Based on the following commit, but modified for rhel-7.9.
Added sysdeps/ieee754/ldbl-96/Makefile to contain test case.
Adjust test case to use #include <math_private.h>.

commit 9333498794cde1d5cca518badf79533a24114b6f
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Wed Feb 12 23:31:56 2020 +0000

    Avoid ldbl-96 stack corruption from range reduction of pseudo-zero (bug 25487).

    Bug 25487 reports stack corruption in ldbl-96 sinl on a pseudo-zero
    argument (an representation where all the significand bits, including
    the explicit high bit, are zero, but the exponent is not zero, which
    is not a valid representation for the long double type).

    Although this is not a valid long double representation, existing
    practice in this area (see bug 4586, originally marked invalid but
    subsequently fixed) is that we still seek to avoid invalid memory
    accesses as a result, in case of programs that treat arbitrary binary
    data as long double representations, although the invalid
    representations of the ldbl-96 format do not need to be consistently
    handled the same as any particular valid representation.

    This patch makes the range reduction detect pseudo-zero and unnormal
    representations that would otherwise go to __kernel_rem_pio2, and
    returns a NaN for them instead of continuing with the range reduction
    process.  (Pseudo-zero and unnormal representations whose unbiased
    exponent is less than -1 have already been safely returned from the
    function before this point without going through the rest of range
    reduction.)  Pseudo-zero representations would previously result in
    the value passed to __kernel_rem_pio2 being all-zero, which is
    definitely unsafe; unnormal representations would previously result in
    a value passed whose high bit is zero, which might well be unsafe
    since that is not a form of input expected by __kernel_rem_pio2.

    Tested for x86_64.

diff -urN a/sysdeps/ieee754/ldbl-96/e_rem_pio2l.c b/sysdeps/ieee754/ldbl-96/e_rem_pio2l.c
--- a/sysdeps/ieee754/ldbl-96/e_rem_pio2l.c	2012-12-24 22:02:13.000000000 -0500
+++ b/sysdeps/ieee754/ldbl-96/e_rem_pio2l.c	2021-01-06 20:39:13.502241328 -0500
@@ -210,6 +210,18 @@
       return 0;
     }
 
+  if ((i0 & 0x80000000) == 0)
+    {
+      /* Pseudo-zero and unnormal representations are not valid
+	 representations of long double.  We need to avoid stack
+	 corruption in __kernel_rem_pio2, which expects input in a
+	 particular normal form, but those representations do not need
+	 to be consistently handled like any particular floating-point
+	 value.  */
+      y[1] = y[0] = __builtin_nanl ("");
+      return 0;
+    }
+
   /* Split the 64 bits of the mantissa into three 24-bit integers
      stored in a double array.  */
   exp = j0 - 23;
diff -urN a/sysdeps/ieee754/ldbl-96/Makefile b/sysdeps/ieee754/ldbl-96/Makefile
--- a/sysdeps/ieee754/ldbl-96/Makefile	1969-12-31 19:00:00.000000000 -0500
+++ b/sysdeps/ieee754/ldbl-96/Makefile	2021-01-06 20:55:49.163141757 -0500
@@ -0,0 +1,22 @@
+# Makefile for sysdeps/ieee754/ldbl-96.
+# Copyright (C) 2016-2021 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/>.
+
+ifeq ($(subdir),math)
+tests += test-sinl-pseudo
+CFLAGS-test-sinl-pseudo.c += -fstack-protector-all
+endif
diff -urN a/sysdeps/ieee754/ldbl-96/test-sinl-pseudo.c b/sysdeps/ieee754/ldbl-96/test-sinl-pseudo.c
--- a/sysdeps/ieee754/ldbl-96/test-sinl-pseudo.c	1969-12-31 19:00:00.000000000 -0500
+++ b/sysdeps/ieee754/ldbl-96/test-sinl-pseudo.c	2021-01-05 16:27:24.658596782 -0500
@@ -0,0 +1,41 @@
+/* Test sinl for pseudo-zeros and unnormals for ldbl-96 (bug 25487).
+   Copyright (C) 2020 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/>.  */
+
+#include <math.h>
+#include <math_private.h>
+#include <stdint.h>
+
+static int
+do_test (void)
+{
+  for (int i = 0; i < 64; i++)
+    {
+      uint64_t sig = i == 63 ? 0 : 1ULL << i;
+      long double ld;
+      SET_LDOUBLE_WORDS (ld, 0x4141,
+                        sig >> 32, sig & 0xffffffffULL);
+      /* The requirement is that no stack overflow occurs when the
+        pseudo-zero or unnormal goes through range reduction.  */
+      volatile long double ldr;
+      ldr = sinl (ld);
+      (void) ldr;
+    }
+  return 0;
+}
+
+#include <support/test-driver.c>