Re: [PATCH RFC 1/1] elf: align the mapping address of LOAD segments with p_align

["H.J. Lu" <hjl.tools@gmail.com>]

On Fri, Dec 3, 2021 at 9:00 PM Rongwei Wang via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> Now, ld.so always map the LOAD segments and aligned by base
> page size (e.g. 4k in x86 or 4k, 16k and 64k in arm64). And
> this patch improve the scheme here. In this patch, ld.so
> can align the mapping address of the first LOAD segment with
> p_align when p_align is greater than the current base page
> size.

This is a bug fix.  Please open a glibc bug:

https://sourceware.org/bugzilla/enter_bug.cgi

with a testcase which should align variables to 2MB in the main
program and a shared library.   Please include the testcase in
your patch and mention the glibc bug in your commit message.

> And this change makes code segments using huge pages become
> simple and available.
>
> Signed-off-by: Rongwei Wang <rongwei.wang@linux.alibaba.com>
> ---
>  elf/dl-load.c         |  1 +
>  elf/dl-map-segments.h | 54 +++++++++++++++++++++++++++++++++++++++++--
>  include/link.h        |  3 +++
>  3 files changed, 56 insertions(+), 2 deletions(-)
>
> diff --git a/elf/dl-load.c b/elf/dl-load.c
> index e39980fb19..136cfe2fa8 100644
> --- a/elf/dl-load.c
> +++ b/elf/dl-load.c
> @@ -1154,6 +1154,7 @@ _dl_map_object_from_fd (const char *name, const char *origname, int fd,
>           c->dataend = ph->p_vaddr + ph->p_filesz;
>           c->allocend = ph->p_vaddr + ph->p_memsz;
>           c->mapoff = ALIGN_DOWN (ph->p_offset, GLRO(dl_pagesize));
> +          l->l_load_align = ph->p_align;
>
>           /* Determine whether there is a gap between the last segment
>              and this one.  */
> diff --git a/elf/dl-map-segments.h b/elf/dl-map-segments.h
> index ac9f09ab4c..ae03236045 100644
> --- a/elf/dl-map-segments.h
> +++ b/elf/dl-map-segments.h
> @@ -18,6 +18,47 @@
>
>  #include <dl-load.h>
>
> +static __always_inline void *
> +_dl_map_segments_align (const struct loadcmd *c,
> +                   ElfW(Addr) mappref, int fd, size_t alignment,
> +                   const size_t maplength)
> +{
> +       unsigned long map_start, map_start_align, map_end;
> +       unsigned long maplen = (maplength >= alignment) ?
> +                               (maplength + alignment) : (2 * alignment);
> +
> +       /* Allocate enough space to ensure that address aligned by
> +           p_align is included. */
> +       map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplen,
> +                                    PROT_NONE,
> +                                    MAP_ANONYMOUS | MAP_PRIVATE,
> +                                    -1, 0);
> +       if (__glibc_unlikely ((void *) map_start == MAP_FAILED)) {
> +               /* If mapping a aligned address failed, then ... */
> +               map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
> +                                    c->prot,
> +                                    MAP_COPY|MAP_FILE,
> +                                    fd, c->mapoff);
> +
> +               return (void *) map_start;
> +       }
> +       map_start_align = ALIGN_UP(map_start, alignment);
> +       map_end = map_start_align + maplength;
> +
> +       /* Remember which part of the address space this object uses.  */
> +       map_start_align = (ElfW(Addr)) __mmap ((void *) map_start_align, maplength,
> +                                    c->prot,
> +                                    MAP_COPY|MAP_FILE|MAP_FIXED,
> +                                    fd, c->mapoff);
> +       if (__glibc_unlikely ((void *) map_start_align == MAP_FAILED))
> +               return MAP_FAILED;
> +       if (map_start_align > map_start)
> +               __munmap((void *)map_start, map_start_align - map_start);
> +       __munmap((void *)map_end, map_start + maplen - map_end);
> +
> +       return (void *) map_start_align;
> +}
> +

Please follow the glibc coding format.

>  /* This implementation assumes (as does the corresponding implementation
>     of _dl_unmap_segments, in dl-unmap-segments.h) that shared objects
>     are always laid out with all segments contiguous (or with gaps
> @@ -52,11 +93,20 @@ _dl_map_segments (struct link_map *l, int fd,
>                                    c->mapstart & GLRO(dl_use_load_bias))
>             - MAP_BASE_ADDR (l));
>
> -      /* Remember which part of the address space this object uses.  */
> -      l->l_map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
> +       /* During mapping, align the mapping address of the LOAD segments
> +          according to own p_align. This helps OS map its code segment to
> +          huge pages. */
> +       if (l->l_load_align > GLRO(dl_pagesize)) {
> +               l->l_map_start = (ElfW(Addr)) _dl_map_segments_align (c,
> +                                            mappref, fd,
> +                                            l->l_load_align, maplength);
> +       } else {
> +               /* Remember which part of the address space this object uses.  */
> +               l->l_map_start = (ElfW(Addr)) __mmap ((void *) mappref, maplength,
>                                              c->prot,
>                                              MAP_COPY|MAP_FILE,
>                                              fd, c->mapoff);

Please follow the glibc coding format.

> +       }
>        if (__glibc_unlikely ((void *) l->l_map_start == MAP_FAILED))
>          return DL_MAP_SEGMENTS_ERROR_MAP_SEGMENT;
>
> diff --git a/include/link.h b/include/link.h
> index aea268439c..fc6ce29fab 100644
> --- a/include/link.h
> +++ b/include/link.h
> @@ -298,6 +298,9 @@ struct link_map
>
>      /* Thread-local storage related info.  */
>
> +    /* Alignment requirement of the LOAD block.  */
> +    size_t l_load_align;
> +
>      /* Start of the initialization image.  */
>      void *l_tls_initimage;
>      /* Size of the initialization image.  */
> --
> 2.27.0
>

Thanks.

-- 
H.J.