[REGRESSION] rseq: refactoring in v6.19 broke everyone on arm64 and tcmalloc everywhere

[Florian Weimer]

* Thomas Gleixner:

> The real question is how to differentiate between the legacy and the
> optimized mode. I have two working variants to achieve that:
>
>    1) The fully safe option requires a new flag for RSEQ
>       registration. It obviously requires a glibc update. (Suggested by
>       PeterZ)

Without glibc changes, RSEQ would keep working, but with the old,
problematic performance, right?

If we don't have a notification in the auxiliary vector, we'd have to do
two system calls at process start, which isn't ideal, but is probably
not a significant issue, either.

I haven't verified this, but it looks like introducing the flag breaks
CRIU?  In dump_thread_rseq, we have this:

        if (rseqc.flags != 0) {
                pr_err("something wrong with ptrace(PTRACE_GET_RSEQ_CONFIGURATION, %d) flags = 0x%x\n", tid,
                       rseqc.flags);
                return -1;
        }

I suppose a workaround could make this behavior flag a prctl flag.  CRIU
wouldn't dump and restore that until taught about it.  If the new
behavior is switched on explicitly by the flag, it would be
backwards-compatible, except that restoring with unpatched CRIU would
lead to a performance loss.

>    2) Determine the requirements of the registering task via the size of
>       the registered RSEQ area.
>
>       The original implementation, which TCMalloc depends on, registers
>       a 32 byte region (ORIG_RSEG_SIZE). This region has 32 byte
>       alignment requirement.
>
>       The extension safe newer variant exposes the kernel RSEQ feature
>       size via getauxval(AT_RSEQ_FEATURE_SIZE) and the alignment
>       requirement via getauxval(AT_RSEQ_ALIGN). The alignment
>       requirement is that the registered rseq region is aligned to the
>       next power of two of the feature size. The kernel currently has a
>       feature size of 33 bytes, which means the alignment requirement is
>       64 bytes.

There are still glibc builds in use that do not use AT_RSEQ_ALIGN, and
instead unconditionally reserve a size of 32.  In some builds, the RSEQ
area is not aligned to a multiple of 64, which makes glibc
indistinguishable from tcmalloc.  You could look at the location of the
thread pointer relative to the RSEQ area at registration to tell them
apart, but that is perhaps too nasty.

Switching to the new extensible RSEQ allocation code in older glibc
builds is not entirely trivial, and I would prefer not doing that.
Registering with a new flag is comparatively simple, and we could
backport it, except that it might not be compatible with CRIU.

Thanks,
Florian