From ffb17e7ba3a5ba9632cee97330b325072fbe41dd Mon Sep 17 00:00:00 2001 From: Szabolcs Nagy Date: Wed, 10 Jun 2020 13:40:40 +0100 Subject: rtld: Avoid using up static TLS surplus for optimizations [BZ #25051] On some targets static TLS surplus area can be used opportunistically for dynamically loaded modules such that the TLS access then becomes faster (TLSDESC and powerpc TLS optimization). However we don't want all surplus TLS to be used for this optimization because dynamically loaded modules with initial-exec model TLS can only use surplus TLS. The new contract for surplus static TLS use is: - libc.so can have up to 192 bytes of IE TLS, - other system libraries together can have up to 144 bytes of IE TLS. - Some "optional" static TLS is available for opportunistic use. The optional TLS is now tunable: rtld.optional_static_tls, so users can directly affect the allocated static TLS size. (Note that module unloading with dlclose does not reclaim static TLS. After the optional TLS runs out, TLS access is no longer optimized to use static TLS.) The default setting of rtld.optional_static_tls is 512 so the surplus TLS is 3*192 + 4*144 + 512 = 1664 by default, the same as before. Fixes BZ #25051. Tested on aarch64-linux-gnu and x86_64-linux-gnu. Reviewed-by: Carlos O'Donell --- elf/tst-tls-ie.c | 111 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 111 insertions(+) create mode 100644 elf/tst-tls-ie.c (limited to 'elf/tst-tls-ie.c') diff --git a/elf/tst-tls-ie.c b/elf/tst-tls-ie.c new file mode 100644 index 0000000..2dc0894 --- /dev/null +++ b/elf/tst-tls-ie.c @@ -0,0 +1,111 @@ +/* Test dlopen of modules with initial-exec TLS. + Copyright (C) 2016-2020 Free Software Foundation, Inc. + This file is part of the GNU C Library. + + The GNU C Library is free software; you can redistribute it and/or + modify it under the terms of the GNU Lesser General Public + License as published by the Free Software Foundation; either + version 2.1 of the License, or (at your option) any later version. + + The GNU C Library is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU + Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public + License along with the GNU C Library; if not, see + . */ + +/* This test tries to check that surplus static TLS is not used up for + dynamic TLS optimizations and 3*192 + 4*144 = 1152 bytes of static + TLS is available for dlopening modules with initial-exec TLS. It + depends on rtld.nns=4 and rtld.optional_static_tls=512 tunable setting. */ + +#include +#include +#include +#include +#include + +static int do_test (void); +#include +#include +#include +#include + +/* Have some big TLS in the main exe: should not use surplus TLS. */ +__thread char maintls[1000]; + +static pthread_barrier_t barrier; + +/* Forces multi-threaded behaviour. */ +static void * +blocked_thread_func (void *closure) +{ + xpthread_barrier_wait (&barrier); + /* TLS load and access tests run here in the main thread. */ + xpthread_barrier_wait (&barrier); + return NULL; +} + +static void * +load_and_access (const char *mod, const char *func) +{ + /* Load module with TLS. */ + void *p = xdlopen (mod, RTLD_NOW); + /* Access the TLS variable to ensure it is allocated. */ + void (*f) (void) = (void (*) (void))xdlsym (p, func); + f (); + return p; +} + +static int +do_test (void) +{ + void *mods[6]; + + { + int ret = pthread_barrier_init (&barrier, NULL, 2); + if (ret != 0) + { + errno = ret; + printf ("error: pthread_barrier_init: %m\n"); + exit (1); + } + } + + pthread_t blocked_thread = xpthread_create (NULL, blocked_thread_func, NULL); + xpthread_barrier_wait (&barrier); + + printf ("maintls[%zu]:\t %p .. %p\n", + sizeof maintls, maintls, maintls + sizeof maintls); + memset (maintls, 1, sizeof maintls); + + /* Load modules with dynamic TLS (may use surplus static TLS + opportunistically). */ + mods[0] = load_and_access ("tst-tls-ie-mod0.so", "access0"); + mods[1] = load_and_access ("tst-tls-ie-mod1.so", "access1"); + mods[2] = load_and_access ("tst-tls-ie-mod2.so", "access2"); + mods[3] = load_and_access ("tst-tls-ie-mod3.so", "access3"); + /* Load modules with initial-exec TLS (can only use surplus static TLS). */ + mods[4] = load_and_access ("tst-tls-ie-mod4.so", "access4"); + mods[5] = load_and_access ("tst-tls-ie-mod5.so", "access5"); + + /* Here 1152 bytes of surplus static TLS is in use and at most 512 bytes + are available (depending on TLS optimizations). */ + printf ("The next dlopen should fail...\n"); + void *p = dlopen ("tst-tls-ie-mod6.so", RTLD_NOW); + if (p != NULL) + FAIL_EXIT1 ("error: expected dlopen to fail because there is " + "not enough surplus static TLS.\n"); + printf ("...OK failed with: %s.\n", dlerror ()); + + xpthread_barrier_wait (&barrier); + xpthread_join (blocked_thread); + + /* Close the modules. */ + for (int i = 0; i < 6; ++i) + xdlclose (mods[i]); + + return 0; +} -- cgit v1.1