rseq: Reenable performance optimizations conditionally

Due to the incompatibility with TCMalloc the RSEQ optimizations and
extended features (time slice extensions) have been disabled and made
run-time conditional.

The original RSEQ implementation, which TCMalloc depends on, registers a 32
byte region (ORIG_RSEG_SIZE). This region has a 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.

The TCMalloc RSEQ region is embedded into a cache line aligned data
structure starting at offset 32 bytes so that bytes 28-31 and the
cpu_id_start field at bytes 32-35 form a 64-bit little endian pointer with
the top-most bit (63 set) to check whether the kernel has overwritten
cpu_id_start with an actual CPU id value, which is guaranteed to not have
the top most bit set.

As this is part of their performance tuned magic, it's a pretty safe
assumption, that TCMalloc won't use a larger RSEQ size.

This allows the kernel to declare that registrations with a size greater
than the original size of 32 bytes, which is the cases since time slice
extensions got introduced, as RSEQ ABI v2 with the following differences to
the original behaviour:

  1) Unconditional updates of the user read only fields (CPU, node, MMCID)
     are removed. Those fields are only updated on registration, task
     migration and MMCID changes.

  2) Unconditional evaluation of the criticial section pointer is
     removed. It's only evaluated when user space was interrupted and was
     scheduled out or before delivering a signal in the interrupted
     context.

  3) The read/only requirement of the ID fields is enforced. When the
     kernel detects that userspace manipulated the fields, the process is
     terminated. This ensures that multiple entities (libraries) can
     utilize RSEQ without interfering.

  4) Todays extended RSEQ feature (time slice extensions) and future
     extensions are only enabled in the v2 enabled mode.

Registrations with the original size of 32 bytes operate in backwards
compatible legacy mode without performance improvements and extended
features.

Unfortunately that also affects users of older GLIBC versions which
register the original size of 32 bytes and do not evaluate the kernel
required size in the auxiliary vector AT_RSEQ_FEATURE_SIZE.

That's the result of the lack of enforcement in the original implementation
and the unwillingness of a single entity to cooperate with the larger
ecosystem for many years.

Implement the required registration changes by restructuring the spaghetti
code and adding the size/version check. Also add documentation about the
differences of legacy and optimized RSEQ V2 mode.

Thanks to Mathieu for pointing out the ORIG_RSEQ_SIZE constraints!

Fixes: d6200245c75e ("rseq: Allow registering RSEQ with slice extension")
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Tested-by: Dmitry Vyukov <dvyukov@google.com>
Link: https://patch.msgid.link/20260428224427.927160119%40kernel.org
Cc: stable@vger.kernel.org

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