riscv: Add the clone3 wrapper

It follows the internal signature:

  extern int clone3 (struct clone_args *__cl_args, size_t __size,
 int (*__func) (void *__arg), void *__arg);

Checked on riscv64-linux-gnu-rv64imafdc-lp64d.

Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>

sysdeps/unix/sysv/linux/riscv/clone3.S

@@ -0,0 +1,79 @@
+/* The clone3 syscall wrapper.  Linux/RISC-V version.
+   Copyright (C) 2023 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 <asm/errno.h>
+#include <sys/asm.h>
+#include <sysdep.h>
+
+/* The userland implementation is:
+   int clone3 (struct clone_args *cl_args, size_t size,
+               int (*func)(void *arg), void *arg);
+
+   the kernel entry is:
+   int clone3 (struct clone_args *cl_args, size_t size);
+
+   The parameters are passed in registers from userland:
+   a0: cl_args
+   a1: size
+   a2: func
+   a3: arg  */
+
+        .text
+ENTRY(__clone3)
+	/* Sanity check args.  */
+	beqz	a0, L(invalid)	/* No NULL cl_args pointer.  */
+	beqz	a2, L(invalid)  /* No NULL function pointer.  */
+
+	/* Do the system call, the kernel expects:
+	   a7: system call number
+	   a0: cl_args
+	   a1: size  */
+	li	a7, __NR_clone3
+	scall
+
+	bltz	a0, L(error)
+	beqz	a0, L(thread_start)
+
+	ret
+
+L(invalid):
+	li	a0, -EINVAL
+L(error):
+	tail	__syscall_error
+END (__clone3)
+
+ENTRY(__thread_start_clone3)
+L(thread_start):
+	/* Terminate call stack by noting ra is undefined.  Use a dummy
+	   .cfi_label to force starting the FDE.  */
+	.cfi_label .Ldummy
+	cfi_undefined (ra)
+
+	/* Restore the arg for user's function and call the user's
+	   function.  */
+	mv		a0, a3	/* Argument pointer.  */
+	jalr		a2
+
+	/* Call exit with the function's return value.  */
+	li		a7, __NR_exit
+	scall
+END(__thread_start_clone3)
+
+libc_hidden_def (__clone3)
+weak_alias (__clone3, clone3)

sysdeps/unix/sysv/linux/riscv/sysdep.h

@@ -151,6 +151,7 @@
 
 /* RV32 does not support the gettime VDSO syscalls.  */
 # endif
+# define HAVE_CLONE3_WRAPPER		1
 
 /* List of system calls which are supported as vsyscalls (for RV32 and
    RV64).  */