[PATCH 3/3] elf: Implement force_first handling in _dl_sort_maps_dfs

[Adhemerval Zanella Netto]

On 15/08/22 11:30, Florian Weimer via Libc-alpha wrote:
> As documented in a comment _dl_close_worker, the skipping is actually
> needed for correctness.  It also seems less surprising if the
> just-opened object is always initialized last, even in the presence
> of cycles.

I think it is BZ#28937, isn't? Also could you extend the explanation as you
did in the last comment, the initial phrase sounds confusing.  Maybe extend
the comment to say that not _dl_sort_maps_dfs will move the main object to
front, so where previous you have the maps input as:

  maps[0].l_name=elf/tst-bz28937-dir/tst-bz28937-a.so
  maps[1].l_name=[...]/elf/tst-bz28937-dir/tst-bz28937-a1.so
  maps[2].l_name=[...]/elf/tst-bz28937-dir/tst-bz28937-a2.so
  maps[3].l_name=./libc.so.6
  maps[4].l_name=elf/ld.so

It will not be properly sorted as:

  maps[0].l_name=elf/tst-bz28937-dir/tst-bz28937-a.so
  maps[1].l_name=[...]/elf/tst-bz28937-dir/tst-bz28937-a1.so
  maps[2].l_name=[...]/elf/tst-bz28937-dir/tst-bz28937-a2.so
  maps[3].l_name=./libc.so.6
  maps[4].l_name=elf/ld.so

Instead of wrongly:

  maps[0].l_name=[...]/elf/tst-bz28937-dir/tst-bz28937-a1.so
  maps[1].l_name=[...]/elf/tst-bz28937-dir/tst-bz28937-a2.so
  maps[2].l_name=elf/tst-bz28937-dir/tst-bz28937-a.so
  maps[3].l_name=./libc.so.6
  maps[4].l_name=elf/ld.so

> ---
>  elf/dl-sort-maps.c       | 35 ++++++++++++++++++++++++++---------
>  elf/dso-sort-tests-1.def |  7 +++++++
>  2 files changed, 33 insertions(+), 9 deletions(-)
> 
> diff --git a/elf/dl-sort-maps.c b/elf/dl-sort-maps.c
> index 5b550b1e94..cd2d9c93fc 100644
> --- a/elf/dl-sort-maps.c
> +++ b/elf/dl-sort-maps.c
> @@ -182,8 +182,9 @@ dfs_traversal (struct link_map ***rpo, struct link_map *map,
>  
>  static void
>  _dl_sort_maps_dfs (struct link_map **maps, unsigned int nmaps,
> -		   bool force_first __attribute__ ((unused)), bool for_fini)
> +		   bool force_first, bool for_fini)
>  {
> +  struct link_map *first_map = maps[0];

Move this to where it is actually used.

>    for (int i = nmaps - 1; i >= 0; i--)
>      maps[i]->l_visited = 0;
>  
> @@ -208,14 +209,6 @@ _dl_sort_maps_dfs (struct link_map **maps, unsigned int nmaps,
>       Adjusting the order so that maps[0] is last traversed naturally avoids
>       this problem.
>  
> -     Further, the old "optimization" of skipping the main object at maps[0]
> -     from the call-site (i.e. _dl_sort_maps(maps+1,nmaps-1)) is in general
> -     no longer valid, since traversing along object dependency-links
> -     may "find" the main object even when it is not included in the initial
> -     order (e.g. a dlopen()'ed shared object can have circular dependencies> -     linked back to itself). In such a case, traversing N-1 objects will
> -     create a N-object result, and raise problems.
> -
>       To summarize, just passing in the full list, and iterating from back
>       to front makes things much more straightforward.  */
>  
> @@ -274,6 +267,30 @@ _dl_sort_maps_dfs (struct link_map **maps, unsigned int nmaps,
>      }
>  
>    memcpy (maps, rpo, sizeof (struct link_map *) * nmaps);
> +
> +  /* Skipping the first object at maps[0] is not valid in general,
> +     since traversing along object dependency-links may "find" that
> +     first object even when it is not included in the initial order
> +     (e.g. a dlopen()'ed shared object can have circular dependencies
> +     linked back to itself). In such a case, traversing N-1 objects

Double space after period and think we do not reference symbol using '()'.

> +     will create a N-object result, and raise problems.  Instead,
> +     force the object back into first place after sorting.  */
> +  if (force_first && maps[0] != first_map)
> +    {
> +      struct link_map *saved = maps[0];
> +      maps[0] = first_map;
> +      int i = 1;
> +      while (true)
> +	{
> +	  assert (i < nmaps);
> +	  struct link_map *current = maps[i];
> +	  maps[i] = saved;
> +	  if (current == first_map)
> +	     break;
> +	  saved = current;
> +	  ++i;
> +	}
> +    }
>  }

It sounds reasonable to keep the main object as the initial map, although
it would slow down a bit normal dlclose.  I think it would be possible
to optimize the memory move with memmove here, although not sure if it is
worth.

>  
>  void
> diff --git a/elf/dso-sort-tests-1.def b/elf/dso-sort-tests-1.def
> index 5f7f18ef27..4bf9052db1 100644
> --- a/elf/dso-sort-tests-1.def
> +++ b/elf/dso-sort-tests-1.def
> @@ -64,3 +64,10 @@ output: b>a>{}<a<b
>  tst-bz15311: {+a;+e;+f;+g;+d;%d;-d;-g;-f;-e;-a};a->b->c->d;d=>[ba];c=>a;b=>e=>a;c=>f=>b;d=>g=>c
>  output(glibc.rtld.dynamic_sort=1): {+a[d>c>b>a>];+e[e>];+f[f>];+g[g>];+d[];%d(b(e(a()))a()g(c(a()f(b(e(a()))))));-d[];-g[];-f[];-e[];-a[<a<c<d<g<f<b<e];}
>  output(glibc.rtld.dynamic_sort=2): {+a[d>c>b>a>];+e[e>];+f[f>];+g[g>];+d[];%d(b(e(a()))a()g(c(a()f(b(e(a()))))));-d[];-g[];-f[];-e[];-a[<g<f<a<b<c<d<e];}
> +
> +# Test that even in the presence of dependency loops involving dlopen'ed
> +# object, that object is initialized last (and not unloaded prematurely).
> +# Final destructor order is indeterminate due to the cycle.
> +tst-bz28937: {+a;+b;-b;+c;%c};a->a1;a->a2;a2->a;b->b1;c->a1;c=>a1

So main program:

  1. dlopen 'a' and 'b';
  2. dclose 'b';
  3. dlopen 'c';
  4. dlsym 'c' and calls fn_a from 'c';

And we have a circle dependency where a depends of a2 and a2 depends on 'a'.

Do we need to add a test for multiple circles? For instance where you have
either another disjointed circle ({+d};d->d2;d2->d) and/or another circle
in same dependency chain (a1->b;b1->a)?

> +output(glibc.rtld.dynamic_sort=1): {+a[a2>a1>a>];+b[b1>b>];-b[<b<b1];+c[c>];%c(a1());}<a<a2<c<a1
> +output(glibc.rtld.dynamic_sort=2): {+a[a2>a1>a>];+b[b1>b>];-b[<b<b1];+c[c>];%c(a1());}<a2<a<c<a1