glibc/csu/libc-tls.c

/* Initialization code for TLS in statically linked application.
   Copyright (C) 2002-2025 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 <startup.h>
#include <errno.h>
#include <ldsodefs.h>
#include <tls.h>
#include <dl-tls.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/param.h>
#include <array_length.h>
#include <pthreadP.h>
#include <dl-call_tls_init_tp.h>
#include <dl-extra_tls.h>

#ifdef SHARED
 #error makefile bug, this file is for static only
#endif

dtv_t _dl_static_dtv[2 + TLS_SLOTINFO_SURPLUS];


static struct dtv_slotinfo_list static_slotinfo =
  {
   /* Allocate an array of 2 + TLS_SLOTINFO_SURPLUS elements.  */
   .slotinfo =  { [array_length (_dl_static_dtv) - 1] = { 0 } },
  };

/* Highest dtv index currently needed.  */
size_t _dl_tls_max_dtv_idx;
/* Flag signalling whether there are gaps in the module ID allocation.  */
bool _dl_tls_dtv_gaps;
/* Information about the dtv slots.  */
struct dtv_slotinfo_list *_dl_tls_dtv_slotinfo_list;
/* Number of modules in the static TLS block.  */
size_t _dl_tls_static_nelem;
/* Size of the static TLS block.  */
size_t _dl_tls_static_size;
/* Size actually allocated in the static TLS block.  */
size_t _dl_tls_static_used;
/* Alignment requirement of the static TLS block.  */
size_t _dl_tls_static_align;
/* Size of surplus space in the static TLS area for dynamically
   loaded modules with IE-model TLS or for TLSDESC optimization.
   See comments in elf/dl-tls.c where it is initialized.  */
size_t _dl_tls_static_surplus;
/* Remaining amount of static TLS that may be used for optimizing
   dynamic TLS access (e.g. with TLSDESC).  */
size_t _dl_tls_static_optional;

/* Generation counter for the dtv.  */
size_t _dl_tls_generation;


/* Additional definitions needed by TLS initialization.  */
#ifdef TLS_INIT_HELPER
TLS_INIT_HELPER
#endif

static void
init_slotinfo (void)
{
  /* Create the slotinfo list.  Note that the type of static_slotinfo
     has effectively a zero-length array, so we cannot use the size of
     static_slotinfo to determine the array length.  */
  static_slotinfo.len = array_length (_dl_static_dtv);
  /* static_slotinfo.next = NULL; -- Already zero.  */

  /* The slotinfo list.  Will be extended by the code doing dynamic
     linking.  */
  GL(dl_tls_max_dtv_idx) = 1;
  GL(dl_tls_dtv_slotinfo_list) = &static_slotinfo;
}

static void
init_static_tls (size_t memsz, size_t align)
{
  /* That is the size of the TLS memory for this object.  */
  GL(dl_tls_static_size) = roundup (memsz + GLRO(dl_tls_static_surplus),
				    TCB_ALIGNMENT);
#if TLS_TCB_AT_TP
  GL(dl_tls_static_size) += TLS_TCB_SIZE;
#endif
  GL(dl_tls_static_used) = memsz;
  /* The alignment requirement for the static TLS block.  */
  GL(dl_tls_static_align) = align;
  /* Number of elements in the static TLS block.  */
  GL(dl_tls_static_nelem) = GL(dl_tls_max_dtv_idx);
}

void
__libc_setup_tls (void)
{
  void *tlsblock;
  size_t memsz = 0;
  size_t filesz = 0;
  void *initimage = NULL;
  size_t align = 0;
  size_t tls_blocks_size = 0;
  size_t max_align = TCB_ALIGNMENT;
  size_t tcb_offset;
  const ElfW(Phdr) *phdr;

  struct link_map *main_map = GL(dl_ns)[LM_ID_BASE]._ns_loaded;

  __tls_pre_init_tp ();

  /* Look through the TLS segment if there is any.  */
  for (phdr = _dl_phdr; phdr < &_dl_phdr[_dl_phnum]; ++phdr)
    if (phdr->p_type == PT_TLS)
      {
	/* Remember the values we need.  */
	memsz = phdr->p_memsz;
	filesz = phdr->p_filesz;
	initimage = (void *) phdr->p_vaddr + main_map->l_addr;
	align = phdr->p_align;
	if (phdr->p_align > max_align)
	  max_align = phdr->p_align;
	break;
      }

  /* Calculate the size of the static TLS surplus, with 0 auditors.  */
  _dl_tls_static_surplus_init (0);

  /* Extra TLS block for internal usage to append at the end of the TLS blocks
     (in allocation order).  The address at which the block is allocated must
     be aligned to 'extra_tls_align'.  The size of the block as returned by
     '_dl_extra_tls_get_size ()' is always a multiple of the aligment.

     On Linux systems this is where the rseq area will be allocated.  On other
     systems it is currently unused and both values will be '0'.  */
  size_t extra_tls_size = _dl_extra_tls_get_size ();
  size_t extra_tls_align = _dl_extra_tls_get_align ();

  /* Increase the maximum alignment with the extra TLS alignment requirements
     if necessary.  */
  max_align = MAX (max_align, extra_tls_align);

  /* We have to set up the TCB block which also (possibly) contains
     'errno'.  Therefore we avoid 'malloc' which might touch 'errno'.
     Instead we use 'sbrk' which would only uses 'errno' if it fails.
     In this case we are right away out of memory and the user gets
     what she/he deserves.  */
#if TLS_TCB_AT_TP
  /* In this layout the TLS blocks are located before the thread pointer.  */

  /* Record the size of the combined TLS blocks.

     First reserve space for 'memsz' while respecting both its alignment
     requirements and those of the extra TLS blocks.  Then add the size of
     the extra TLS block.  Both values respect the extra TLS alignment
     requirements and so does the resulting size and the offset that will
     be derived from it.  */
  tls_blocks_size = roundup (memsz, MAX (align, extra_tls_align) ?: 1)
		  + extra_tls_size;

 /* Record the extra TLS block offset from the thread pointer.

    With TLS_TCB_AT_TP the TLS blocks are allocated before the thread pointer
    in reverse order.  Our block is added last which results in it being the
    first in the static TLS block, thus record the most negative offset.

    The alignment requirements of the pointer resulting from this offset and
    the thread pointer are enforced by 'max_align' which is used to align the
    tcb_offset.  */
  _dl_extra_tls_set_offset (-tls_blocks_size);

  /* Align the TCB offset to the maximum alignment, as
     _dl_allocate_tls_storage (in elf/dl-tls.c) does using __libc_memalign
     and dl_tls_static_align.  */
  tcb_offset = roundup (tls_blocks_size + GLRO(dl_tls_static_surplus), max_align);
  tlsblock = _dl_early_allocate (tcb_offset + TLS_INIT_TCB_SIZE + max_align);
  if (tlsblock == NULL)
    _startup_fatal_tls_error ();
#elif TLS_DTV_AT_TP
  /* In this layout the TLS blocks are located after the thread pointer.  */

  /* Record the tcb_offset including the aligment requirements of 'memsz'
     that comes after it.  */
  tcb_offset = roundup (TLS_INIT_TCB_SIZE, align ?: 1);

  /* Record the size of the combined TLS blocks.

     First reserve space for TLS_INIT_TCB_SIZE and 'memsz' while respecting
     both its alignment requirements and those of the extra TLS blocks.  Then
     add the size of the extra TLS block.  Both values respect the extra TLS
     alignment requirements and so does the resulting size and the offset that
     will be derived from it.  */
  tls_blocks_size = roundup (TLS_INIT_TCB_SIZE + memsz,
		  MAX (align, extra_tls_align) ?: 1) + extra_tls_size;

 /* Record the extra TLS block offset from the thread pointer.

    With TLS_DTV_AT_TP the TLS blocks are allocated after the thread pointer in
    order. Our block is added last which results in it being the last in the
    static TLS block, thus record the offset as the size of the static TLS
    block minus the size of our block.

    On some architectures the TLS blocks are offset from the thread pointer,
    include this offset in the extra TLS block offset.

    The alignment requirements of the pointer resulting from this offset and
    the thread pointer are enforced by 'max_align' which is used to align the
    tcb_offset.  */
  _dl_extra_tls_set_offset (tls_blocks_size - extra_tls_size - TLS_TP_OFFSET);

  tlsblock = _dl_early_allocate (tls_blocks_size + max_align
				 + TLS_PRE_TCB_SIZE
				 + GLRO(dl_tls_static_surplus));
  if (tlsblock == NULL)
    _startup_fatal_tls_error ();
  tlsblock += TLS_PRE_TCB_SIZE;
#else
  /* In case a model with a different layout for the TCB and DTV
     is defined add another #elif here and in the following #ifs.  */
# error "Either TLS_TCB_AT_TP or TLS_DTV_AT_TP must be defined"
#endif

  /* Align the TLS block.  */
  tlsblock = (void *) (((uintptr_t) tlsblock + max_align - 1)
		       & ~(max_align - 1));

  /* Initialize the dtv.  [0] is the length, [1] the generation counter.  */
  _dl_static_dtv[0].counter = (sizeof (_dl_static_dtv) / sizeof (_dl_static_dtv[0])) - 2;
  // _dl_static_dtv[1].counter = 0;		would be needed if not already done

  /* Initialize the TLS block.  */
#if TLS_TCB_AT_TP
  _dl_static_dtv[2].pointer.val = ((char *) tlsblock + tcb_offset
			       - roundup (memsz, align ?: 1));
  main_map->l_tls_offset = roundup (memsz, align ?: 1);
#elif TLS_DTV_AT_TP
  _dl_static_dtv[2].pointer.val = (char *) tlsblock + tcb_offset;
  main_map->l_tls_offset = tcb_offset;
#else
# error "Either TLS_TCB_AT_TP or TLS_DTV_AT_TP must be defined"
#endif
  _dl_static_dtv[2].pointer.to_free = NULL;
  /* sbrk gives us zero'd memory, so we don't need to clear the remainder.  */
  memcpy (_dl_static_dtv[2].pointer.val, initimage, filesz);

  /* Install the pointer to the dtv.  */

  /* Initialize the thread pointer.  */
#if TLS_TCB_AT_TP
  INSTALL_DTV ((char *) tlsblock + tcb_offset, _dl_static_dtv);

  call_tls_init_tp ((char *) tlsblock + tcb_offset);
#elif TLS_DTV_AT_TP
  INSTALL_DTV (tlsblock, _dl_static_dtv);
  call_tls_init_tp (tlsblock);
#endif

  /* Update the executable's link map with enough information to make
     the TLS routines happy.  */
  main_map->l_tls_align = align;
  main_map->l_tls_blocksize = memsz;
  main_map->l_tls_initimage = initimage;
  main_map->l_tls_initimage_size = filesz;
  main_map->l_tls_modid = 1;

  init_slotinfo ();
  /* static_slotinfo.slotinfo[1].gen = 0; -- Already zero.  */
  static_slotinfo.slotinfo[1].map = main_map;

  init_static_tls (tls_blocks_size, MAX (TCB_ALIGNMENT, max_align));
}