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Date: Sun, 19 Jan 2020 19:42:26 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: [RFC] removing __NR_clock_gettime / SYS_clock_gettime
* Rich Felker <dalias@...c.org> [2020-01-19 13:16:43 -0500]:
> On Sun, Jan 19, 2020 at 06:51:17PM +0100, Szabolcs Nagy wrote:
> > * Rich Felker <dalias@...c.org> [2020-01-19 11:36:16 -0500]:
> > > Today we discovered that libstdc++ std::chrono is broken because it's
> > > making direct syscalls to SYS_clock_gettime to work around glibc
> > > putting clock_gettime in librt. This is exactly the same issue as
> > > busybox https://bugs.busybox.net/show_bug.cgi?id=12091 and I would not
> > > be surprised if it exists in more software. It's a silent bug that's
> > > easy to find and fix if you know what to look for, but very confusing
> > > and hard to find if you don't, and it can easily slip into software
> > > that's not well-tested on time64.
> > >
> > > What I'd like to propose doing is removing __NR_clock_gettime and
> > > SYS_clock_gettime from the public sys/syscall.h (via bits headers) on
> > > 32-bit archs, and moving SYS_clock_gettime to
> > > arch/$(ARCH)/syscall_arch.h for musl-internal use. This would make it
> > > a hard compile-time error for any software attempting to use the
> > > syscall directly, and in the case of libstdc++ I think it would even
> > > fix the problem without patching gcc, since they have a configure
> > > check for the syscall.
> > >
> > > Thoughts? Is this too big a hammer?
> >
> > i think you should build gcc with --enable-libstdcxx-time so
> > it does not try to do raw syscalls (which is bad on 64bit
> > targets too, not just for time64, i thought distros already
> > do this or patch out that entire thing)
>
> It does raw syscalls with that as I understand it. You need =rt to
> make it do the right thing.
--enable-libstdcxx-time is default in mcm since
commit 0291cc44eee410270a97efb6258394c1f1f8352a
Commit: Rich Felker <dalias@...ifal.cx>
CommitDate: 2016-05-06 18:37:09 +0000
and the libstdc++ i built with that only has SYS_futex
syscalls in it on all targets.
now i see that alpine libstdc++ has a raw clock_gettime
syscall in it too, alpine should fix that.
>
> But we know how to fix this for gcc now. I'm more concerned that if we
> already caught busybox and libstdc++ doing this, there may be lots
> more apps doing it that we don't know about...
i see,
i'm not sure what's the right solution.
we can try to fix them or break their build.
some usage may be valid though.
> > i'd ask the glibc folks if they want to do something about this
> > when building for the time64 abi.
>
> I think they just use the kernel headers to provide sys/syscall.h.
well if there are really raw syscall users with
libc type then glibc will have a problem too,
so either the user code gets fixed or glibc
does some workaround.
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