rtld: Add rtld.nns tunable for the number of supported namespaces

TLS_STATIC_SURPLUS is 1664 bytes currently which is not enough to
support DL_NNS (== 16) number of dynamic link namespaces, if we
assume 192 bytes of TLS are reserved for libc use and 144 bytes
are reserved for other system libraries that use IE TLS.

A new tunable is introduced to control the number of supported
namespaces and to adjust the surplus static TLS size as follows:

surplus_tls = 192 * (rtld.nns-1) + 144 * rtld.nns + 512

The default is rtld.nns == 4 and then the surplus TLS size is the
same as before, so the behaviour is unchanged by default. If an
application creates more namespaces than the rtld.nns setting
allows, then it is not guaranteed to work, but the limit is not
checked. So existing usage will continue to work, but in the
future if an application creates more than 4 dynamic link
namespaces then the tunable will need to be set.

In this patch DL_NNS is a fixed value and provides a maximum to
the rtld.nns setting.

Static linking used fixed 2048 bytes surplus TLS, this is changed
so the same contract is used as for dynamic linking.  With static
linking DL_NNS == 1 so rtld.nns tunable is forced to 1, so by
default the surplus TLS is reduced to 144 + 512 = 656 bytes. This
change is not expected to cause problems.

Tested on aarch64-linux-gnu and x86_64-linux-gnu.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/manual/tunables.texi b/manual/tunables.texi
index 4e68c7ff91..dbfb3308d1 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -31,6 +31,7 @@ their own namespace.
 @menu
 * Tunable names::  The structure of a tunable name
 * Memory Allocation Tunables::  Tunables in the memory allocation subsystem
+* Dynamic Linking Tunables:: Tunables in the dynamic linking subsystem
 * Elision Tunables::  Tunables in elision subsystem
 * POSIX Thread Tunables:: Tunables in the POSIX thread subsystem
 * Hardware Capability Tunables::  Tunables that modify the hardware
@@ -226,6 +227,26 @@ pointer, so add 4 on 32-bit systems or 8 on 64-bit systems to the size
 passed to @code{malloc} for the largest bin size to enable.
 @end deftp
 
+@node Dynamic Linking Tunables
+@section Dynamic Linking Tunables
+@cindex dynamic linking tunables
+@cindex rtld tunables
+
+@deftp {Tunable namespace} glibc.rtld
+Dynamic linker behavior can be modified by setting the
+following tunables in the @code{rtld} namespace:
+@end deftp
+
+@deftp Tunable glibc.rtld.nns
+Sets the number of supported dynamic link namespaces (see @code{dlmopen}).
+Currently this limit can be set between 1 and 16 inclusive, the default is 4.
+Each link namespace consumes some memory in all thread, and thus raising the
+limit will increase the amount of memory each thread uses. Raising the limit
+is useful when your application uses more than 4 dynamic linker audit modules
+e.g. @env{LD_AUDIT}, or will use more than 4 dynamic link namespaces as created
+by @code{dlmopen} with an lmid argument of @code{LM_ID_NEWLM}.
+@end deftp
+
 @node Elision Tunables
 @section Elision Tunables
 @cindex elision tunables