer, but it would
affect ABI between pairs of libc-consumers using the type (which seems
pretty far-fetched. Also, we would never be able to access the
additional space it's expanded to, because the caller could plausibly
be an old binary that allocated less space. So I think just getting
rid of it all is best.
Note that we lack a proper type (what would essentially be
"__old_time_t") to define same-size replacement for the members to be
removed and replaced with "reserved" space. This is the same type we
need in rusage, timex, and possibly utmpx as well.
I could make alltypes.h define __OLD_TIME_T or something, and
conditionally use it if it's defined. But I think it may be cleaner to
use _REDIR_TIME64 as a proxy for this. If _REDIR_TIME64 is defined,
that means there's an old ABI to be matched, and "old time_t"
necessarily has type long, which is necessarily 32-bit in such a case.
So it could be something like:
struct sched_param {
int sched_priority;
int __reserved1;
#if _REDIR_TIME64
long __reserved2[4];
#else
long long __reserved2[4];
#endif
int __reserved3;
};
Note that the only reason this is needed is x32; otherwise, long would
always be the right size to match the old time_t. But in the other
places, it's more useful:
struct rusage {
#if _REDIR_TIME64
struct {
long tv_sec, tv_usec;
} __ru_utime32, __ru_stime32;
#else
struct timeval ru_utime;
struct timeval ru_stime;
#endif
/* linux extentions, but useful */
long ru_maxrss;
long ru_ixrss;
long ru_idrss;
long ru_isrss;
long ru_minflt;
long ru_majflt;
long ru_nswap;
long ru_inblock;
long ru_oublock;
long ru_msgsnd;
long ru_msgrcv;
long ru_nsignals;
long ru_nvcsw;
long ru_nivcsw;
#if _REDIR_TIME64
long __reserved[8];
struct timeval ru_utime;
struct timeval ru_stime;
#else
long __reserved[16];
#endif
};
Note that using the reserved space to avoid increasing the size here
isn't really useful, since the alignment requirement may change too
due to introduction of 64-bit members, but it might make things more
robust against ABI mismatches (likely SIGBUS instead of OOB
load/store).
Rich
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