[Bug network/19690] New: Do not store resolver state (_res) in TCB

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19690

            Bug ID: 19690
           Summary: Do not store resolver state (_res) in TCB
           Product: glibc
           Version: 2.24
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---
             Flags: security-

struct __res_state is fairly large:

(gdb) print sizeof (struct __res_state)
$3 = 568

A copy is included in struct pthread (see nptl/descr.h):

    378   /* Resolver state.  */
    379   struct __res_state res;

Lazy allocation is somewhat difficult because allocation is not allowed to
fail.  We can avoid the allocation with some linker hacks, for binaries which
do not reference the thread-local _res structure.  I think this is a worthwhile
optimization.

nss_dns and nss_files do not actually have to use the _res structure if we
provide a better internal interface for their needs.  Almost all of the state
used by these NSS modules is global, not thread-local because it comes from
/etc/resolv.conf.

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