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

[Szabolcs Nagy <nsz@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=572bd547d57a39b6cf0ea072545dc4048921f4c3

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:
---
 elf/dl-close.c |  6 +++++-
 elf/dl-open.c  | 10 ----------
 elf/dl-tls.c   |  5 +----
 3 files changed, 6 insertions(+), 15 deletions(-)

diff --git a/elf/dl-close.c b/elf/dl-close.c
index 3720e47dd1..9f31532f41 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 bb79ef00f1..0887fc5cc5 100644
--- a/elf/dl-open.c
+++ b/elf/dl-open.c
@@ -890,16 +890,6 @@ 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 dc69cd984e..67781bc108 100644
--- a/elf/dl-tls.c
+++ b/elf/dl-tls.c
@@ -191,10 +191,7 @@ _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);