Re: Rseq registration: Google tcmalloc vs glibc

[Joel Fernandes]

On Thu, Feb 20, 2020 at 04:33:30PM -0500, Mathieu Desnoyers wrote:
> Hi Chris,
> 
> As one of the maintainers of the Rseq system call in the Linux kernel, I would
> like to thank the Google team for open sourcing a tcmalloc implementation based
> on Rseq!
> 
> I've looked into some critical integration aspects of the tcmalloc implementation,
> and would like to bring up a topic which involves both tcmalloc developers and the
> glibc community.
> 
> I have been discussing aspects of co-existence between early Rseq adopter libraries
> and glibc for the past year with the glibc community, and tcmalloc happens to be the
> first project to publicly use Rseq outside of prototype branches or selftests code.
> Considering that there can only be one Rseq registration per thread (as imposed by
> the rseq ABI), there needs to be some kind of protocol between libraries to ensure we
> don't introduce regressions when we eventually combine a newer glibc which takes care
> of registration of the __rseq_abi TLS along with tcmalloc which also try to perform
> that registration within the same thread.
> 
> Throughout the various rounds of review of the glibc Rseq integration patch set,
> there were a few solutions envisioned. Here is a brief history:
> 
> 1) Introduce a __rseq_refcount TLS variable.
> 
> - Currently used by Linux tools/testing/selftests/rseq/rseq.c,
> - Currently used by Google tcmalloc,
> - Emitted by glibc as well my the original patchset (but was later removed),
> 
> A user incrementing the refcount from 0 -> 1 performs rseq registration.
> The last user decrementing from 1 -> 0 performs rseq unregistration.
> 
> Works for co-existence of dlopen'd/dlclose'd libraries, for dynamically
> linked libraries, and for the main executable.
> 
> The refcounting was deemed too complex for glibc's needs (it always
> exists for the entire executable's lifetime), so we moved to
> __rseq_handled instead.
> 
> 
> 2) Introduce a __rseq_handled global variable.
> 
> - Currently used by Linux tools/testing/selftests/rseq/rseq.c,
> - At some point emitted by glibc as well in my patch set (but was later
>   removed),
> 
> A library may take rseq ownership if it is still 0 when executing the
> library constructor. Set to 1 by library constructor when handling rseq.
> Set to 0 in destructor if handling rseq.
> 
> Not meant to be set by dlopen'd/dlclose'd libraries, only by libraries
> existing for the whole lifetime of the executable and/or the main executable.
> 
> This __rseq_handled symbol has been identified as being somewhat redundant
> with the information provided in the __rseq_abi.cpu_id field (uninitialized
> state), which motivated removing this symbol from the glibc integration
> entirely. The only reason for having __rseq_handled separate from
> __rseq_abi.cpu_id was because it was then impossible to touch TLS data
> early in the glibc initialization. This issue was later resolved within
> glibc.
> 
> 
> 3) Use the  __rseq_abi TLS cpu_id field to know whether Rseq has been
> registered.
> 
> - Current protocol in the most recent glibc integration patch set.
> - Not supported yet by Linux kernel rseq selftests,
> - Not supported yet by tcmalloc,
> 
> Use the per-thread state to figure out whether each thread need to register
> Rseq individually.
> 
> Works for integration between a library which exists for the entire lifetime
> of the executable (e.g. glibc) and other libraries. However, it does not
> allow a set of libraries which are dlopen'd/dlclose'd to co-exist without
> having a library like glibc handling the registration present.

Mathieu, could you share more details about why during dlopen/close
libraries we cannot use the same __rseq_abi TLS to detect that rseq was
registered?

Thanks!

  - Joel


> 
> So overall, I suspect the protocol we want for early adopters is that they
> only register Rseq if __rseq_abi.cpu_id == RSEQ_CPU_ID_UNINITIALIZED, which
> ensure they do not get -1, errno = EBUSY when linked against a newer glibc
> which handles Rseq registration. In order to handle multiple early adopters
> dlopen'd/dlclose'd in the same executable, those should synchronize with a
> __rseq_refcount TLS reference count, but it does not have to be taken into
> account by the main executable or libraries present for the entire executable
> lifetime (like glibc).
> 
> Based on this, what I think would be missing from the current Google tcmalloc
> implementation is a check for __rseq_abi.cpu_id == RSEQ_CPU_ID_UNINITIALIZED
> in InitThreadPerCpu().
> 
> Is tcmalloc ever meant to be dlopen'd/dlclose'd (either directly or indirectly),
> or is it required to exist for the entire executable lifetime ? The check and
> increment of __rseq_refcount is only useful to co-exist with dlopen'd/dlclose'd
> libraries, but it would not allow discovering the presence of a glibc which
> takes care of the rseq registration with the planned protocol. A dlopen'd
> library should then only perform rseq unregistration if if brings the
> __rseq_refcount back to 0 (e.g. in a pthread_key destructor).
> 
> Adding this check for __rseq_abi.cpu_id == RSEQ_CPU_ID_UNINITIALIZED is something
> I need to do in the Linux rseq selftests, but I refrained from submitting any
> further change to those tests until the glibc rseq integration gets finally
> merged.
> 
> Is it something that could be easily changed at this stage in Google tcmalloc,
> or should we reconsider adding back __rseq_refcount within the glibc integration
> patch set, even though it is not strictly useful to glibc ?
> 
> Thanks,
> 
> Mathieu
> 
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com