glibc/nptl/allocatestack.c

/* 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 <assert.h>
#include <errno.h>
#include <signal.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/param.h>
#include <dl-sysdep.h>
#include <dl-tls.h>
#include <tls.h>
#include <list.h>
#include <lowlevellock.h>
#include <futex-internal.h>
#include <kernel-features.h>
#include <nptl-stack.h>
#include <libc-lock.h>
#include <tls-internal.h>
#include <intprops.h>
#include <setvmaname.h>

/* Default alignment of stack.  */
#ifndef STACK_ALIGN
# define STACK_ALIGN __alignof__ (long double)
#endif

/* Default value for minimal stack size after allocating thread
   descriptor and guard.  */
#ifndef MINIMAL_REST_STACK
# define MINIMAL_REST_STACK	4096
#endif


/* Newer kernels have the MAP_STACK flag to indicate a mapping is used for
   a stack.  Use it when possible.  */
#ifndef MAP_STACK
# define MAP_STACK 0
#endif

/* Get a stack frame from the cache.  We have to match by size since
   some blocks might be too small or far too large.  */
static struct pthread *
get_cached_stack (size_t *sizep, void **memp)
{
  size_t size = *sizep;
  struct pthread *result = NULL;
  list_t *entry;

  lll_lock (GL (dl_stack_cache_lock), LLL_PRIVATE);

  /* Search the cache for a matching entry.  We search for the
     smallest stack which has at least the required size.  Note that
     in normal situations the size of all allocated stacks is the
     same.  As the very least there are only a few different sizes.
     Therefore this loop will exit early most of the time with an
     exact match.  */
  list_for_each (entry, &GL (dl_stack_cache))
    {
      struct pthread *curr;

      curr = list_entry (entry, struct pthread, list);
      if (__nptl_stack_in_use (curr) && curr->stackblock_size >= size)
	{
	  if (curr->stackblock_size == size)
	    {
	      result = curr;
	      break;
	    }

	  if (result == NULL
	      || result->stackblock_size > curr->stackblock_size)
	    result = curr;
	}
    }

  if (__builtin_expect (result == NULL, 0)
      /* Make sure the size difference is not too excessive.  In that
	 case we do not use the block.  */
      || __builtin_expect (result->stackblock_size > 4 * size, 0))
    {
      /* Release the lock.  */
      lll_unlock (GL (dl_stack_cache_lock), LLL_PRIVATE);

      return NULL;
    }

  /* Don't allow setxid until cloned.  */
  result->setxid_futex = -1;

  /* Dequeue the entry.  */
  __nptl_stack_list_del (&result->list);

  /* And add to the list of stacks in use.  */
  __nptl_stack_list_add (&result->list, &GL (dl_stack_used));

  /* And decrease the cache size.  */
  GL (dl_stack_cache_actsize) -= result->stackblock_size;

  /* Release the lock early.  */
  lll_unlock (GL (dl_stack_cache_lock), LLL_PRIVATE);

  /* Report size and location of the stack to the caller.  */
  *sizep = result->stackblock_size;
  *memp = result->stackblock;

  /* Cancellation handling is back to the default.  */
  result->cancelhandling = 0;
  result->cleanup = NULL;
  result->setup_failed = 0;

  /* No pending event.  */
  result->nextevent = NULL;

  result->exiting = false;
  __libc_lock_init (result->exit_lock);
  memset (&result->tls_state, 0, sizeof result->tls_state);

  result->getrandom_buf = NULL;

  /* Clear the DTV.  */
  dtv_t *dtv = GET_DTV (TLS_TPADJ (result));
  for (size_t cnt = 0; cnt < dtv[-1].counter; ++cnt)
    free (dtv[1 + cnt].pointer.to_free);
  memset (dtv, '\0', (dtv[-1].counter + 1) * sizeof (dtv_t));

  /* Re-initialize the TLS.  */
  _dl_allocate_tls_init (TLS_TPADJ (result), false);

  return result;
}

/* Assume support for MADV_ADVISE_GUARD, setup_stack_prot will disable it
   and fallback to ALLOCATE_GUARD_PROT_NONE if the madvise call fails.  */
static int allocate_stack_mode = ALLOCATE_GUARD_MADV_GUARD;

static inline int stack_prot (void)
{
  return (PROT_READ | PROT_WRITE
	  | ((GL(dl_stack_flags) & PF_X) ? PROT_EXEC : 0));
}

static void *
allocate_thread_stack (size_t size, size_t guardsize)
{
  /* MADV_ADVISE_GUARD does not require an additional PROT_NONE mapping.  */
  int prot = stack_prot ();

  if (atomic_load_relaxed (&allocate_stack_mode) == ALLOCATE_GUARD_PROT_NONE)
    /* If a guard page is required, avoid committing memory by first allocate
       with PROT_NONE and then reserve with required permission excluding the
       guard page.  */
    prot = guardsize == 0 ? prot : PROT_NONE;

  return __mmap (NULL, size, prot, MAP_PRIVATE | MAP_ANONYMOUS | MAP_STACK, -1,
		 0);
}


/* Return the guard page position on allocated stack.  */
static inline char *
__attribute ((always_inline))
guard_position (void *mem, size_t size, size_t guardsize, const struct pthread *pd,
		size_t pagesize_m1)
{
#if _STACK_GROWS_DOWN
  return mem;
#elif _STACK_GROWS_UP
  return (char *) (((uintptr_t) pd - guardsize) & ~pagesize_m1);
#endif
}

/* Setup the MEM thread stack of SIZE bytes with the required protection flags
   along with a guard area of GUARDSIZE size.  It first tries with
   MADV_GUARD_INSTALL, and then fallback to setup the guard area using the
   extra PROT_NONE mapping.  Update PD with the type of guard area setup.  */
static inline bool
setup_stack_prot (char *mem, size_t size, struct pthread *pd,
		  size_t guardsize, size_t pagesize_m1)
{
  if (__glibc_unlikely (guardsize == 0))
    return true;

  char *guard = guard_position (mem, size, guardsize, pd, pagesize_m1);
  if (atomic_load_relaxed (&allocate_stack_mode) == ALLOCATE_GUARD_MADV_GUARD)
    {
      if (__madvise (guard, guardsize, MADV_GUARD_INSTALL) == 0)
	{
	  pd->stack_mode = ALLOCATE_GUARD_MADV_GUARD;
	  return true;
	}

      /* If madvise fails it means the kernel does not support the guard
	 advise (we assume that the syscall is available, guard is page-aligned
	 and length is non negative).  The stack has already the expected
	 protection flags, so it just need to PROT_NONE the guard area.  */
      atomic_store_relaxed (&allocate_stack_mode, ALLOCATE_GUARD_PROT_NONE);
      if (__mprotect (guard, guardsize, PROT_NONE) != 0)
	return false;
    }
  else
    {
      const int prot = stack_prot ();
      char *guardend = guard + guardsize;
#if _STACK_GROWS_DOWN
      /* As defined at guard_position, for architectures with downward stack
	 the guard page is always at start of the allocated area.  */
      if (__mprotect (guardend, size - guardsize, prot) != 0)
	return false;
#else
      size_t mprots1 = (uintptr_t) guard - (uintptr_t) mem;
      if (__mprotect (mem, mprots1, prot) != 0)
	return false;
      size_t mprots2 = ((uintptr_t) mem + size) - (uintptr_t) guardend;
      if (__mprotect (guardend, mprots2, prot) != 0)
	return false;
#endif
    }

  pd->stack_mode = ALLOCATE_GUARD_PROT_NONE;
  return true;
}

/* Update the guard area of the thread stack MEM of size SIZE with the new
   GUARDISZE.  It uses the method defined by PD stack_mode.  */
static inline bool
adjust_stack_prot (char *mem, size_t size, const struct pthread *pd,
		   size_t guardsize, size_t pagesize_m1)
{
  /* The required guard area is larger than the current one.  For
     _STACK_GROWS_DOWN it means the guard should increase as:

     |guard|---------------------------------stack|
     |new guard--|---------------------------stack|

     while for _STACK_GROWS_UP:

     |stack---------------------------|guard|-----|
     |stack--------------------|new guard---|-----|

     Both madvise and mprotect allows overlap the required region,
     so use the new guard placement with the new size.  */
  if (guardsize > pd->guardsize)
    {
      char *guard = guard_position (mem, size, guardsize, pd, pagesize_m1);
      if (pd->stack_mode == ALLOCATE_GUARD_MADV_GUARD)
	return __madvise (guard, guardsize, MADV_GUARD_INSTALL) == 0;
      else if (pd->stack_mode == ALLOCATE_GUARD_PROT_NONE)
	return __mprotect (guard, guardsize, PROT_NONE) == 0;
    }
  /* The current guard area is larger than the required one.  For
     _STACK_GROWS_DOWN is means change the guard as:

     |guard-------|-------------------------stack|
     |new guard|----------------------------stack|

     And for _STACK_GROWS_UP:

     |stack---------------------|guard-------|---|
     |stack------------------------|new guard|---|

     For ALLOCATE_GUARD_MADV_GUARD it means remove the slack area
     (disjointed region of guard and new guard), while for
     ALLOCATE_GUARD_PROT_NONE it requires to mprotect it with the stack
     protection flags.  */
  else if (pd->guardsize > guardsize)
    {
      size_t slacksize = pd->guardsize - guardsize;
      if (pd->stack_mode == ALLOCATE_GUARD_MADV_GUARD)
	{
	  void *slack =
#if _STACK_GROWS_DOWN
	    mem + guardsize;
#else
	    guard_position (mem, size, pd->guardsize, pd, pagesize_m1);
#endif
	  return __madvise (slack, slacksize, MADV_GUARD_REMOVE) == 0;
	}
      else if (pd->stack_mode == ALLOCATE_GUARD_PROT_NONE)
	{
	  const int prot = stack_prot ();
#if _STACK_GROWS_DOWN
	  return __mprotect (mem + guardsize, slacksize, prot) == 0;
#else
	  char *new_guard = (char *)(((uintptr_t) pd - guardsize)
				     & ~pagesize_m1);
	  char *old_guard = (char *)(((uintptr_t) pd - pd->guardsize)
				     & ~pagesize_m1);
	  /* The guard size difference might be > 0, but once rounded
	     to the nearest page the size difference might be zero.  */
	  if (new_guard > old_guard
	      && __mprotect (old_guard, new_guard - old_guard, prot) != 0)
	    return false;
#endif
	}
    }
  return true;
}

/* Mark the memory of the stack as usable to the kernel.  It frees everything
   except for the space used for the TCB itself.  */
static __always_inline void
advise_stack_range (void *mem, size_t size, uintptr_t pd, size_t guardsize)
{
  uintptr_t sp = (uintptr_t) CURRENT_STACK_FRAME;
  size_t pagesize_m1 = __getpagesize () - 1;
#if _STACK_GROWS_DOWN
  size_t freesize = (sp - (uintptr_t) mem) & ~pagesize_m1;
  assert (freesize < size);
  if (freesize > PTHREAD_STACK_MIN)
    __madvise (mem, freesize - PTHREAD_STACK_MIN, MADV_DONTNEED);
#else
  /* Page aligned start of memory to free (higher than or equal
     to current sp plus the minimum stack size).  */
  uintptr_t freeblock = (sp + PTHREAD_STACK_MIN + pagesize_m1) & ~pagesize_m1;
  uintptr_t free_end = (pd - guardsize) & ~pagesize_m1;
  if (free_end > freeblock)
    {
      size_t freesize = free_end - freeblock;
      assert (freesize < size);
      __madvise ((void*) freeblock, freesize, MADV_DONTNEED);
    }
#endif
}

/* Returns a usable stack for a new thread either by allocating a
   new stack or reusing a cached stack of sufficient size.
   ATTR must be non-NULL and point to a valid pthread_attr.
   PDP must be non-NULL.  */
static int
allocate_stack (const struct pthread_attr *attr, struct pthread **pdp,
		void **stack, size_t *stacksize)
{
  struct pthread *pd;
  size_t size;
  size_t pagesize_m1 = __getpagesize () - 1;
  size_t tls_static_size_for_stack = __nptl_tls_static_size_for_stack ();
  size_t tls_static_align_m1 = GLRO (dl_tls_static_align) - 1;

  assert (powerof2 (pagesize_m1 + 1));
  assert (TCB_ALIGNMENT >= STACK_ALIGN);

  /* Get the stack size from the attribute if it is set.  Otherwise we
     use the default we determined at start time.  */
  if (attr->stacksize != 0)
    size = attr->stacksize;
  else
    {
      lll_lock (__default_pthread_attr_lock, LLL_PRIVATE);
      size = __default_pthread_attr.internal.stacksize;
      lll_unlock (__default_pthread_attr_lock, LLL_PRIVATE);
    }

  /* Get memory for the stack.  */
  if (__glibc_unlikely (attr->flags & ATTR_FLAG_STACKADDR))
    {
      uintptr_t adj;
      char *stackaddr = (char *) attr->stackaddr;

      /* Assume the same layout as the _STACK_GROWS_DOWN case, with struct
	 pthread at the top of the stack block.  Later we adjust the guard
	 location and stack address to match the _STACK_GROWS_UP case.  */
      if (_STACK_GROWS_UP)
	stackaddr += attr->stacksize;

      /* If the user also specified the size of the stack make sure it
	 is large enough.  */
      if (attr->stacksize != 0
	  && attr->stacksize < (tls_static_size_for_stack
				+ MINIMAL_REST_STACK))
	return EINVAL;

      /* Adjust stack size for alignment of the TLS block.  */
#if TLS_TCB_AT_TP
      adj = ((uintptr_t) stackaddr - TLS_TCB_SIZE)
	    & tls_static_align_m1;
      assert (size > adj + TLS_TCB_SIZE);
#elif TLS_DTV_AT_TP
      adj = ((uintptr_t) stackaddr - tls_static_size_for_stack)
	    & tls_static_align_m1;
      assert (size > adj);
#endif

      /* The user provided some memory.  Let's hope it matches the
	 size...  We do not allocate guard pages if the user provided
	 the stack.  It is the user's responsibility to do this if it
	 is wanted.  */
#if TLS_TCB_AT_TP
      pd = (struct pthread *) ((uintptr_t) stackaddr
			       - TLS_TCB_SIZE - adj);
#elif TLS_DTV_AT_TP
      pd = (struct pthread *) (((uintptr_t) stackaddr
				- tls_static_size_for_stack - adj)
			       - TLS_PRE_TCB_SIZE);
#endif

      /* The user provided stack memory needs to be cleared.  */
      memset (pd, '\0', sizeof (struct pthread));

      /* The first TSD block is included in the TCB.  */
      pd->specific[0] = pd->specific_1stblock;

      /* Remember the stack-related values.  */
      pd->stackblock = (char *) stackaddr - size;
      pd->stackblock_size = size;

      /* This is a user-provided stack.  It will not be queued in the
	 stack cache nor will the memory (except the TLS memory) be freed.  */
      pd->stack_mode = ALLOCATE_GUARD_USER;

      /* This is at least the second thread.  */
      pd->header.multiple_threads = 1;

#ifdef NEED_DL_SYSINFO
      SETUP_THREAD_SYSINFO (pd);
#endif

      /* Don't allow setxid until cloned.  */
      pd->setxid_futex = -1;

      /* Allocate the DTV for this thread.  */
      if (_dl_allocate_tls (TLS_TPADJ (pd)) == NULL)
	{
	  /* Something went wrong.  */
	  assert (errno == ENOMEM);
	  return errno;
	}


      /* Prepare to modify global data.  */
      lll_lock (GL (dl_stack_cache_lock), LLL_PRIVATE);

      /* And add to the list of stacks in use.  */
      list_add (&pd->list, &GL (dl_stack_user));

      lll_unlock (GL (dl_stack_cache_lock), LLL_PRIVATE);
    }
  else
    {
      /* Allocate some anonymous memory.  If possible use the cache.  */
      size_t guardsize;
      size_t reported_guardsize;
      void *mem;

      /* Adjust the stack size for alignment.  */
      size &= ~tls_static_align_m1;
      assert (size != 0);

      /* Make sure the size of the stack is enough for the guard and
	 eventually the thread descriptor.  On some targets there is
	 a minimum guard size requirement, ARCH_MIN_GUARD_SIZE, so
	 internally enforce it (unless the guard was disabled), but
	 report the original guard size for backward compatibility:
	 before POSIX 2008 the guardsize was specified to be one page
	 by default which is observable via pthread_attr_getguardsize
	 and pthread_getattr_np.  */
      guardsize = (attr->guardsize + pagesize_m1) & ~pagesize_m1;
      reported_guardsize = guardsize;
      if (guardsize > 0 && guardsize < ARCH_MIN_GUARD_SIZE)
	guardsize = ARCH_MIN_GUARD_SIZE;
      if (guardsize < attr->guardsize || size + guardsize < guardsize)
	/* Arithmetic overflow.  */
	return EINVAL;
      size += guardsize;
      if (__builtin_expect (size < ((guardsize + tls_static_size_for_stack
				     + MINIMAL_REST_STACK + pagesize_m1)
				    & ~pagesize_m1),
			    0))
	/* The stack is too small (or the guard too large).  */
	return EINVAL;

      /* Try to get a stack from the cache.  */
      pd = get_cached_stack (&size, &mem);
      if (pd == NULL)
	{
	  mem = allocate_thread_stack (size, guardsize);
	  if (__glibc_unlikely (mem == MAP_FAILED))
	    return errno;

	  /* Do madvise in case the tunable glibc.pthread.stack_hugetlb is
	     set to 0, disabling hugetlb.  */
	  if (__glibc_unlikely (__nptl_stack_hugetlb == 0)
	      && __madvise (mem, size, MADV_NOHUGEPAGE) != 0)
	    return errno;

	  /* SIZE is guaranteed to be greater than zero.
	     So we can never get a null pointer back from mmap.  */
	  assert (mem != NULL);

	  /* Place the thread descriptor at the end of the stack.  */
#if TLS_TCB_AT_TP
	  pd = (struct pthread *) ((((uintptr_t) mem + size)
				    - TLS_TCB_SIZE)
				   & ~tls_static_align_m1);
#elif TLS_DTV_AT_TP
	  pd = (struct pthread *) ((((uintptr_t) mem + size
				    - tls_static_size_for_stack)
				    & ~tls_static_align_m1)
				   - TLS_PRE_TCB_SIZE);
#endif

	  /* Now mprotect the required region excluding the guard area.  */
	  if (!setup_stack_prot (mem, size, pd, guardsize, pagesize_m1))
	    {
	      __munmap (mem, size);
	      return errno;
	    }

	  /* Remember the stack-related values.  */
	  pd->stackblock = mem;
	  pd->stackblock_size = size;
	  /* Update guardsize for newly allocated guardsize to avoid
	     an mprotect in guard resize below.  */
	  pd->guardsize = guardsize;

	  /* We allocated the first block thread-specific data array.
	     This address will not change for the lifetime of this
	     descriptor.  */
	  pd->specific[0] = pd->specific_1stblock;

	  /* This is at least the second thread.  */
	  pd->header.multiple_threads = 1;

#ifdef NEED_DL_SYSINFO
	  SETUP_THREAD_SYSINFO (pd);
#endif

	  /* Don't allow setxid until cloned.  */
	  pd->setxid_futex = -1;

	  /* Allocate the DTV for this thread.  */
	  if (_dl_allocate_tls (TLS_TPADJ (pd)) == NULL)
	    {
	      /* Something went wrong.  */
	      assert (errno == ENOMEM);

	      /* Free the stack memory we just allocated.  */
	      (void) __munmap (mem, size);

	      return errno;
	    }


	  /* Prepare to modify global data.  */
	  lll_lock (GL (dl_stack_cache_lock), LLL_PRIVATE);

	  /* And add to the list of stacks in use.  */
	  __nptl_stack_list_add (&pd->list, &GL (dl_stack_used));

	  lll_unlock (GL (dl_stack_cache_lock), LLL_PRIVATE);

	  /* Note that all of the stack and the thread descriptor is
	     zeroed.  This means we do not have to initialize fields
	     with initial value zero.  This is specifically true for
	     the 'tid' field which is always set back to zero once the
	     stack is not used anymore and for the 'guardsize' field
	     which will be read next.  */
	}

      /* Create or resize the guard area if necessary on an already
	 allocated stack.  */
      if (!adjust_stack_prot (mem, size, pd, guardsize, pagesize_m1))
	{
	  lll_lock (GL (dl_stack_cache_lock), LLL_PRIVATE);

	  /* Remove the thread from the list.  */
	  __nptl_stack_list_del (&pd->list);

	  lll_unlock (GL (dl_stack_cache_lock), LLL_PRIVATE);

	  /* Get rid of the TLS block we allocated.  */
	  _dl_deallocate_tls (TLS_TPADJ (pd), false);

	  /* Free the stack memory regardless of whether the size
	     of the cache is over the limit or not.  If this piece
	     of memory caused problems we better do not use it
	     anymore.  Uh, and we ignore possible errors.  There
	     is nothing we could do.  */
	  (void) __munmap (mem, size);

	  return errno;
	}

      pd->guardsize = guardsize;
      /* The pthread_getattr_np() calls need to get passed the size
	 requested in the attribute, regardless of how large the
	 actually used guardsize is.  */
      pd->reported_guardsize = reported_guardsize;
    }

  /* Initialize the lock.  We have to do this unconditionally since the
     stillborn thread could be canceled while the lock is taken.  */
  pd->lock = LLL_LOCK_INITIALIZER;

  /* The robust mutex lists also need to be initialized
     unconditionally because the cleanup for the previous stack owner
     might have happened in the kernel.  */
  pd->robust_head.futex_offset = (offsetof (pthread_mutex_t, __data.__lock)
				  - offsetof (pthread_mutex_t,
					      __data.__list.__next));
  pd->robust_head.list_op_pending = NULL;
#if __PTHREAD_MUTEX_HAVE_PREV
  pd->robust_prev = &pd->robust_head;
#endif
  pd->robust_head.list = &pd->robust_head;

  /* We place the thread descriptor at the end of the stack.  */
  *pdp = pd;

  void *stacktop;

#if TLS_TCB_AT_TP
  /* The stack begins before the TCB and the static TLS block.  */
  stacktop = ((char *) (pd + 1) - tls_static_size_for_stack);
#elif TLS_DTV_AT_TP
  stacktop = (char *) (pd - 1);
#endif

  *stacksize = stacktop - pd->stackblock;
  *stack = pd->stackblock;

  return 0;
}

#define SET_STACK_NAME(__prefix, __stack, __stacksize, __tid)		\
  ({									\
     char __stack_name[sizeof (__prefix) +				\
		       INT_BUFSIZE_BOUND (unsigned int)];		\
     _Static_assert (sizeof __stack_name <= ANON_VMA_NAME_MAX_LEN,	\
		     "VMA name size larger than maximum supported");	\
     __snprintf (__stack_name, sizeof (__stack_name), __prefix "%u",	\
		 (unsigned int) __tid);					\
     __set_vma_name (__stack, __stacksize, __stack_name);		\
   })

/* Add or remove an associated name to the PD VMA stack.  */
static void
name_stack_maps (struct pthread *pd, bool set)
{
  size_t adjust = pd->stack_mode == ALLOCATE_GUARD_PROT_NONE ?
    pd->guardsize : 0;
#if _STACK_GROWS_DOWN
  void *stack = pd->stackblock + adjust;
#else
  void *stack = pd->stackblock;
#endif
  size_t stacksize = pd->stackblock_size - adjust;

  if (!set)
    __set_vma_name (stack, stacksize, " glibc: unused stack");
  else
    {
      unsigned int tid = pd->tid;
      if (pd->stack_mode == ALLOCATE_GUARD_USER)
	SET_STACK_NAME (" glibc: pthread user stack: ", stack, stacksize, tid);
      else
	SET_STACK_NAME (" glibc: pthread stack: ", stack, stacksize, tid);
    }
}