commit 572bd547d57a39b6cf0ea072545dc4048921f4c3

Author: Szabolcs Nagy <szabolcs.nagy@arm.com>

Date:   Thu Dec 31 13:59:38 2020 +0000

    elf: Fix DTV gap reuse logic [BZ #27135]

    For some reason only dlopen failure caused dtv gaps to be reused.

    It is possible that the intent was to never reuse modids for a

    different module, but after dlopen failure all gaps are reused

    not just the ones caused by the unfinished dlopened.

    So the code has to handle reused modids already which seems to

    work, however the data races at thread creation and tls access

    (see bug 19329 and bug 27111) may be more severe if slots are

    reused so this is scheduled after those fixes. I think fixing

    the races are not simpler if reuse is disallowed and reuse has

    other benefits, so set GL(dl_tls_dtv_gaps) whenever entries are

    removed from the middle of the slotinfo list. The value does

    not have to be correct: incorrect true value causes the next

    modid query to do a slotinfo walk, incorrect false will leave

    gaps and new entries are added at the end.

    Fixes bug 27135.

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

diff --git a/elf/dl-close.c b/elf/dl-close.c

index 7d2dc2272cd643f5..41cb6c58491c364b 100644

--- a/elf/dl-close.c

+++ b/elf/dl-close.c

@@ -88,7 +88,11 @@ remove_slotinfo (size_t idx, struct dtv_slotinfo_list *listp, size_t disp,

       /* If this is not the last currently used entry no need to look

 	 further.  */

       if (idx != GL(dl_tls_max_dtv_idx))

-	return true;

+	{

+	  /* There is an unused dtv entry in the middle.  */

+	  GL(dl_tls_dtv_gaps) = true;

+	  return true;

+	}

     }

   while (idx - disp > (disp == 0 ? 1 + GL(dl_tls_static_nelem) : 0))

diff --git a/elf/dl-open.c b/elf/dl-open.c

index a67fb3aee40860e1..54727402750f4c0c 100644

--- a/elf/dl-open.c

+++ b/elf/dl-open.c

@@ -896,16 +896,6 @@ no more namespaces available for dlmopen()"));

 	 state if relocation failed, for example.  */

       if (args.map)

 	{

-	  /* Maybe some of the modules which were loaded use TLS.

-	     Since it will be removed in the following _dl_close call

-	     we have to mark the dtv array as having gaps to fill the

-	     holes.  This is a pessimistic assumption which won't hurt

-	     if not true.  There is no need to do this when we are

-	     loading the auditing DSOs since TLS has not yet been set

-	     up.  */

-	  if ((mode & __RTLD_AUDIT) == 0)

-	    GL(dl_tls_dtv_gaps) = true;

-

 	  _dl_close_worker (args.map, true);

 	  /* All l_nodelete_pending objects should have been deleted

diff --git a/elf/dl-tls.c b/elf/dl-tls.c

index 801eafad3961573c..bacb4101e2e2c4e5 100644

--- a/elf/dl-tls.c

+++ b/elf/dl-tls.c

@@ -187,10 +187,7 @@ _dl_next_tls_modid (void)

 size_t

 _dl_count_modids (void)

 {

-  /* It is rare that we have gaps; see elf/dl-open.c (_dl_open) where

-     we fail to load a module and unload it leaving a gap.  If we don't

-     have gaps then the number of modids is the current maximum so

-     return that.  */

+  /* The count is the max unless dlclose or failed dlopen created gaps.  */

   if (__glibc_likely (!GL(dl_tls_dtv_gaps)))

     return GL(dl_tls_max_dtv_idx);