Re: [PATCH v2 03/11] Linux: Do not align the stack for __clone3

[Carlos O'Donell via Libc-alpha <libc-alpha@sourceware.org>]

On 11/4/22 15:01, Adhemerval Zanella via Libc-alpha wrote:
> Adjusting the stack on child thread is not async-signal safe, since
> kernel does not enforce stack alignmnet.  Although, all the internal
> usage of __clone_internal are done with all signal masked, removing
> stack alignment of __clone3 is still a net gain: is simplifies the
> function contract detail (mask/unmarsk signals) and simplifies the
> arch-specific code.

Please post v4.

Suggest:

All internal callers of __clone3 should provide an already aligned stack.
Removing the stack alignment in __clone3 is a net gain: it simplifies the
internal function contract (mask/unmask signals) along with the arch-specific
code.

> 
> The internal users are responsible for correct stack alignment.  If
> glibc ever exports a clone3 wrapper, it should adjust stack alignment.
> 
> Checked on x86_64-linux-gnu.
> ---
>  include/clone_internal.h                      |  5 ++
>  .../sysv/linux/tst-misalign-clone-internal.c  | 74 -------------------
>  sysdeps/unix/sysv/linux/x86_64/clone3.S       |  3 -
>  3 files changed, 5 insertions(+), 77 deletions(-)
>  delete mode 100644 sysdeps/unix/sysv/linux/tst-misalign-clone-internal.c
> 
> diff --git a/include/clone_internal.h b/include/clone_internal.h
> index 73b8114df4..5a48c7caa5 100644
> --- a/include/clone_internal.h
> +++ b/include/clone_internal.h
> @@ -11,6 +11,11 @@
>     Different than kernel, the implementation also returns EINVAL for an
>     invalid NULL __CL_ARGS or __FUNC (similar to __clone).
>  
> +   The stack is not aligned prior the syscall, different than exported
> +   __clone.  Adjusting the stack on child is not async-signal safe and since
> +   it is used only internally the caller is responsible for proper stack
> +   alignment.

Suggest:

All callers are responsible for correctly aligning the stack.  The stack is not aligned
prior to the syscall (this differs from the exported __clone).


> +
>     This function is only implemented if the ABI defines HAVE_CLONE3_WRAPPER.
>  */
>  extern int __clone3 (struct clone_args *__cl_args, size_t __size,
> diff --git a/sysdeps/unix/sysv/linux/tst-misalign-clone-internal.c b/sysdeps/unix/sysv/linux/tst-misalign-clone-internal.c
> deleted file mode 100644
> index e039ca5a3a..0000000000
> --- a/sysdeps/unix/sysv/linux/tst-misalign-clone-internal.c
> +++ /dev/null
> @@ -1,74 +0,0 @@
> -/* Verify that __clone_internal properly aligns the child stack.
> -   Copyright (C) 2021-2022 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 <sched.h>
> -#include <stdbool.h>
> -#include <stdint.h>
> -#include <stdio.h>
> -#include <string.h>
> -#include <sys/wait.h>
> -#include <unistd.h>
> -#include <libc-pointer-arith.h>
> -#include <tst-stack-align.h>
> -#include <clone_internal.h>
> -#include <support/xunistd.h>
> -#include <support/check.h>
> -
> -static int
> -check_stack_alignment (void *arg)
> -{
> -  puts ("in f");
> -
> -  return TEST_STACK_ALIGN () ? 1 : 0;
> -}
> -
> -static int
> -do_test (void)
> -{
> -  puts ("in do_test");
> -
> -  if (TEST_STACK_ALIGN ())
> -    FAIL_EXIT1 ("stack isn't aligned\n");
> -
> -#ifdef __ia64__
> -# define STACK_SIZE (256 * 1024)
> -#else
> -# define STACK_SIZE (128 * 1024)
> -#endif
> -  char st[STACK_SIZE + 1];
> -  /* NB: Align child stack to 1 byte.  */
> -  char *stack = PTR_ALIGN_UP (&st[0], 2) + 1;
> -  struct clone_args clone_args =
> -    {
> -      .stack = (uintptr_t) stack,
> -      .stack_size = STACK_SIZE,
> -    };
> -  pid_t p = __clone_internal (&clone_args, check_stack_alignment, 0);
> -
> -  /* Clone must not fail.  */
> -  TEST_VERIFY_EXIT (p != -1);
> -
> -  int e;
> -  xwaitpid (p, &e, __WCLONE);
> -  TEST_VERIFY (WIFEXITED (e));
> -  TEST_COMPARE (WEXITSTATUS (e), 0);
> -
> -  return 0;
> -}
> -
> -#include <support/test-driver.c>

OK.

> diff --git a/sysdeps/unix/sysv/linux/x86_64/clone3.S b/sysdeps/unix/sysv/linux/x86_64/clone3.S
> index 18fc2ee966..989b62bdf6 100644
> --- a/sysdeps/unix/sysv/linux/x86_64/clone3.S
> +++ b/sysdeps/unix/sysv/linux/x86_64/clone3.S
> @@ -73,9 +73,6 @@ L(thread_start):
>  	   the outermost frame obviously.  */
>  	xorl	%ebp, %ebp
>  
> -	/* Align stack to 16 bytes per the x86-64 psABI.  */
> -	and	$-16, %RSP_LP

OK.

> -
>  	/* Set up arguments for the function call.  */
>  	mov	%R8_LP, %RDI_LP	/* Argument.  */
>  	call	*%rdx		/* Call function.  */

-- 
Cheers,
Carlos.