[Bug dynamic-link/30981] New: dlclose does not properly implement force-first handling

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30981

            Bug ID: 30981
           Summary: dlclose does not properly implement force-first
                    handling
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: dynamic-link
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

The ELF destructor for the dlclose'd object should run first, but that does not
happen due to two factors:

The array passed to _dl_sort_maps is gathered from the linked list of all
shared objects. In case of cycles and an unfortuate dlopen/dlclose order, the
first array element may not be the link map passed to dlclose, so the
force_first flag does not have the intended effect.

Inside _dl_sort_maps_dfs, the relocation dependency sorting (for_fini mode) has
early returns which bypass the force_first flag processing further down in the
function's source code. This means that force_first may not have any effect if
for_fini is active.

Fixing these two issues changes the results of the existing tst-bz15311 test
case:

-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];}
+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<b<c<d<g<f<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[<a<g<f<b<c<d<e];}

In the glibc.rtld.dynamic_sort=1, the lack of nice abcd destructor ordering was
historically blamed on a sorting algorithm deficiency, while in reality it
seems to be a result of the incorrect way _dl_sort_maps was called by dlclose
(the first issue described above).

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