Add Linux 'extra TLS'

Add the Linux implementation of 'extra TLS' which will allocate space
for the rseq area at the end of the TLS blocks in allocation order.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Reviewed-by: Florian Weimer <fweimer@redhat.com>

1 files changed, 71 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/dl-extra_tls.h b/sysdeps/unix/sysv/linux/dl-extra_tls.h
new file mode 100644
index 0000000..d48a5e9
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/dl-extra_tls.h
@@ -0,0 +1,71 @@
+/* extra tls block utils for the dynamic linker.  Linux version.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#ifndef _DL_EXTRA_TLS_H
+#define _DL_EXTRA_TLS_H 1
+
+#include <stddef.h>
+#include <sys/rseq.h>
+#include <rseq-internal.h>
+#include <elf/dl-tunables.h>
+
+/* Returns the size of the extra TLS block, it must always be a multiple of the
+   alignment.  */
+static inline size_t
+_dl_extra_tls_get_size (void)
+{
+  bool do_rseq = TUNABLE_GET_FULL (glibc, pthread, rseq, int, NULL);
+  if (do_rseq)
+    {
+      /* Make sure the rseq area size is at least the minimum ABI size and a
+         multiple of the requested aligment.  */
+      return roundup (MAX (_rseq_size, RSEQ_AREA_SIZE_INITIAL), _rseq_align);
+    }
+
+  /* Even when disabled by tunable, an rseq area will be allocated to allow
+     application code to test the registration status with 'rseq->cpu_id >= 0'.
+     Default to the rseq ABI minimum size, this will ensure we don't use more
+     TLS than necessary.  */
+  return RSEQ_AREA_SIZE_INITIAL;
+}
+
+/* Returns the alignment requirements of the extra TLS block.  */
+static inline size_t
+_dl_extra_tls_get_align (void)
+{
+  bool do_rseq = TUNABLE_GET_FULL (glibc, pthread, rseq, int, NULL);
+  if (do_rseq)
+    {
+      return _rseq_align;
+    }
+
+  /* Even when disabled by tunable, an rseq area will be allocated to allow
+     application code to test the registration status with 'rseq->cpu_id >= 0'.
+     Default to the rseq ABI minimum alignment, this will ensure we don't use
+     more TLS than necessary.  */
+  return RSEQ_MIN_ALIGN;
+}
+
+/* Record the offset of the extra TLS block from the thread pointer.  */
+static inline void
+_dl_extra_tls_set_offset (ptrdiff_t tls_offset)
+{
+    _rseq_offset = tls_offset;
+}
+
+#endif