Re: [PATCH v5 4/5] x86-64: Add the clone3 wrapper

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Sat, May 15, 2021 at 9:23 AM H.J. Lu via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> extern int clone3 (struct clone_args *__cl_args,
>                    int (*__func) (void *__arg), void *__arg);
> ---
>  sysdeps/unix/sysv/linux/x86_64/clone3.S | 92 +++++++++++++++++++++++++
>  sysdeps/unix/sysv/linux/x86_64/sysdep.h |  2 +
>  2 files changed, 94 insertions(+)
>  create mode 100644 sysdeps/unix/sysv/linux/x86_64/clone3.S
>
> diff --git a/sysdeps/unix/sysv/linux/x86_64/clone3.S b/sysdeps/unix/sysv/linux/x86_64/clone3.S
> new file mode 100644
> index 0000000000..f7d4036a6a
> --- /dev/null
> +++ b/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>
> +#include <clone-offsets.h>
> +
> +/* The userland implementation is:
> +   int clone3 (struct clone_args *cl_args, 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: func
> +   rdx: arg
> +
> +   The kernel expects:
> +   rax: system call number
> +   rdi: cl_args
> +   rsi: size  */
> +
> +        .text
> +ENTRY (__clone3)
> +       /* Sanity check arguments.  */
> +       movq    $-EINVAL, %rax

Can this be movl?

> +       testq   %rdi, %rdi              /* No NULL cl_args pointer.  */
> +       jz      SYSCALL_ERROR_LABEL
> +       testq   %rsi, %rsi              /* No NULL function pointer.  */
> +       jz      SYSCALL_ERROR_LABEL
> +
> +       /* Save the function pointer in R8 which is preserved by the
> +          syscall.  */
> +       movq    %rsi, %r8
> +
> +       /* Put sizeof (struct clone_args) in ESI.  */
> +       movl    $CLONE_ARGS_SIZE , %esi
> +
> +       /* 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)
> +

Is expectation to go to L(thread_start)? If so
think jnz L(ret) and fallthrough is probably
better.

> +       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
> +
> +       /* Set up arguments for the function call.  */
> +       movq    %rdx, %rdi      /* Argument.  */
> +       call    *%r8            /* 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)
> diff --git a/sysdeps/unix/sysv/linux/x86_64/sysdep.h b/sysdeps/unix/sysv/linux/x86_64/sysdep.h
> index dbad2c788a..f26ffc68ae 100644
> --- a/sysdeps/unix/sysv/linux/x86_64/sysdep.h
> +++ b/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__ */
> --
> 2.31.1
>