Re: [PATCH v2] elf: Implement force_first handling in _dl_sort_maps_dfs (bug 28937)

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 06/09/22 04:01, Florian Weimer via Libc-alpha wrote:
> The implementation in _dl_close_worker requires that the first
> element of l_initfini is always this very map (“We are always the
> zeroth entry, and since we don't include ourselves in the
> dependency analysis start at 1.”).  Rather than fixing that
> assumption, this commit adds an implementation of the force_first
> argument to the new dependency sorting algorithm.  This also means
> that the directly dlopen'ed shared object is always initialized last,
> which is the least surprising behavior in the presence of cycles.

LGTM, thanks.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

> 
> ---
> v2: Incorporate Adhemerval's review comments.  Retested on i386-linux-gnu
>     and x86_64-linux-gnu.
>  elf/dl-sort-maps.c       | 32 +++++++++++++++++++++++---------
>  elf/dso-sort-tests-1.def |  7 +++++++
>  2 files changed, 30 insertions(+), 9 deletions(-)
> 
> diff --git a/elf/dl-sort-maps.c b/elf/dl-sort-maps.c
> index 5b550b1e94..3e2a6a584e 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];
>    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 -     no longer valid, since traversing along object dependency-links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 +267,27 @@ _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
> +     will create a N-object result, and raise problems.  Instead,
> +     force the object back into first place after sorting.  This naive
> +     approach may introduce further dependency ordering violations
> +     compared to rotating the cycle until the first map is again in
> +     the first position, but as there is a cycle, at least one
> +     violation is already present.  */
> +  if (force_first && maps[0] != first_map)
> +    {
> +      int i;
> +      for (i = 0; maps[i] != first_map; ++i)
> +	;
> +      assert (i < nmaps);
> +      memmove (&maps[1], maps, i * sizeof (maps[0]));
> +      maps[0] = first_map;
> +    }
>  }
>  
>  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
> +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
> 
> base-commit: dbb75513f5cf9285c77c9e55777c5c35b653f890
>