[PATCH v2] elf: Re-initialise static TLS after .tdata relocation (BZ 34164)
H.J. Lu
hjl.tools@gmail.com
Tue May 26 23:07:56 GMT 2026
On Wed, May 27, 2026 at 7:04 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
> The af34b1376a37fa27e1de9d869ed9493fc569bfa6 (BZ 34164) changed the
> TLS setup from:
>
> relocation loop (applies relocations to .tdata in DSO memory)
> _dl_allocate_tls_init copies relocated .tdata -> main thread TLS
>
> to a new order:
>
> _dl_allocate_tls_init copies unrelocated .tdata -> main thread TLS
> relocation loop (relocates .tdata in DSO memory, but the TLS block
> has stale copies)
>
> This broke file-scope thread-local initialised with the address of a
> function (for instance the cache structs in libmpfr).
>
> Fix it by splitting ELF_DYNAMIC_RELOCATE inside
> _dl_relocate_object_no_relro into the non-IRELATIVE and IRELATIVE
> sub-passes (similar as done on static-pie startup by b75ad99d45b)
> and call _dl_init_static_tls between them. By the time the IFUNC
> pass fires, .tdata is fully relocated.
>
> Checked on x86_64-linux-gnu and aarch64-linux-gnu.
> --
> Changes from v1:
> * Mention BZ 34164 on commit body.
> ---
> elf/Makefile | 3 +++
> elf/dl-reloc.c | 24 ++++++++++++++++++--
> elf/tst-tls-tdata-reloc-lib.c | 42 +++++++++++++++++++++++++++++++++++
> elf/tst-tls-tdata-reloc.c | 32 ++++++++++++++++++++++++++
> 4 files changed, 99 insertions(+), 2 deletions(-)
> create mode 100644 elf/tst-tls-tdata-reloc-lib.c
> create mode 100644 elf/tst-tls-tdata-reloc.c
>
> diff --git a/elf/Makefile b/elf/Makefile
> index d7bab52cd97..a58a902451c 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -1297,6 +1297,7 @@ tests += \
> tst-ifunc-resolver-protector \
> tst-ifunc-tls-init \
> tst-ifunc-tls-write \
> + tst-tls-tdata-reloc \
> # tests
> # Note: sysdeps/x86_64/ifuncmain8.c uses ifuncmain8.
> tests-internal += \
> @@ -1365,6 +1366,7 @@ modules-names += \
> tst-ifunc-tls-init-lib1 \
> tst-ifunc-tls-init-lib2 \
> tst-ifunc-tls-write-lib \
> + tst-tls-tdata-reloc-lib \
> # modules-names
> ifeq (no,$(with-lld))
> modules-names += ifuncmod5
> @@ -2518,6 +2520,7 @@ $(objpfx)tst-ifunc-tls-init: $(objpfx)tst-ifunc-tls-init-lib1.so
> $(objpfx)tst-ifunc-tls-init.out: \
> $(objpfx)tst-ifunc-tls-init-lib2.so
> $(objpfx)tst-ifunc-tls-write: $(objpfx)tst-ifunc-tls-write-lib.so
> +$(objpfx)tst-tls-tdata-reloc: $(objpfx)tst-tls-tdata-reloc-lib.so
>
> $(objpfx)tst-unique1.out: $(objpfx)tst-unique1mod1.so \
> $(objpfx)tst-unique1mod2.so
> diff --git a/elf/dl-reloc.c b/elf/dl-reloc.c
> index 91497b38397..15a6a4cffef 100644
> --- a/elf/dl-reloc.c
> +++ b/elf/dl-reloc.c
> @@ -269,9 +269,29 @@ _dl_relocate_object_no_relro (struct link_map *l, struct r_scope_elem *scope[],
> }
>
> {
> - /* Do the actual relocation of the object's GOT and other data. */
> + /* Do the actual relocation of the object's GOT and other data.
>
> - ELF_DYNAMIC_RELOCATE (l, scope, lazy, consider_profiling, skip_ifunc);
> + Process the non-IRELATIVE pass first so .tdata is fully relocated
> + (including R_*_RELATIVE / R_*_64 fixups for TLS initialisers, e.g. a
> + file-scope thread-local initialised with the address of a function),
> + then refresh the static TLS slot before the IRELATIVE pass runs the
> + IFUNC resolvers. Without this, a resolver would see the unrelocated
> + initialiser bytes that were placed into the slot by the early
> + _dl_allocate_tls_init. */
> + ELF_DYNAMIC_RELOCATE_NOIFUNC (l, scope, lazy, consider_profiling);
> +
> +#ifdef SHARED
> + /* Re-initialise the static TLS slot with the .tdata so the IRELATIVE
> + pass observes a fully-relocated initialiser image. Skipped for objects
> + without static TLS or before the main thread TCB has been set up. */
> + if (l->l_tls_blocksize != 0
> + && __rtld_tls_init_tp_called
> + && l->l_tls_offset != NO_TLS_OFFSET
> + && l->l_tls_offset != FORCED_DYNAMIC_TLS_OFFSET)
> + _dl_init_static_tls (l);
> +#endif
> +
> + ELF_DYNAMIC_RELOCATE_IFUNC (l, scope, lazy, skip_ifunc);
>
> if ((consider_profiling || consider_symbind)
> && l->l_info[DT_PLTRELSZ] != NULL)
> diff --git a/elf/tst-tls-tdata-reloc-lib.c b/elf/tst-tls-tdata-reloc-lib.c
> new file mode 100644
> index 00000000000..73aa58b0722
> --- /dev/null
> +++ b/elf/tst-tls-tdata-reloc-lib.c
> @@ -0,0 +1,42 @@
> +/* Shared library for tst-tls-tdata-reloc.
> + Copyright (C) 2026 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/>. */
> +
> +#define EXPECTED 0x12345678
> +
> +static int
> +callee (void)
> +{
> + return EXPECTED;
> +}
> +
> +typedef struct
> +{
> + int (*func) (void);
> +} cache_t;
> +
> +/* General-dynamic TLS: the initialiser of '.func' produces an
> + R_*_RELATIVE on .tdata. */
> +__thread cache_t cache = { .func = callee };
> +
> +int
> +call_through_tls (void)
> +{
> + if (cache.func == 0)
> + return -1;
> + return cache.func ();
> +}
> diff --git a/elf/tst-tls-tdata-reloc.c b/elf/tst-tls-tdata-reloc.c
> new file mode 100644
> index 00000000000..1d38726e364
> --- /dev/null
> +++ b/elf/tst-tls-tdata-reloc.c
> @@ -0,0 +1,32 @@
> +/* Check that .tdata relocations are applied to the static TLS slot.
> + Copyright (C) 2026 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/>. */
> +
> +#include <support/check.h>
> +
> +#define EXPECTED 0x12345678
> +
> +extern int call_through_tls (void);
> +
> +static int
> +do_test (void)
> +{
> + TEST_COMPARE (call_through_tls (), EXPECTED);
> + return 0;
> +}
> +
> +#include <support/test-driver.c>
> --
> 2.43.0
>
LGTM.
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
Thanks.
--
H.J.
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