diff mbox series

[v2,08/14] elf: Fix DTV gap reuse logic [BZ #27135]

Message ID 299d28c6695cd2e76f222b0d36b17b7124c549e7.1618301209.git.szabolcs.nagy@arm.com
State Committed
Commit 572bd547d57a39b6cf0ea072545dc4048921f4c3
Delegated to: Adhemerval Zanella Netto
Headers show
Series Dynamic TLS related data race fixes | expand

Commit Message

Szabolcs Nagy April 13, 2021, 8:20 a.m. UTC
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.
---
 elf/dl-close.c |  6 +++++-
 elf/dl-open.c  | 10 ----------
 elf/dl-tls.c   |  5 +----
 3 files changed, 6 insertions(+), 15 deletions(-)

Comments

Adhemerval Zanella April 15, 2021, 7:45 p.m. UTC | #1
On 13/04/2021 05:20, Szabolcs Nagy via Libc-alpha wrote:
> 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.

LGTM, thanks.

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

> ---
>  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))

Ok.

> diff --git a/elf/dl-open.c b/elf/dl-open.c
> index 83b8e96a5c..661f26977e 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

Ok.

> diff --git a/elf/dl-tls.c b/elf/dl-tls.c
> index c4466bd9fc..b0257185e9 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);
>  
> 

Ok.
diff mbox series

Patch

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 83b8e96a5c..661f26977e 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 c4466bd9fc..b0257185e9 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);