[PATCH v6 2/9] elf: Parse gnu properties for statically linked binaries

[Yury Khrustalev]

On Tue, Mar 11, 2025 at 02:09:49PM -0300, Adhemerval Zanella wrote:
> It allows static binary to opt-in of memory sealing.  The aarch64
> already does it for GCS, so refactor it to use
> __libc_process_gnu_attributes instead.
> 
> Checked on x86_64-linux-gnu.
> ---
>  csu/libc-start.c                             |  4 ++
>  elf/dl-support.c                             | 13 ++++++
>  sysdeps/generic/libc-prop.h                  | 44 ++++++++++++++++++++
>  sysdeps/unix/sysv/linux/aarch64/libc-start.h | 11 -----
>  sysdeps/x86/dl-prop.h                        |  4 +-
>  5 files changed, 64 insertions(+), 12 deletions(-)
>  create mode 100644 sysdeps/generic/libc-prop.h
> 
> +  /* Process program headers again, but scan them backwards so
> +     that PT_NOTE can be skipped if PT_GNU_PROPERTY exits.  */

Why do we need to process headers again?

> +  for (const ElfW(Phdr) *ph = &_dl_phdr[_dl_phnum]; ph != _dl_phdr; --ph)
> +    switch (ph[-1].p_type)
> +      {
> +      case PT_NOTE:
> +	_dl_process_pt_note (&_dl_main_map, -1, &ph[-1]);
> +	break;
> +      case PT_GNU_PROPERTY:
> +	_dl_process_pt_gnu_property (&_dl_main_map, -1, &ph[-1]);
> +	break;
> +      }
>  
>    if ((__glibc_unlikely (GL(dl_stack_flags)) & PF_X)
>        && TUNABLE_GET (glibc, rtld, execstack, int32_t, NULL) == 0)
> diff --git a/sysdeps/generic/libc-prop.h b/sysdeps/generic/libc-prop.h
> new file mode 100644
> index 0000000000..723575d29b
> --- /dev/null
> +++ b/sysdeps/generic/libc-prop.h
> @@ -0,0 +1,44 @@
> +/* Support for GNU properties for static builds.  Generic version.
> +   Copyright (C) 2025 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/>.  */
> +
> +#ifndef _LIBC_PROP_H
> +#define _LIBC_PROP_H
> +
> +#include <dl-prop.h>
> +
> +/* Called at the start of program execution to handle GNU attribute from
> +   PT_NOTE / PT_GNU_PROPERTY.  Must be on a top-level stack frame that does
> +   not return.  */
> +static __always_inline void
> +__libc_process_gnu_attributes (void)

I think that "__libc_process_gnu_properties" may be a better name. There may
be a need in the future to process build attributes [1] and the name of the
function above might be confusing.

[1] https://github.com/smithp35/abi-aa/blob/build-attributes/buildattr64/buildattr64.rst

Kind regardsm
Yury