[PATCH 2/3] elf: Set l_contiguous to 1 for the main map in more cases

[H.J. Lu]

On Mon, Jan 3, 2022 at 9:12 AM Florian Weimer via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> l_contiguous was not initialized at all for the main map and
> always 0.  This commit adds code to check if the LOAD segments
> are adjacent to each other, and sets l_contiguous accordingly.
> This helps _dl_find_object because it is more efficient if the
> main mapping is contiguous.
>
> Note that not all (PIE or non-PIE) binaries are contiguous in this
> way because BFD ld creates executables with LOAD holes:
>
> ELF LOAD segments creating holes in the process image on GNU/Linux
> https://sourceware.org/pipermail/binutils/2022-January/119082.html
> https://sourceware.org/bugzilla/show_bug.cgi?id=28743
> ---
>  elf/rtld.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> diff --git a/elf/rtld.c b/elf/rtld.c
> index ba6e31377d..53293f5b13 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -1144,6 +1144,22 @@ rtld_setup_main_map (struct link_map *main_map)
>    main_map->l_map_start = ~0;
>    /* And it was opened directly.  */
>    ++main_map->l_direct_opencount;
> +  main_map->l_contiguous = 1;
> +
> +  /* A PT_LOAD segment at an unexpected address will clear the
> +     l_contiguous flag.  The ELF specification says that PT_LOAD
> +     segments need to be sorted in in increasing order, but perhaps
> +     not all executables follow this requirement.  Having l_contiguous
> +     equal to 1 is just an optimization, so the code below does not
> +     try to sort the segments in case they are unordered.
> +
> +     There is one corner case in which l_contiguous is not set to 1,
> +     but where it could be set: If a PIE (ET_DYN) binary is loaded by
> +     glibc itself (not the kernel), it is always contiguous due to the
> +     way the glibc loader works.  However, the kernel loader may still
> +     create holes in this case, and the code here still uses 0
> +     conservatively for the glibc-loaded case, too.  */
> +  ElfW(Addr) expected_load_address = 0;
>
>    /* Scan the program header table for the dynamic section.  */
>    for (const ElfW(Phdr) *ph = phdr; ph < &phdr[phnum]; ++ph)
> @@ -1207,12 +1223,21 @@ rtld_setup_main_map (struct link_map *main_map)
>           if (main_map->l_map_start > mapstart)
>             main_map->l_map_start = mapstart;
>
> +         if (main_map->l_contiguous && expected_load_address != 0
> +             && expected_load_address != mapstart)
> +           main_map->l_contiguous = 0;
> +
>           /* Also where it ends.  */
>           allocend = main_map->l_addr + ph->p_vaddr + ph->p_memsz;
>           if (main_map->l_map_end < allocend)
>             main_map->l_map_end = allocend;
>           if ((ph->p_flags & PF_X) && allocend > main_map->l_text_end)
>             main_map->l_text_end = allocend;
> +
> +         /* The next expected address is the page following this load
> +            segment.  */
> +         expected_load_address = ((allocend + GLRO(dl_pagesize) - 1)
> +                                  & ~(GLRO(dl_pagesize) - 1));
>         }
>         break;
>
> --
> 2.33.1
>

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.

-- 
H.J.