system/lib/libc/sbrk.c

/*
 * Copyright 2019 The Emscripten Authors.  All rights reserved.
 * Emscripten is available under two separate licenses, the MIT license and the
 * University of Illinois/NCSA Open Source License.  Both these licenses can be
 * found in the LICENSE file.
 *
*/

// libc files are compiled as -std=c99 which doesn't normally declare
// max_align_t.
#if __STDC_VERSION__ < 201112L
#define __NEED_max_align_t
#endif

#include <errno.h>
#include <limits.h>
#include <stdatomic.h>
#include <stddef.h>
#include <stdint.h>
#ifdef __EMSCRIPTEN_SHARED_MEMORY__
#include <stdlib.h> // for abort
#endif

#include <emscripten/console.h>
#include <emscripten/heap.h>
#include <emscripten/trace.h>

extern size_t __heap_base;

static uintptr_t sbrk_val = (uintptr_t)&__heap_base;

uintptr_t* emscripten_get_sbrk_ptr() {
#ifdef __PIC__
  // In relocatable code we may call emscripten_get_sbrk_ptr() during startup,
  // potentially *before* the setup of the dynamically-linked __heap_base, when
  // using SAFE_HEAP. (SAFE_HEAP instruments *all* memory accesses, so even the
  // code doing dynamic linking itself ends up instrumented, which is why we can
  // get such an instrumented call before sbrk_val has its proper value.)
  if (sbrk_val == 0) {
    sbrk_val = (uintptr_t)&__heap_base;
  }
#endif
  return &sbrk_val;
}

// Enforce preserving a minimal alignof(maxalign_t) alignment for sbrk.
#define SBRK_ALIGNMENT (__alignof__(max_align_t))

void *_sbrk64(int64_t increment) {
  if (increment >= 0) {
    increment = (increment + (SBRK_ALIGNMENT-1)) & ~((int64_t)SBRK_ALIGNMENT-1);
  } else {
    increment = -(-increment & ~((int64_t)SBRK_ALIGNMENT-1));
  }

  _Atomic uintptr_t *sbrk_ptr = (_Atomic uintptr_t *)emscripten_get_sbrk_ptr();

  // To make sbrk thread-safe, implement a CAS loop to update the
  // value of sbrk_ptr.
  while (1) {
    uintptr_t old_brk = *sbrk_ptr;
    int64_t new_brk64 = (int64_t)old_brk + increment;
    uintptr_t new_brk = (uintptr_t)new_brk64;
    // Check for a) an over/underflow, which would indicate that we are
    // allocating over maximum addressable memory. and b) if necessary,
    // increase the WebAssembly Memory size, and abort if that fails.
    if (new_brk < 0 || new_brk64 != (int64_t)new_brk
     || (new_brk > emscripten_get_heap_size() && !emscripten_resize_heap(new_brk))) {
      errno = ENOMEM;
      return (void*)-1;
    }
#ifdef __EMSCRIPTEN_SHARED_MEMORY__
    // Attempt to update the dynamic top to new value. Another thread may have
    // beat this one to the update, in which case we will need to start over
    // by iterating the loop body again.
    uintptr_t expected = old_brk;

    atomic_compare_exchange_strong(sbrk_ptr, &expected, new_brk);

    if (expected != old_brk) continue; // CAS failed, another thread raced in between.
#else
    *sbrk_ptr = new_brk;
#endif

    emscripten_trace_sbrk_grow(old_brk, new_brk);
    return (void*)old_brk;
  }
}

void *sbrk(intptr_t increment_) {
#if defined(__wasm64__) // TODO || !defined(wasm2gb)
  // In the correct https://linux.die.net/man/2/sbrk spec, sbrk() parameter is
  // intended to be treated as signed, meaning that it is not possible in a
  // 32-bit program to sbrk alloc (or dealloc) more than 2GB of memory at once.

  // Treat sbrk() parameter as signed.
  return _sbrk64((int64_t)increment_);
#else
  // BUG: Currently the Emscripten test suite codifies expectations that sbrk()
  // values passed to this function are to be treated as unsigned, which means
  // that in 2GB and 4GB build modes, it is not possible to shrink memory.
  // To satisfy that mode, treat sbrk() parameters in 32-bit builds as unsigned.
  // https://github.com/emscripten-core/emscripten/issues/25138

  // Treat sbrk() parameter as unsigned.
  return _sbrk64((int64_t)(uintptr_t)increment_);
#endif
}

int brk(void* ptr) {
#ifdef __EMSCRIPTEN_SHARED_MEMORY__
  // FIXME
#ifndef NDEBUG
  emscripten_err("brk() is not threadsafe yet, https://github.com/emscripten-core/emscripten/issues/10006");
#endif
  abort();
#else
  uintptr_t last = (uintptr_t)sbrk(0);
  if (sbrk((uintptr_t)ptr - last) == (void*)-1) {
    return -1;
  }
  return 0;
#endif
}