elf: Record libc.so link map when it is the main program (bug 20972)

  Otherwise, it will not participate in the dependency sorting.

Fixes commit 9ffa50b26b0cb5d3043adf6d3d0b1ea735acc147
("elf: Include libc.so.6 as main program in dependency sort
(bug 20972)").

---
 elf/rtld.c | 11 +++++++++++
 1 file changed, 11 insertions(+)
  

On 12/15/20 5:57 AM, Florian Weimer via Libc-alpha wrote:
> Otherwise, it will not participate in the dependency sorting.
> 
> Fixes commit 9ffa50b26b0cb5d3043adf6d3d0b1ea735acc147
> ("elf: Include libc.so.6 as main program in dependency sort
> (bug 20972)").

Thanks for catching this, I hadn't noticed that the map won't get
assigned.

For the record I think all of this code should be deleted. We should
not be allowing libc.so.6 to be run as a binary, instead all of that
functionality should move into ld.so where we should query libc.so.6
when run in --verbose to provide the same information. We might
have to keep libc.so.6 as a binary, but it should syscall write
"Please run ld.so --help." :-)

LGTM.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
 
> ---
>  elf/rtld.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/elf/rtld.c b/elf/rtld.c
> index 526360237f..38f11f5b73 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -49,6 +49,7 @@
>  #include <libc-early-init.h>
>  #include <dl-main.h>
>  #include <list.h>
> +#include <gnu/lib-names.h>
>  
>  #include <assert.h>
>  
> @@ -1609,6 +1610,16 @@ dl_main (const ElfW(Phdr) *phdr,
>      {
>        /* Extract the contents of the dynamic section for easy access.  */
>        elf_get_dynamic_info (main_map, NULL);
> +
> +      /* If the main map is libc.so, update the base namespace to
> +	 refer to this map.  If libc.so is loaded later, this happens
> +	 in _dl_map_object_from_fd.  */
> +      if (main_map->l_info[DT_SONAME] != NULL
> +	  && (strcmp (((const char *) D_PTR (main_map, l_info[DT_STRTAB])
> +		      + main_map->l_info[DT_SONAME]->d_un.d_val), LIBC_SO)
> +	      == 0))
> +	GL(dl_ns)[LM_ID_BASE].libc_map = main_map;

OK.

> +
>        /* Set up our cache of pointers into the hash table.  */
>        _dl_setup_hash (main_map);
>      }
>
  

The patch LGTM.
And I agree with Carlos, running libc.so.6 as binary shouldn't be allowed.

On 15/12/2020 07:57, Florian Weimer via Libc-alpha wrote:
> Otherwise, it will not participate in the dependency sorting.
> 
> Fixes commit 9ffa50b26b0cb5d3043adf6d3d0b1ea735acc147
> ("elf: Include libc.so.6 as main program in dependency sort
> (bug 20972)").
> 
> ---
>   elf/rtld.c | 11 +++++++++++
>   1 file changed, 11 insertions(+)
> 
> diff --git a/elf/rtld.c b/elf/rtld.c
> index 526360237f..38f11f5b73 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -49,6 +49,7 @@
>   #include <libc-early-init.h>
>   #include <dl-main.h>
>   #include <list.h>
> +#include <gnu/lib-names.h>
> 
>   #include <assert.h>
> 
> @@ -1609,6 +1610,16 @@ dl_main (const ElfW(Phdr) *phdr,
>       {
>         /* Extract the contents of the dynamic section for easy access.  */
>         elf_get_dynamic_info (main_map, NULL);
> +
> +      /* If the main map is libc.so, update the base namespace to
> +	 refer to this map.  If libc.so is loaded later, this happens
> +	 in _dl_map_object_from_fd.  */
> +      if (main_map->l_info[DT_SONAME] != NULL
> +	  && (strcmp (((const char *) D_PTR (main_map, l_info[DT_STRTAB])
> +		      + main_map->l_info[DT_SONAME]->d_un.d_val), LIBC_SO)
> +	      == 0))
> +	GL(dl_ns)[LM_ID_BASE].libc_map = main_map;
> +
>         /* Set up our cache of pointers into the hash table.  */
>         _dl_setup_hash (main_map);
>       }
>