[PATCH v8 3/6] elf: Introduce _dl_map_segment_align hook for segment alignment tuning

[H.J. Lu]

On Sun, Apr 5, 2026 at 11:54 AM WANG Rui <wangrui@loongson.cn> wrote:
>
> Introduce a new helper function, _dl_map_segment_align, to allow
> architecture-specific adjustment of ELF load segment alignment during
> object mapping.
>
> The generic ELF loader now calls this hook when determining the maximum
> segment alignment.  The generic implementation is a no-op and preserves
> existing behavior.
>
> This provides a well-defined extension point for architectures that
> need to adjust segment alignment policies (for example, to improve
> mapping efficiency or enable platform-specific optimizations) without
> embedding such logic directly in the generic loader.
>
> Reviewed-by: Wilco Dijkstra  <Wilco.Dijkstra@arm.com>
> Signed-off-by: WANG Rui <wangrui@loongson.cn>
> ---
>  elf/dl-load.c                          |  4 ++++
>  sysdeps/generic/dl-map-segment-align.h | 26 ++++++++++++++++++++++++++
>  2 files changed, 30 insertions(+)
>  create mode 100644 sysdeps/generic/dl-map-segment-align.h
>
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index 7355eef8e76..f3d943e99c0 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -71,6 +71,7 @@ struct filebuf
>  #include <dl-dst.h>
>  #include <dl-load.h>
>  #include <dl-map-segments.h>
> +#include <dl-map-segment-align.h>
>  #include <dl-unmap-segments.h>
>  #include <dl-machine-reject-phdr.h>
>  #include <dl-prop.h>
> @@ -1171,6 +1172,9 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>
>           /* Optimize a common case.  */
>           c->prot = pf_to_prot (ph->p_flags);
> +
> +         /* Architecture-specific adjustment of segment alignment. */
> +         p_align_max = _dl_map_segment_align (c, p_align_max);
>           break;

Can you make THP to work for

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           0x000034 0x00400034 0x00400034 0x001a0 0x001a0 R   0x4
  INTERP         0x0001f8 0x004001f8 0x004001f8 0x0001a 0x0001a R   0x1
      [Requesting program interpreter: /libx32/ld-linux-x32.so.2]
  LOAD           0x000000 0x00400000 0x00400000 0x16d110 0x16d110 R   0x1000
  LOAD           0x16e000 0x0056e000 0x0056e000 0x1055d9 0x1055d9 R E 0x1000
  LOAD           0x273ebc 0x00674ebc 0x00674ebc 0x02540 0x039fc RW  0x1000
  DYNAMIC        0x273ec8 0x00674ec8 0x00674ec8 0x00110 0x00110 RW  0x4
  NOTE           0x0001d4 0x004001d4 0x004001d4 0x00024 0x00024 R   0x4
  NOTE           0x16d0b0 0x0056d0b0 0x0056d0b0 0x00060 0x00060 R   0x4
  TLS            0x273ebc 0x00674ebc 0x00674ebc 0x00000 0x0000c R   0x4
  GNU_PROPERTY   0x16d0b0 0x0056d0b0 0x0056d0b0 0x00040 0x00040 R   0x4
  GNU_EH_FRAME   0x1493e0 0x005493e0 0x005493e0 0x05b94 0x05b94 R   0x4
  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x10
  GNU_RELRO      0x273ebc 0x00674ebc 0x00674ebc 0x00144 0x00144 R   0x1

As an option, we can combine

  LOAD           0x000000 0x00400000 0x00400000 0x16d110 0x16d110 R   0x1000
  LOAD           0x16e000 0x0056e000 0x0056e000 0x1055d9 0x1055d9 R E 0x1000

into a single RE THP map.

>         case PT_TLS:
> diff --git a/sysdeps/generic/dl-map-segment-align.h b/sysdeps/generic/dl-map-segment-align.h
> new file mode 100644
> index 00000000000..f4a671f25f8
> --- /dev/null
> +++ b/sysdeps/generic/dl-map-segment-align.h
> @@ -0,0 +1,26 @@
> +/* _dl_map_segment_align.  Generic version.
> +   Copyright (C) 2026 Free Software Foundation, Inc.
> +   Copyright The GNU Toolchain Authors.
> +   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 <dl-load.h>
> +
> +static inline ElfW(Addr)
> +_dl_map_segment_align (const struct loadcmd *c, ElfW(Addr) p_align_max)
> +{
> +  return p_align_max;
> +}
> --
> 2.53.0
>


-- 
H.J.