commit 233e667e1a upstream.
glibc-2.35 (upcoming release date 2022-02-01) exposes the rseq per-thread
data in the TCB, accessible at an offset from the thread pointer, rather
than through an actual Thread-Local Storage (TLS) variable, as the
Linux kernel selftests initially expected.
The __rseq_abi TLS and glibc-2.35's ABI for per-thread data cannot
actively coexist in a process, because the kernel supports only a single
rseq registration per thread.
Here is the scheme introduced to ensure selftests can work both with an
older glibc and with glibc-2.35+:
- librseq exposes its own "rseq_offset, rseq_size, rseq_flags" ABI.
- librseq queries for glibc rseq ABI (__rseq_offset, __rseq_size,
__rseq_flags) using dlsym() in a librseq library constructor. If those
are found, copy their values into rseq_offset, rseq_size, and
rseq_flags.
- Else, if those glibc symbols are not found, handle rseq registration
from librseq and use its own IE-model TLS to implement the rseq ABI
per-thread storage.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220124171253.22072-8-mathieu.desnoyers@efficios.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
e85fdae4df