[PATCH v2 02/13] elf, nptl: Resolve recursive lock implementation early
Carlos O'Donell
carlos@redhat.com
Sun May 9 21:42:23 GMT 2021
On 5/6/21 2:09 PM, Florian Weimer via Libc-alpha wrote:
> If libpthread is included in libc, it is not necessary to delay
> initialization of the lock/unlock function pointers until libpthread
> is loaded. This eliminates two unprotected function pointers
> from _rtld_global and removes some initialization code from
> libpthread.
This version looks good to me, and the early initialization makes it
logically easier to follow when reading the code. Despite the removal
of the unprotected function pointesr in _rtld_global, we still need
some function pointer in order to lookup the function symbols from libc.so
and remember their values, but data placement is harder to discover than
the fixed offset from a public symbol.
Tested on x86_64 and i686 without regression.
Tested-by: Carlos O'Donell <carlos@redhat.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
> ---
> v2: Rename dl-lock.c into dl-mutex.c and use a sysdeps override instead
> of a preprocessor conditional.
>
> elf/Makefile | 3 ++-
> elf/dl-mutex.c | 19 ++++++++++++++
> elf/rtld.c | 18 +++++++++++++
> nptl/nptl-init.c | 9 -------
> sysdeps/generic/ldsodefs.h | 25 +++++++++++++++++-
> sysdeps/nptl/dl-mutex.c | 53 ++++++++++++++++++++++++++++++++++++++
> sysdeps/nptl/libc-lockP.h | 17 +++---------
> 7 files changed, 120 insertions(+), 24 deletions(-)
> create mode 100644 elf/dl-mutex.c
> create mode 100644 sysdeps/nptl/dl-mutex.c
>
> diff --git a/elf/Makefile b/elf/Makefile
> index 4f99af626f..d3e909637a 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -66,7 +66,8 @@ elide-routines.os = $(all-dl-routines) dl-support enbl-secure dl-origin \
> # interpreter and operating independent of libc.
> rtld-routines = rtld $(all-dl-routines) dl-sysdep dl-environ dl-minimal \
> dl-error-minimal dl-conflict dl-hwcaps dl-hwcaps_split dl-hwcaps-subdirs \
> - dl-usage dl-diagnostics dl-diagnostics-kernel dl-diagnostics-cpu
> + dl-usage dl-diagnostics dl-diagnostics-kernel dl-diagnostics-cpu \
> + dl-mutex
> all-rtld-routines = $(rtld-routines) $(sysdep-rtld-routines)
>
> CFLAGS-dl-runtime.c += -fexceptions -fasynchronous-unwind-tables
> diff --git a/elf/dl-mutex.c b/elf/dl-mutex.c
> new file mode 100644
> index 0000000000..2cd9d49c2e
> --- /dev/null
> +++ b/elf/dl-mutex.c
> @@ -0,0 +1,19 @@
> +/* Recursive locking implementation for the dynamic loader. Generic version.
> + Copyright (C) 2021 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
> + <https://www.gnu.org/licenses/>. */
> +
> +/* The generic version initialization happpens in dl_main. */
> diff --git a/elf/rtld.c b/elf/rtld.c
> index ad325d4c10..a359167f8a 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -857,6 +857,14 @@ rtld_lock_default_unlock_recursive (void *lock)
> __rtld_lock_default_unlock_recursive (lock);
> }
> #endif
> +#if PTHREAD_IN_LIBC
> +/* Dummy implementation. See __rtld_mutex_init. */
> +static int
> +rtld_mutex_dummy (pthread_mutex_t *lock)
> +{
> + return 0;
> +}
> +#endif
>
>
> static void
> @@ -1148,6 +1156,10 @@ dl_main (const ElfW(Phdr) *phdr,
> GL(dl_rtld_lock_recursive) = rtld_lock_default_lock_recursive;
> GL(dl_rtld_unlock_recursive) = rtld_lock_default_unlock_recursive;
> #endif
> +#if PTHREAD_IN_LIBC
> + ___rtld_mutex_lock = rtld_mutex_dummy;
> + ___rtld_mutex_unlock = rtld_mutex_dummy;
> +#endif
>
> /* The explicit initialization here is cheaper than processing the reloc
> in the _rtld_local definition's initializer. */
> @@ -2363,6 +2375,9 @@ dl_main (const ElfW(Phdr) *phdr,
> loader. */
> __rtld_malloc_init_real (main_map);
>
> + /* Likewise for the locking implementation. */
> + __rtld_mutex_init ();
> +
> /* Mark all the objects so we know they have been already relocated. */
> for (struct link_map *l = main_map; l != NULL; l = l->l_next)
> {
> @@ -2468,6 +2483,9 @@ dl_main (const ElfW(Phdr) *phdr,
> at this point. */
> __rtld_malloc_init_real (main_map);
>
> + /* Likewise for the locking implementation. */
> + __rtld_mutex_init ();
> +
> RTLD_TIMING_VAR (start);
> rtld_timer_start (&start);
>
> diff --git a/nptl/nptl-init.c b/nptl/nptl-init.c
> index fcab5a0904..2724770533 100644
> --- a/nptl/nptl-init.c
> +++ b/nptl/nptl-init.c
> @@ -179,15 +179,6 @@ __pthread_initialize_minimal_internal (void)
> lll_unlock (__default_pthread_attr_lock, LLL_PRIVATE);
>
> #ifdef SHARED
> - /* Make __rtld_lock_{,un}lock_recursive use pthread_mutex_{,un}lock,
> - keep the lock count from the ld.so implementation. */
> - GL(dl_rtld_lock_recursive) = (void *) __pthread_mutex_lock;
> - GL(dl_rtld_unlock_recursive) = (void *) __pthread_mutex_unlock;
> - unsigned int rtld_lock_count = GL(dl_load_lock).mutex.__data.__count;
> - GL(dl_load_lock).mutex.__data.__count = 0;
> - while (rtld_lock_count-- > 0)
> - __pthread_mutex_lock (&GL(dl_load_lock).mutex);
> -
> GL(dl_make_stack_executable_hook) = &__make_stacks_executable;
> #endif
>
> diff --git a/sysdeps/generic/ldsodefs.h b/sysdeps/generic/ldsodefs.h
> index 1b064c5894..6d590d1335 100644
> --- a/sysdeps/generic/ldsodefs.h
> +++ b/sysdeps/generic/ldsodefs.h
> @@ -403,7 +403,7 @@ struct rtld_global
> struct auditstate _dl_rtld_auditstate[DL_NNS];
> #endif
>
> -#if defined SHARED && defined _LIBC_REENTRANT \
> +#if !PTHREAD_IN_LIBC && defined SHARED \
> && defined __rtld_lock_default_lock_recursive
> EXTERN void (*_dl_rtld_lock_recursive) (void *);
> EXTERN void (*_dl_rtld_unlock_recursive) (void *);
> @@ -1318,6 +1318,29 @@ link_map_audit_state (struct link_map *l, size_t index)
> }
> #endif /* SHARED */
>
> +#if PTHREAD_IN_LIBC && defined SHARED
> +/* Recursive locking implementation for use within the dynamic loader.
> + Used to define the __rtld_lock_lock_recursive and
> + __rtld_lock_unlock_recursive via <libc-lock.h>. Initialized to a
> + no-op dummy implementation early. Similar
> + to GL (dl_rtld_lock_recursive) and GL (dl_rtld_unlock_recursive)
> + in !PTHREAD_IN_LIBC builds. */
> +extern int (*___rtld_mutex_lock) (pthread_mutex_t *) attribute_hidden;
> +extern int (*___rtld_mutex_unlock) (pthread_mutex_t *lock) attribute_hidden;
> +
> +/* Called after libc has been loaded, but before RELRO is activated.
> + Used to initialize the function pointers to the actual
> + implementations. */
> +void __rtld_mutex_init (void) attribute_hidden;
> +#else /* !PTHREAD_IN_LIBC */
> +static inline void
> +__rtld_mutex_init (void)
> +{
> + /* The initialization happens later (!PTHREAD_IN_LIBC) or is not
> + needed at all (!SHARED). */
> +}
> +#endif /* !PTHREAD_IN_LIBC */
> +
> #if THREAD_GSCOPE_IN_TCB
> void __thread_gscope_wait (void) attribute_hidden;
> # define THREAD_GSCOPE_WAIT() __thread_gscope_wait ()
> diff --git a/sysdeps/nptl/dl-mutex.c b/sysdeps/nptl/dl-mutex.c
> new file mode 100644
> index 0000000000..08b71dc21b
> --- /dev/null
> +++ b/sysdeps/nptl/dl-mutex.c
> @@ -0,0 +1,53 @@
> +/* Recursive locking implementation for the dynamic loader. NPTL version.
> + Copyright (C) 2021 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
> + <https://www.gnu.org/licenses/>. */
> +
> +/* Use the mutex implementation in libc (assuming PTHREAD_IN_LIBC). */
> +
> +#include <assert.h>
> +#include <first-versions.h>
> +#include <ldsodefs.h>
> +
> +__typeof (pthread_mutex_lock) *___rtld_mutex_lock attribute_relro;
> +__typeof (pthread_mutex_unlock) *___rtld_mutex_unlock attribute_relro;
> +
> +void
> +__rtld_mutex_init (void)
> +{
> + /* There is an implicit assumption here that the lock counters are
> + zero and this function is called while nothing is locked. For
> + early initialization of the mutex functions this is true because
> + it happens directly in dl_main in elf/rtld.c, and not some ELF
> + constructor while holding loader locks. */
> +
> + struct link_map *libc_map = GL (dl_ns)[LM_ID_BASE].libc_map;
> +
> + const ElfW(Sym) *sym
> + = _dl_lookup_direct (libc_map, "pthread_mutex_lock",
> + 0x4f152227, /* dl_new_hash output. */
> + FIRST_VERSION_libc_pthread_mutex_lock_STRING,
> + FIRST_VERSION_libc_pthread_mutex_lock_HASH);
> + assert (sym != NULL);
> + ___rtld_mutex_lock = DL_SYMBOL_ADDRESS (libc_map, sym);
> +
> + sym = _dl_lookup_direct (libc_map, "pthread_mutex_unlock",
> + 0x7dd7aaaa, /* dl_new_hash output. */
> + FIRST_VERSION_libc_pthread_mutex_unlock_STRING,
> + FIRST_VERSION_libc_pthread_mutex_unlock_HASH);
> + assert (sym != NULL);
> + ___rtld_mutex_unlock = DL_SYMBOL_ADDRESS (libc_map, sym);
> +}
> diff --git a/sysdeps/nptl/libc-lockP.h b/sysdeps/nptl/libc-lockP.h
> index ae9691d40e..ec7b02bbdd 100644
> --- a/sysdeps/nptl/libc-lockP.h
> +++ b/sysdeps/nptl/libc-lockP.h
> @@ -151,9 +151,6 @@ _Static_assert (LLL_LOCK_INITIALIZER == 0, "LLL_LOCK_INITIALIZER != 0");
> __libc_maybe_call (__pthread_mutex_trylock, (&(NAME)), 0)
> #endif
>
> -#define __rtld_lock_trylock_recursive(NAME) \
> - __libc_maybe_call (__pthread_mutex_trylock, (&(NAME).mutex), 0)
> -
> /* Unlock the named lock variable. */
> #if IS_IN (libc) || IS_IN (libpthread)
> # define __libc_lock_unlock(NAME) \
> @@ -163,19 +160,13 @@ _Static_assert (LLL_LOCK_INITIALIZER == 0, "LLL_LOCK_INITIALIZER != 0");
> #endif
> #define __libc_rwlock_unlock(NAME) __pthread_rwlock_unlock (&(NAME))
>
> -#ifdef SHARED
> -# define __rtld_lock_default_lock_recursive(lock) \
> - ++((pthread_mutex_t *)(lock))->__data.__count;
> -
> -# define __rtld_lock_default_unlock_recursive(lock) \
> - --((pthread_mutex_t *)(lock))->__data.__count;
> -
> +#if IS_IN (rtld)
> # define __rtld_lock_lock_recursive(NAME) \
> - GL(dl_rtld_lock_recursive) (&(NAME).mutex)
> + ___rtld_mutex_lock (&(NAME).mutex)
>
> # define __rtld_lock_unlock_recursive(NAME) \
> - GL(dl_rtld_unlock_recursive) (&(NAME).mutex)
> -#else
> + ___rtld_mutex_unlock (&(NAME).mutex)
> +#else /* Not in the dynamic loader. */
> # define __rtld_lock_lock_recursive(NAME) \
> __pthread_mutex_lock (&(NAME).mutex)
>
>
--
Cheers,
Carlos.
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