[PATCH v6 1/2] elf(tls): Add debug logging for TLS operations
Adhemerval Zanella Netto
adhemerval.zanella@linaro.org
Mon Mar 2 20:34:50 GMT 2026
On 25/02/26 15:51, Frédéric Bérat wrote:
> This commit introduces extensive debug logging for thread-local storage
> (TLS) operations within the dynamic linker. When `LD_DEBUG=tls` is
> enabled, messages are printed for:
> - TLS module assignment and release.
> - DTV (Dynamic Thread Vector) resizing events.
> - TLS block allocations and deallocations.
> - `__tls_get_addr` slow path events (DTV updates, lazy allocations, and
> static TLS usage).
>
> The log format is standardized to use a "tls: " prefix and identifies
> modules using the "modid %lu" convention. To aid in debugging
> multithreaded applications, thread-specific logs include the Thread
> Control Block (TCB) address to identify the context of the operation.
>
> A new test module `tst-tls-debug-mod.c` and a corresponding shell script
> `tst-tls-debug-recursive.sh` have been added. Additionally, the existing
> `tst-dl-debug-tid` NPTL test has been updated to verify these TLS debug
> messages in a multithreaded context.
This version looks good to me.
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
> ---
> Changes in v6:
> - Increase TLS size in elf/tst-recursive-tlsmodN.c.
> - Moved tests-special additions before Rules inclusion
>
> elf/Makefile | 14 ++++++
> elf/dl-close.c | 5 ++
> elf/dl-tls.c | 83 +++++++++++++++++++++++++++++-----
> elf/rtld.c | 5 ++
> elf/tst-recursive-tlsmodN.c | 4 +-
> elf/tst-tls-debug-recursive.sh | 83 ++++++++++++++++++++++++++++++++++
> nptl/Makefile | 5 +-
> nptl/allocatestack.c | 10 ++--
> nptl/nptl-stack.c | 6 +--
> nptl/pthread_create.c | 2 +-
> nptl/tst-dl-debug-tid.c | 13 ++++++
> nptl/tst-dl-debug-tid.sh | 33 +++++++++++---
> nptl/tst-tls-debug-mod.c | 26 +++++++++++
> sysdeps/x86_64/dl-tls.c | 33 ++++++++++++--
> 14 files changed, 289 insertions(+), 33 deletions(-)
> create mode 100755 elf/tst-tls-debug-recursive.sh
> create mode 100644 nptl/tst-tls-debug-mod.c
>
> diff --git a/elf/Makefile b/elf/Makefile
> index 1a0ed49bc3..caa5ff025f 100644
> --- a/elf/Makefile
> +++ b/elf/Makefile
> @@ -1433,6 +1433,7 @@ $(objpfx)tst-glibcelf.out: tst-glibcelf.py elf.h $(..)/scripts/glibcelf.py \
>
> ifeq ($(run-built-tests),yes)
> tests-special += $(objpfx)tst-tls-allocation-failure-static-patched.out
> +tests-special += $(objpfx)tst-tls-debug-recursive.out
> endif
>
> # The test requires shared _and_ PIE because the executable
> @@ -3533,3 +3534,16 @@ $(objpfx)tst-origin.out: tst-origin.sh $(objpfx)tst-origin
> $(evaluate-test)
>
> $(objpfx)tst-dlopen-sgid.out: $(objpfx)tst-dlopen-sgid-mod.so
> +
> +
> +ifeq ($(run-built-tests),yes)
> +$(objpfx)tst-tls-debug-recursive.out: tst-tls-debug-recursive.sh \
> + $(objpfx)tst-recursive-tls \
> + $(objpfx)tst-recursive-tlsmallocmod.so \
> + $(patsubst %,$(objpfx)tst-recursive-tlsmod%.so, \
> + 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15)
> + $(SHELL) $< $(common-objpfx) '$(test-wrapper-env)' \
> + '$(rtld-prefix)' '$(run_program_env)' \
> + $(objpfx)tst-recursive-tls > $@; \
> + $(evaluate-test)
> +endif
> diff --git a/elf/dl-close.c b/elf/dl-close.c
> index fca877534e..92bce07c7a 100644
> --- a/elf/dl-close.c
> +++ b/elf/dl-close.c
> @@ -74,6 +74,11 @@ remove_slotinfo (size_t idx, struct dtv_slotinfo_list *listp, size_t disp,
> if (__glibc_likely (old_map != NULL))
> {
> /* Mark the entry as unused. These can be read concurrently. */
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf (
> + "tls: release modid %lu from %s [%ld]\n",
> + (unsigned long int) idx, DSO_FILENAME (old_map->l_name),
> + (long int) old_map->l_ns);
> atomic_store_relaxed (&listp->slotinfo[idx - disp].gen,
> GL(dl_tls_generation) + 1);
> atomic_store_relaxed (&listp->slotinfo[idx - disp].map, NULL);
> diff --git a/elf/dl-tls.c b/elf/dl-tls.c
> index 8cef809261..5bfcb184ed 100644
> --- a/elf/dl-tls.c
> +++ b/elf/dl-tls.c
> @@ -220,6 +220,12 @@ _dl_assign_tls_modid (struct link_map *l)
> }
>
> l->l_tls_modid = result;
> +
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf ("tls: assign modid %lu to %s [%ld]\n",
> + (unsigned long int) result,
> + DSO_FILENAME (l->l_name),
> + (long int) l->l_ns);
> }
>
>
> @@ -538,7 +544,7 @@ _dl_allocate_tls_storage (void)
> if (result == NULL)
> free (allocated);
> else if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> - _dl_debug_printf ("TCB allocated: 0x%lx\n", (unsigned long int) result);
> + _dl_debug_printf ("tls: allocate TCB 0x%lx\n", (unsigned long int) result);
>
> _dl_tls_allocate_end ();
> return result;
> @@ -551,13 +557,18 @@ extern dtv_t _dl_static_dtv[];
> #endif
>
> static dtv_t *
> -_dl_resize_dtv (dtv_t *dtv, size_t max_modid)
> +_dl_resize_dtv (dtv_t *dtv, size_t max_modid, void *tcb)
> {
> /* Resize the dtv. */
> dtv_t *newp;
> size_t newsize = max_modid + DTV_SURPLUS;
> size_t oldsize = dtv[-1].counter;
>
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf ("tls: DTV resized for TCB 0x%lx: oldsize=%lu, newsize=%lu\n",
> + (unsigned long int) tcb,
> + (unsigned long int) oldsize, (unsigned long int) newsize);
> +
> _dl_tls_allocate_begin ();
> if (dtv == GL(dl_initial_dtv))
> {
> @@ -626,7 +637,7 @@ _dl_allocate_tls_init (void *result, bool main_thread)
> if (dtv[-1].counter < GL(dl_tls_max_dtv_idx))
> {
> /* Resize the dtv. */
> - dtv = _dl_resize_dtv (dtv, GL(dl_tls_max_dtv_idx));
> + dtv = _dl_resize_dtv (dtv, GL(dl_tls_max_dtv_idx), result);
>
> /* Install this new dtv in the thread data structures. */
> INSTALL_DTV (result, &dtv[-1]);
> @@ -717,9 +728,14 @@ rtld_hidden_def (_dl_allocate_tls_init)
> void *
> _dl_allocate_tls (void *mem)
> {
> - return _dl_allocate_tls_init (mem == NULL
> - ? _dl_allocate_tls_storage ()
> - : allocate_dtv (mem), false);
> + void *result = _dl_allocate_tls_init (mem == NULL
> + ? _dl_allocate_tls_storage ()
> + : allocate_dtv (mem), false);
> + if (__glibc_unlikely (result != NULL
> + && (GLRO (dl_debug_mask) & DL_DEBUG_TLS)))
> + _dl_debug_printf ("tls: TLS initialized for TCB 0x%lx\n",
> + (unsigned long int) result);
> + return result;
> }
> rtld_hidden_def (_dl_allocate_tls)
>
> @@ -728,14 +744,22 @@ void
> _dl_deallocate_tls (void *tcb, bool dealloc_tcb)
> {
> if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> - _dl_debug_printf ("TCB deallocating: 0x%lx (dealloc_tcb=%d)\n",
> + _dl_debug_printf ("tls: deallocate TCB 0x%lx (dealloc_tcb=%d)\n",
> (unsigned long int) tcb, dealloc_tcb);
>
> dtv_t *dtv = GET_DTV (tcb);
>
> /* We need to free the memory allocated for non-static TLS. */
> for (size_t cnt = 0; cnt < dtv[-1].counter; ++cnt)
> - free (dtv[1 + cnt].pointer.to_free);
> + {
> + if (dtv[1 + cnt].pointer.to_free != NULL
> + && __glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf (
> + "tls: deallocate block 0x%lx for modid %lu; TCB=0x%lx\n",
> + (unsigned long int) dtv[1 + cnt].pointer.to_free,
> + (unsigned long int) (1 + cnt), (unsigned long int) tcb);
> + free (dtv[1 + cnt].pointer.to_free);
> + }
>
> /* The array starts with dtv[-1]. */
> if (dtv != GL(dl_initial_dtv))
> @@ -790,6 +814,12 @@ allocate_and_init (struct link_map *map)
> (map->l_tls_align, map->l_tls_blocksize);
> if (result.val == NULL)
> oom ();
> + else if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf ("tls: allocate block 0x%lx for modid %lu; size=%lu, TCB=0x%lx\n",
> + (unsigned long int) result.to_free,
> + (unsigned long int) map->l_tls_modid,
> + (unsigned long int) map->l_tls_blocksize,
> + (unsigned long int) THREAD_SELF);
>
> /* Initialize the memory. */
> memset (__mempcpy (result.val, map->l_tls_initimage,
> @@ -891,7 +921,7 @@ _dl_update_slotinfo (unsigned long int req_modid, size_t new_gen)
> continue;
>
> /* Resizing the dtv aborts on failure: bug 16134. */
> - dtv = _dl_resize_dtv (dtv, max_modid);
> + dtv = _dl_resize_dtv (dtv, max_modid, THREAD_SELF);
>
> assert (modid <= dtv[-1].counter);
>
> @@ -912,6 +942,12 @@ _dl_update_slotinfo (unsigned long int req_modid, size_t new_gen)
> least some dynamic TLS usage by interposed mallocs. */
> if (dtv[modid].pointer.to_free != NULL)
> {
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf (
> + "tls: DTV update for TCB 0x%lx: modid %lu deallocated block 0x%lx\n",
> + (unsigned long int) THREAD_SELF,
> + (unsigned long int) modid,
> + (unsigned long int) dtv[modid].pointer.to_free);
> _dl_tls_allocate_begin ();
> free (dtv[modid].pointer.to_free);
> _dl_tls_allocate_end ();
> @@ -1004,6 +1040,11 @@ tls_get_addr_tail (tls_index *ti, dtv_t *dtv, struct link_map *the_map)
> dtv[ti->ti_module].pointer.to_free = NULL;
> dtv[ti->ti_module].pointer.val = p;
>
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf ("tls: modid %lu using static TLS; TCB=0x%lx\n",
> + (unsigned long int) ti->ti_module,
> + (unsigned long int) THREAD_SELF);
> +
> return tls_get_addr_adjust (p, ti);
> }
> else
> @@ -1059,18 +1100,28 @@ __tls_get_addr (tls_index *ti)
> {
> if (_dl_tls_allocate_active ()
> && ti->ti_module < _dl_tls_initial_modid_limit)
> + {
> /* This is a reentrant __tls_get_addr call, but we can
> satisfy it because it's an initially-loaded module ID.
> These TLS slotinfo slots do not change, so the
> out-of-date generation counter does not matter. However,
> if not in a TLS update, still update_get_addr below, to
> get off the slow path eventually. */
> - ;
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf ("tls: modid %lu reentrant usage; TCB=0x%lx\n",
> + (unsigned long int) ti->ti_module,
> + (unsigned long int) THREAD_SELF);
> + }
> else
> {
> /* Update DTV up to the global generation, see CONCURRENCY NOTES
> in _dl_update_slotinfo. */
> gen = atomic_load_acquire (&GL(dl_tls_generation));
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf (
> + "tls: modid %lu update DTV to generation %lu; TCB=0x%lx\n",
> + (unsigned long int) ti->ti_module, (unsigned long int) gen,
> + (unsigned long int) THREAD_SELF);
> return update_get_addr (ti, gen);
> }
> }
> @@ -1078,7 +1129,13 @@ __tls_get_addr (tls_index *ti)
> void *p = dtv[ti->ti_module].pointer.val;
>
> if (__glibc_unlikely (p == TLS_DTV_UNALLOCATED))
> - return tls_get_addr_tail (ti, dtv, NULL);
> + {
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf ("tls: modid %lu lazy allocation; TCB=0x%lx\n",
> + (unsigned long int) ti->ti_module,
> + (unsigned long int) THREAD_SELF);
> + return tls_get_addr_tail (ti, dtv, NULL);
> + }
>
> return tls_get_addr_adjust (p, ti);
> }
> @@ -1149,6 +1206,10 @@ _dl_tls_initial_modid_limit_setup (void)
> break;
> }
> _dl_tls_initial_modid_limit = idx;
> +
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf ("tls: initial modid limit set to %lu\n",
> + (unsigned long int) idx);
> }
>
>
> diff --git a/elf/rtld.c b/elf/rtld.c
> index 29e7a4ddfa..1fc32ebefa 100644
> --- a/elf/rtld.c
> +++ b/elf/rtld.c
> @@ -1192,6 +1192,11 @@ rtld_setup_main_map (struct link_map *main_map)
>
> /* This image gets the ID one. */
> GL(dl_tls_max_dtv_idx) = main_map->l_tls_modid = 1;
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf ("tls: assign modid %lu to %s [%ld]\n",
> + (unsigned long int) main_map->l_tls_modid,
> + DSO_FILENAME (main_map->l_name),
> + (long int) main_map->l_ns);
> }
> break;
>
> diff --git a/elf/tst-recursive-tlsmodN.c b/elf/tst-recursive-tlsmodN.c
> index 0eb89f6fe1..20abc93cab 100644
> --- a/elf/tst-recursive-tlsmodN.c
> +++ b/elf/tst-recursive-tlsmodN.c
> @@ -19,10 +19,10 @@
> /* Compiled with VAR and FUNC set via -D. FUNC requires some
> relocation against TLS variable VAR. */
>
> -__thread int VAR;
> +__thread char VAR[32768];
>
> int
> FUNC (void)
> {
> - return VAR;
> + return VAR[0];
> }
> diff --git a/elf/tst-tls-debug-recursive.sh b/elf/tst-tls-debug-recursive.sh
> new file mode 100755
> index 0000000000..083e716f72
> --- /dev/null
> +++ b/elf/tst-tls-debug-recursive.sh
> @@ -0,0 +1,83 @@
> +#!/bin/sh
> +# Test for TLS logging in dynamic linker.
> +# 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/>.
> +
> +# This script runs the tst-tls-debug-recursive test case and verifies its
> +# LD_DEBUG=tls output. It checks for various TLS-related messages
> +# to ensure the dynamic linker's TLS logging is working correctly.
> +
> +set -e
> +common_objpfx="$1"
> +test_wrapper_env="$2"
> +rtld_prefix="$3"
> +run_program_env="$4"
> +test_program="$5"
> +
> +debug_output="${common_objpfx}elf/tst-tls-debug-recursive.debug"
> +rm -f "${debug_output}".*
> +
> +# Run the test program with LD_DEBUG=tls.
> +eval "${test_wrapper_env}" LD_DEBUG=tls LD_DEBUG_OUTPUT="${debug_output}" \
> + "${rtld_prefix}" "${test_program}"
> +
> +debug_output=$(ls "${debug_output}".*)
> +
> +fail=0
> +
> +# Check for expected messages
> +if ! grep -q 'tls: DTV resized for TCB 0x.*: oldsize' "${debug_output}"; then
> + echo "FAIL: DTV resized message not found"
> + fail=1
> +fi
> +
> +if ! grep -q 'tls: DTV update for TCB 0x.*: modid .* deallocated block' "${debug_output}"; then
> + echo "FAIL: module deallocated during DTV update message not found"
> + fail=1
> +fi
> +
> +if ! grep -q 'tls: assign modid .* to' "${debug_output}"; then
> + echo "FAIL: module assigned message not found"
> + fail=1
> +fi
> +
> +if ! grep -q 'tls: allocate block .* for modid .* size=.*, TCB=0x' "${debug_output}"; then
> + echo "FAIL: module allocated message not found"
> + fail=1
> +fi
> +
> +if ! grep -q 'tls: modid .* update DTV to generation .* TCB=0x' "${debug_output}"; then
> + echo "FAIL: update DTV message not found"
> + fail=1
> +fi
> +
> +if ! grep -q 'tls: initial modid limit set to' "${debug_output}"; then
> + echo "FAIL: initial modid limit message not found"
> + fail=1
> +fi
> +
> +if [ $fail -ne 0 ]; then
> + echo "Test FAILED"
> + cat "${debug_output}"
> + rm -f "${debug_output}"
> + exit 1
> +fi
> +
> +echo "Test PASSED"
> +cat "${debug_output}"
> +rm -f "${debug_output}"
> +exit 0
> diff --git a/nptl/Makefile b/nptl/Makefile
> index 08b8ba8a31..85f95dd0cf 100644
> --- a/nptl/Makefile
> +++ b/nptl/Makefile
> @@ -266,6 +266,7 @@ CFLAGS-tst-thread-exit-clobber.o = -std=gnu++11
> LDLIBS-tst-thread-exit-clobber = -lstdc++
> CFLAGS-tst-minstack-throw.o = -std=gnu++11
> LDLIBS-tst-minstack-throw = -lstdc++
> +LDLIBS-tst-dl-debug-tid = $(libdl)
>
> tests = \
> tst-attr2 \
> @@ -485,6 +486,7 @@ modules-names = \
> tst-audit-threads-mod2 \
> tst-compat-forwarder-mod \
> tst-stack4mod \
> + tst-tls-debug-mod \
> tst-tls3mod \
> tst-tls5mod \
> tst-tls5moda \
> @@ -710,7 +712,8 @@ tst-stackguard1-ARGS = --command "$(host-test-program-cmd) --child"
> tst-stackguard1-static-ARGS = --command "$(objpfx)tst-stackguard1-static --child"
>
> ifeq ($(run-built-tests),yes)
> -$(objpfx)tst-dl-debug-tid.out: tst-dl-debug-tid.sh $(objpfx)tst-dl-debug-tid
> +$(objpfx)tst-dl-debug-tid.out: tst-dl-debug-tid.sh $(objpfx)tst-dl-debug-tid \
> + $(objpfx)tst-tls-debug-mod.so
> $(SHELL) $< $(common-objpfx) '$(test-wrapper-env)' '$(rtld-prefix)' \
> '$(run-program-env)' \
> $(objpfx)tst-dl-debug-tid > $@; $(evaluate-test)
> diff --git a/nptl/allocatestack.c b/nptl/allocatestack.c
> index d3a9dbd3d2..b2ecb00113 100644
> --- a/nptl/allocatestack.c
> +++ b/nptl/allocatestack.c
> @@ -117,7 +117,7 @@ get_cached_stack (size_t *sizep, void **memp)
> lll_unlock (GL (dl_stack_cache_lock), LLL_PRIVATE);
>
> if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> - GLRO (dl_debug_printf) ("TLS TCB reused from cache: 0x%lx\n",
> + GLRO (dl_debug_printf) ("tls: TCB reused from cache: 0x%lx\n",
> (unsigned long int) result);
>
> /* Report size and location of the stack to the caller. */
> @@ -436,9 +436,9 @@ allocate_stack (const struct pthread_attr *attr, struct pthread **pdp,
>
> if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> GLRO (dl_debug_printf) (
> - "TCB for user-supplied stack created: 0x%lx, stack=0x%lx, size=%lu\n",
> - (unsigned long int) pd, (unsigned long int) pd->stackblock,
> - (unsigned long int) pd->stackblock_size);
> + "tls: TCB created (user-supplied stack); stack=0x%lx, size=%lu, TCB=0x%lx\n",
> + (unsigned long int) pd->stackblock,
> + (unsigned long int) pd->stackblock_size, (unsigned long int) pd);
>
> /* This is at least the second thread. */
> pd->header.multiple_threads = 1;
> @@ -561,7 +561,7 @@ allocate_stack (const struct pthread_attr *attr, struct pthread **pdp,
> pd->setxid_futex = -1;
>
> if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> - GLRO (dl_debug_printf) ("TCB for new stack allocated: 0x%lx\n",
> + GLRO (dl_debug_printf) ("tls: TCB allocated (new stack): 0x%lx\n",
> (unsigned long int) pd);
>
> /* Allocate the DTV for this thread. */
> diff --git a/nptl/nptl-stack.c b/nptl/nptl-stack.c
> index 4ae081c55e..d327c7f7be 100644
> --- a/nptl/nptl-stack.c
> +++ b/nptl/nptl-stack.c
> @@ -77,7 +77,7 @@ __nptl_free_stacks (size_t limit)
>
> if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> GLRO (dl_debug_printf) (
> - "TCB cache full, deallocating: TID=%ld, TCB=0x%lx\n",
> + "tls: TCB deallocating from full cache; TID=%ld, TCB=0x%lx\n",
> (long int) curr->tid, (unsigned long int) curr);
>
> /* Free the memory associated with the ELF TLS. */
> @@ -104,7 +104,7 @@ queue_stack (struct pthread *stack)
> /* The 'stack' parameter is a pointer to the TCB (struct pthread),
> not just the stack. */
> if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> - GLRO (dl_debug_printf) ("TCB deallocated into cache: TID=%ld, TCB=0x%lx\n",
> + GLRO (dl_debug_printf) ("tls: TCB deallocated into cache; TID=%ld, TCB=0x%lx\n",
> (long int) stack->tid, (unsigned long int) stack);
>
> /* We unconditionally add the stack to the list. The memory may
> @@ -139,7 +139,7 @@ __nptl_deallocate_stack (struct pthread *pd)
> the TLS memory. */
> if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> GLRO (dl_debug_printf) (
> - "TCB for user-supplied stack deallocated: TID=%ld, TCB=0x%lx\n",
> + "tls: TCB deallocated (user-supplied stack); TID=%ld, TCB=0x%lx\n",
> (long int) pd->tid, (unsigned long int) pd);
> /* Free the memory associated with the ELF TLS. */
> _dl_deallocate_tls (TLS_TPADJ (pd), false);
> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
> index 6e7e0c6435..9a0cefb0f5 100644
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c
> @@ -367,7 +367,7 @@ start_thread (void *arg)
> }
>
> if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> - GLRO (dl_debug_printf) ("Thread starting: TID=%ld, TCB=0x%lx\n",
> + GLRO (dl_debug_printf) ("tls: thread starting; TID=%ld, TCB=0x%lx\n",
> (long int) pd->tid, (unsigned long int) pd);
>
> /* Initialize resolver state pointer. */
> diff --git a/nptl/tst-dl-debug-tid.c b/nptl/tst-dl-debug-tid.c
> index b530d2170a..faa3b795b5 100644
> --- a/nptl/tst-dl-debug-tid.c
> +++ b/nptl/tst-dl-debug-tid.c
> @@ -27,12 +27,25 @@
> #include <support/xthread.h>
> #include <stdio.h>
> #include <unistd.h>
> +#include <dlfcn.h>
> +#include <support/xdlfcn.h>
> +#include <support/check.h>
>
> static void *
> thread_function (void *arg)
> {
> if (arg)
> pthread_barrier_wait ((pthread_barrier_t *) arg);
> +
> + /* Load a module with TLS to verify allocation/deallocation logs. */
> + void *h = xdlopen ("tst-tls-debug-mod.so", RTLD_NOW);
> +
> + /* Call a function that accesses TLS. */
> + int (*fp) (void) = (int (*) (void)) xdlsym (h, "in_dso");
> + TEST_COMPARE (fp (), 0);
> +
> + xdlclose (h);
> +
> return NULL;
> }
>
> diff --git a/nptl/tst-dl-debug-tid.sh b/nptl/tst-dl-debug-tid.sh
> index 12a4aa2b34..a748865a25 100644
> --- a/nptl/tst-dl-debug-tid.sh
> +++ b/nptl/tst-dl-debug-tid.sh
> @@ -39,7 +39,7 @@ eval "${test_wrapper_env}" LD_DEBUG=tls LD_DEBUG_OUTPUT="${debug_output}" \
>
> debug_output=$(ls "${debug_output}".*)
> # Check for the "Thread starting" message.
> -if ! grep -q 'Thread starting: TID=' "${debug_output}"; then
> +if ! grep -q 'tls: thread starting; TID=.*, TCB=0x' "${debug_output}"; then
> echo "error: 'Thread starting' message not found"
> cat "${debug_output}"
> exit 1
> @@ -47,10 +47,10 @@ fi
>
> # Check that we have a message where the PID (from prefix) is different
> # from the TID (in the message). This indicates a worker thread log.
> -if ! grep 'Thread starting: TID=' "${debug_output}" | awk -F '[ \t:]+' '{
> - sub(/,/, "", $4);
> - sub(/TID=/, "", $4);
> - if ($1 != $4)
> +if ! grep 'tls: thread starting; TID=.*, TCB=0x' "${debug_output}" | awk -F '[ \t:]+' '{
> + sub(/TID=/, "", $5);
> + sub(/,/, "", $5);
> + if ($1 != $5)
> exit 0;
> exit 1
> }'; then
> @@ -60,12 +60,33 @@ if ! grep 'Thread starting: TID=' "${debug_output}" | awk -F '[ \t:]+' '{
> fi
>
> # We expect messages from thread creation and destruction.
> -if ! grep -q 'TCB allocated\|TCB deallocating\|TCB reused\|TCB deallocated' \
> +if ! grep -q 'tls: allocate TCB 0x\|tls: deallocate TCB 0x\|tls: TCB reused from cache\|tls: TCB deallocated' \
> "${debug_output}"; then
> echo "error: Expected TCB allocation/deallocation message not found"
> cat "${debug_output}"
> exit 1
> fi
>
> +# Check for TLS module ID assignment.
> +if ! grep -q 'tls: assign modid .* to' "${debug_output}"; then
> + echo "error: Expected 'modid ... assigned to' message not found"
> + cat "${debug_output}"
> + exit 1
> +fi
> +
> +# Check for TLS block allocation.
> +if ! grep -q 'tls: allocate block .* for modid .* size=.*, TCB=0x' "${debug_output}"; then
> + echo "error: Expected 'modid ... allocated' message not found"
> + cat "${debug_output}"
> + exit 1
> +fi
> +
> +# TLS block deallocation might be skipped due to DTV surplus.
> +if grep -q 'tls: deallocate block .* for modid .* TCB=0x' "${debug_output}"; then
> + echo "INFO: module deallocated message found"
> +else
> + echo "INFO: module deallocated message not found (may be due to DTV surplus)"
> +fi
> +
> cat "${debug_output}"
> rm -f "${debug_output}"
> diff --git a/nptl/tst-tls-debug-mod.c b/nptl/tst-tls-debug-mod.c
> new file mode 100644
> index 0000000000..0308c8a3e2
> --- /dev/null
> +++ b/nptl/tst-tls-debug-mod.c
> @@ -0,0 +1,26 @@
> +/* Test for TLS logging in dynamic linker.
> + 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/>. */
> +
> +__thread char tls_var[32768] __attribute__ ((tls_model ("global-dynamic")));
> +
> +int
> +in_dso (void)
> +{
> + tls_var[0] = 42;
> + return tls_var[0] - 42;
> +}
> diff --git a/sysdeps/x86_64/dl-tls.c b/sysdeps/x86_64/dl-tls.c
> index a1877eeaed..6aee0972bb 100644
> --- a/sysdeps/x86_64/dl-tls.c
> +++ b/sysdeps/x86_64/dl-tls.c
> @@ -41,11 +41,36 @@ __tls_get_addr_slow (tls_index *ti)
> dtv_t *dtv = THREAD_DTV ();
>
> size_t gen = atomic_load_acquire (&GL(dl_tls_generation));
> - if (__glibc_unlikely (dtv[0].counter != gen)
> + if (__glibc_unlikely (dtv[0].counter != gen))
> + {
> /* See comment in __tls_get_addr in elf/dl-tls.c. */
> - && !(_dl_tls_allocate_active ()
> - && ti->ti_module < _dl_tls_initial_modid_limit))
> - return update_get_addr (ti, gen);
> + if (_dl_tls_allocate_active ()
> + && ti->ti_module < _dl_tls_initial_modid_limit)
> + {
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf (
> + "tls: modid %lu reentrant usage; TCB=0x%lx\n",
> + (unsigned long int) ti->ti_module,
> + (unsigned long int) THREAD_SELF);
> + }
> + else
> + {
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf (
> + "tls: modid %lu update DTV to generation %lu; TCB=0x%lx\n",
> + (unsigned long int) ti->ti_module, (unsigned long int) gen,
> + (unsigned long int) THREAD_SELF);
> + return update_get_addr (ti, gen);
> + }
> + }
> +
> + if (__glibc_unlikely (dtv[ti->ti_module].pointer.val == TLS_DTV_UNALLOCATED))
> + {
> + if (__glibc_unlikely (GLRO (dl_debug_mask) & DL_DEBUG_TLS))
> + _dl_debug_printf ("tls: modid %lu lazy allocation; TCB=0x%lx\n",
> + (unsigned long int) ti->ti_module,
> + (unsigned long int) THREAD_SELF);
> + }
>
> return tls_get_addr_tail (ti, dtv, NULL);
> }
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