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Date: Thu, 18 Jul 2019 16:52:37 -0400
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: time_t progress/findings
On Thu, Jul 18, 2019 at 12:37:45PM -0400, Rich Felker wrote:
> Second bit of progress here: stat. First change can be done before any
> actual time64 work is done or even decided upon: changing all the
> stat-family functions to use fstatat (possibly with AT_EMPTY_PATH) as
> their backend, and changing fstatat to do proper fallbacks if
> SYS_fstatat is missing. Now there's a single point of potential stat
> conversion rather than 4 functions.
>
> Next, add an internal, arch-provided kstat type and make fstatat
> translate from this to the public stat type. This eliminates the need
> for all the mips*/syscall_arch.h hacks.
This step admits a few questions about how to do it best, inspired in
part by a related question:
What should the new time64 stat structures look like?
There are at least three possible goals:
1. Make them as clean and uniform as possible, same for all archs.
2. Avoid increasing the size at all cost so as to maximize
memory-safety of mismatched interfaces between libc consumers
defined in terms of struct stat.
3. Make the start of the new struct match the old struct to minimize
behavioral errors under mismatched interfaces between libc
consumers defined in terms of struct stat.
Choice 2 is pretty much out because I think it's impossible on at
least one arch, and would impose really ugly constraints (making
timespec 24-byte, relying on non-64bit-alignment) on others. In many
ways choice 3 is actually more appealing, because when third-party
libraries *do* use stat in public interfaces, it's usually understood
that the same party both allocates and fills it in, and shares the
contents with the other party.
There are actually 2 subvariants of choice 3: either keep exposing the
32-bit time in the old locations so that mismatched consumers just
work, or fill it in with something like INT_MIN (year~=1902) so that
breakage is caught quickly.
Now, back to kstat and the above-quoted text. If we go with option 3,
we don't actually need a kstat struct. The existing stat syscalls just
write into the beginning of the buffer, and then we copy the result to
the time64 timespecs at the end that make up the new public interface.
This results in the smallest code, and the least amount of new
per-arch definitions. But it doesn't clean up the existing mips
stat-translation hell (currently buried in mips*/syscall_arch.h), and
it imposes assumptions about the relationship between kernel types and
public libc types.
On the other hand, if we make archs define a struct kstat and always
translate everything, the code is a bit larger, but we:
- don't impose any particular choice 1/2/3 above.
- make it easy to cleanup the mips brokenness.
- facilitate future musl archs/ABIs (e.g. a ".2 ABI") where userspace
stat has nothing to do with the legacy kernel stat structs.
So I'm leaning strongly towards just always doing the translation,
even though I'm also leaning towards choice 3 above that won't require
it. If nothing else, it allows me to do the prep work that will set
the stage for time64 transition now, without having finalize the
decisions about how time64 will look.
Rich
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