x86-64: Add the clone3 wrapper

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

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

sysdeps/unix/sysv/linux/x86_64/clone3.S

@@ -0,0 +1,92 @@
+/* The clone3 syscall wrapper.  Linux/x86-64 version.
+   Copyright (C) 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/>.  */
+
+/* clone3() is even more special than fork() as it mucks with stacks
+   and invokes a function in the right context after its all over.  */
+
+#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:
+   rdi: cl_args
+   rsi: size
+   rdx: func
+   rcx: arg
+
+   The kernel expects:
+   rax: system call number
+   rdi: cl_args
+   rsi: size  */
+
+        .text
+ENTRY (__clone3)
+	/* Sanity check arguments.  */
+	movl	$-EINVAL, %eax
+	test	%RDI_LP, %RDI_LP	/* No NULL cl_args pointer.  */
+	jz	SYSCALL_ERROR_LABEL
+	test	%RDX_LP, %RDX_LP	/* No NULL function pointer.  */
+	jz	SYSCALL_ERROR_LABEL
+
+	/* Save the cl_args pointer in R8 which is preserved by the
+	   syscall.  */
+	mov	%RCX_LP, %R8_LP
+
+	/* Do the system call.  */
+	movl	$SYS_ify(clone3), %eax
+
+	/* End FDE now, because in the child the unwind info will be
+	   wrong.  */
+	cfi_endproc
+	syscall
+
+	test	%RAX_LP, %RAX_LP
+	jl	SYSCALL_ERROR_LABEL
+	jz	L(thread_start)
+
+	ret
+
+L(thread_start):
+	cfi_startproc
+	/* Clearing frame pointer is insufficient, use CFI.  */
+	cfi_undefined (rip)
+	/* Clear the frame pointer.  The ABI suggests this be done, to mark
+	   the outermost frame obviously.  */
+	xorl	%ebp, %ebp
+
+	/* Align stack to 16 bytes per the x86-64 psABI.  */
+	and	$-16, %RSP_LP
+
+	/* Set up arguments for the function call.  */
+	mov	%R8_LP, %RDI_LP	/* Argument.  */
+	call	*%rdx		/* Call function.  */
+	/* Call exit with return value from function call. */
+	movq	%rax, %rdi
+	movl	$SYS_ify(exit), %eax
+	syscall
+	cfi_endproc
+
+	cfi_startproc
+PSEUDO_END (__clone3)
+
+libc_hidden_def (__clone3)
+weak_alias (__clone3, clone3)

sysdeps/unix/sysv/linux/x86_64/sysdep.h

@@ -377,6 +377,8 @@
 # define HAVE_GETCPU_VSYSCALL		"__vdso_getcpu"
 # define HAVE_CLOCK_GETRES64_VSYSCALL   "__vdso_clock_getres"
 
+# define HAVE_CLONE3_WAPPER			1
+
 # define SINGLE_THREAD_BY_GLOBAL		1
 
 #endif	/* __ASSEMBLER__ */