[1/4] nptl: fix potential merge of __rseq_* relro symbols

While working on a patch to add support for the extensible rseq ABI, we
came across an issue where a new 'const' variable would be merged with
the existing '__rseq_size' variable. We tracked this to the use of
'-fmerge-all-constants' which allows the compiler to merge identical
constant variables. This means that all 'const' variables in a compile
unit that are of the same size and are initialized to the same value can
be merged.

In this specific case, on 32 bit systems 'unsigned int' and 'ptrdiff_t'
are both 4 bytes and initialized to 0 which should trigger the merge.
However for reasons we haven't delved into when the attribute 'section
(".data.rel.ro")' is added to the mix, only variables of the same exact
types are merged. As far as we know this behavior is not specified
anywhere and could change with a new compiler version, hence this patch.

Move the definitions of these variables into an assembler file and add
hidden writable aliases for internal use. This has the added bonus of
removing the asm workaround to set the values on rseq registration.

Tested on Debian 12 with GCC 12.2.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
Changes sinve v8:
- Remove superfluous attributes on externs
---
 csu/Makefile                  |  2 +-
 csu/rseq-sizes.sym            |  8 +++++
 elf/Makefile                  |  1 +
 elf/dl-rseq-symbols.S         | 55 +++++++++++++++++++++++++++++++++++
 sysdeps/nptl/dl-tls_init_tp.c | 14 ++++-----
 5 files changed, 71 insertions(+), 9 deletions(-)
 create mode 100644 csu/rseq-sizes.sym
 create mode 100644 elf/dl-rseq-symbols.S

LGTM
Reviewed-by: DJ Delorie <dj@redhat.com>


Michael Jeanson <mjeanson@efficios.com> writes:
> diff --git a/csu/Makefile b/csu/Makefile
> -gen-as-const-headers += rtld-sizes.sym
> +gen-as-const-headers += rtld-sizes.sym rseq-sizes.sym

Ok.

> diff --git a/csu/rseq-sizes.sym b/csu/rseq-sizes.sym
> +#include <stddef.h>
> +
> +--
> +RSEQ_SIZE_SIZE		sizeof (unsigned int)
> +RSEQ_SIZE_ALIGN		__alignof (unsigned int)
> +
> +RSEQ_OFFSET_SIZE	sizeof (ptrdiff_t)
> +RSEQ_OFFSET_ALIGN	__alignof (ptrdiff_t)

Ok.

> diff --git a/elf/Makefile b/elf/Makefile
> +  dl-rseq-symbols \

Ok.

> diff --git a/elf/dl-rseq-symbols.S b/elf/dl-rseq-symbols.S
> +/* Define symbols used by rseq.
> +   Copyright (C) 2024 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 <rseq-sizes.h>
> +#include <sysdep.h>
> +
> +/* Some targets define a macro to denote the zero register.  */
> +#undef zero

Ok.

> +/* Define 2 symbols, __rseq_size is public const and _rseq_size, which is an
> +   alias of __rseq_size, but hidden and writable for internal use.  */
> +
> +	.globl	__rseq_size
> +	.type	__rseq_size, %object
> +	.size	__rseq_size, RSEQ_SIZE_SIZE
> +	.hidden _rseq_size
> +	.globl	_rseq_size
> +	.type	_rseq_size, %object
> +	.size	_rseq_size, RSEQ_SIZE_SIZE
> +	.section .data.rel.ro
> +	.balign	RSEQ_SIZE_ALIGN
> +__rseq_size:
> +_rseq_size:
> +	.zero	RSEQ_SIZE_SIZE

Ok.

> +/* Define 2 symbols, __rseq_offset is public const and _rseq_offset, which is an
> +   alias of __rseq_offset, but hidden and writable for internal use.  */
> +
> +	.globl	__rseq_offset
> +	.type	__rseq_offset, %object
> +	.size	__rseq_offset, RSEQ_OFFSET_SIZE
> +	.hidden _rseq_offset
> +	.globl	_rseq_offset
> +	.type	_rseq_offset, %object
> +	.size	_rseq_offset, RSEQ_OFFSET_SIZE
> +	.section .data.rel.ro
> +	.balign	RSEQ_OFFSET_ALIGN
> +__rseq_offset:
> +_rseq_offset:
> +	.zero	RSEQ_OFFSET_SIZE

Ok.

> diff --git a/sysdeps/nptl/dl-tls_init_tp.c b/sysdeps/nptl/dl-tls_init_tp.c
>  #endif
>  
>  const unsigned int __rseq_flags;
> -const unsigned int __rseq_size attribute_relro;
> -const ptrdiff_t __rseq_offset attribute_relro;
> +
> +/* The variables are in .data.relro but are not yet write-protected.  */
> +extern unsigned int _rseq_size;
> +extern ptrdiff_t _rseq_offset;

Ok.

> @@ -105,10 +107,7 @@ __tls_init_tp (void)
>      do_rseq = TUNABLE_GET (rseq, int, NULL);
>      if (rseq_register_current_thread (pd, do_rseq))
>        {
> -        /* We need a writable view of the variables.  They are in
> -           .data.relro and are not yet write-protected.  */
> -        extern unsigned int size __asm__ ("__rseq_size");
> -        size = sizeof (pd->rseq_area);
> +        _rseq_size = sizeof (pd->rseq_area);
>        }

Ok.

> @@ -117,8 +116,7 @@ __tls_init_tp (void)
>         all targets support __thread_pointer, so set __rseq_offset only
>         if the rseq registration may have happened because RSEQ_SIG is
>         defined.  */
> -    extern ptrdiff_t offset __asm__ ("__rseq_offset");
> -    offset = (char *) &pd->rseq_area - (char *) __thread_pointer ();
> +    _rseq_offset = (char *) &pd->rseq_area - (char *) __thread_pointer ();
>  #endif
>    }

Ok.

On 2024-02-29 23:08, DJ Delorie wrote:
> 
> LGTM
> Reviewed-by: DJ Delorie <dj@redhat.com>

Hi,

I'm just wondering if there is something actionable on my side here to get 
this merged. If not that's fine I just don't want to be in the way.

Thanks,

Michael