Move malloc hooks into a compat DSO

Remove all malloc hook uses from core malloc functions and move it
into a new library libc_malloc_debug.so.  With this, the hooks now no
longer have any effect on the core library.

libc_malloc_debug.so is a malloc interposer that needs to be preloaded
to get hooks functionality back so that the debugging features that
depend on the hooks, i.e. malloc-check, mcheck and mtrace work again.
Without the preloaded DSO these debugging features will be nops.
These features will be ported away from hooks in subsequent patches.

Similarly, legacy applications that need hooks functionality need to
preload libc_malloc_debug.so.

The symbols exported by libc_malloc_debug.so are maintained at exactly
the same version as libc.so.

Finally, static binaries will no longer be able to use malloc
debugging features since they cannot preload the debugging DSO.

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

1 files changed, 6 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 75ad8a9b95..fa80c9685b 100644
--- a/NEWS
+++ b/NEWS
@@ -132,6 +132,12 @@ Deprecated and removed features, and other changes affecting compatibility:
   applications will continue to link against these symbols but the interfaces
   no longer have any effect on malloc.
 
+* Debugging features in malloc such as the MALLOC_CHECK_ environment variable
+  (or the glibc.malloc.check tunable), mtrace() and mcheck() have now been
+  disabled by default in the main C library.  Users looking to use these
+  features now need to preload a new debugging DSO libc_malloc_debug.so to get
+  this functionality back.
+
 Changes to build and runtime requirements:
 
 * On Linux, the shm_open, sem_open, and related functions now expect the