[PATCH 3/3] elf: Implement force_first handling in _dl_sort_maps_dfs
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Wed Aug 31 16:37:10 GMT 2022
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 - no longer valid, since traversing along object dependency-links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
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