Re: [RFC PATCH v4 1/5] glibc: Perform rseq(2) registration at nptl init and thread creation

Rich Felker
2018-11-23T17:31:26Z

On Fri, Nov 23, 2018 at 12:05:20PM -0500, Mathieu Desnoyers wrote:
> ----- On Nov 23, 2018, at 9:28 AM, Rich Felker dalias@libc.org wrote:
> [...]
> >
> > Absolutely. As long as it's in libc, implicit destruction will happen.
> > Actually I think the glibc code shound unconditionally unregister the
> > rseq address at exit (after blocking signals, so no application code
> > can run) in case a third-party rseq library was linked and failed to
> > do so before thread exit (e.g. due to mismatched ref counts) rather
> > than respecting the reference count, since it knows it's the last
> > user. This would make potentially-buggy code safer.
>
> OK, let me go ahead with a few ideas/questions along that path.
^^^^^^^^^^^^^^^
>
> Let's say our stated goal is to let the "exit" system call from the
> glibc thread exit path perform rseq unregistration (without explicit
> unregistration beforehand). Let's look at what we need.

This is not "along that path". The above-quoted text is not about
assuming it's safe to make SYS_exit without unregistering the rseq
object, but rather about glibc being able to perform the
rseq-unregister syscall without caring about reference counts, since
it knows no other code that might depend on rseq can run after it.

> First, we need the TLS area to be valid until the exit system call
> is invoked by the thread. If glibc defines __rseq_abi as a weak symbol,
> I'm not entirely sure we can guarantee the IE model if another library
> gets its own global-dynamic weak symbol elected at execution time. Would
> it be better to switch to a "strong" symbol for the glibc __rseq_abi
> rather than weak ?

This doesn't help; still whichever comes first in link order would
override. Either way __rseq_abi would be in static TLS, though,
because any dynamically-loaded library is necessarily loaded after
libc, which is loaded at initial exec time.

> There has been presumptions about signals being blocked when the thread
> exits throughout this email thread. Out of curiosity, what code is
> responsible for disabling signals in this situation ? Related to this,
> is it valid to access a IE model TLS variable from a signal handler at
> _any_ point where the signal handler nests over thread's execution ?
> This includes early start and just before invoking the exit system call.

It should be valid to access *any* TLS object like this, but the
standards don't cover it well. Right now access to dynamic TLS from
signal handlers is unsafe in glibc, but static is safe.

Rich