BLI_mmap.c

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/* SPDX-FileCopyrightText: 2020 Blender Authors
 *
 * SPDX-License-Identifier: GPL-2.0-or-later */
 
#include "BLI_mmap.h"
#include "BLI_fileops.h"
#include "BLI_listbase.h"
#include "MEM_guardedalloc.h"
 
#include <string.h>
 
#ifndef WIN32
#  include <signal.h>
#  include <stdlib.h>
#  include <sys/mman.h> /* For mmap. */
#  include <unistd.h>   /* For read close. */
#else
#  include "BLI_winstuff.h"
#  include <io.h> /* For open close read. */
#endif
 
struct BLI_mmap_file {
  /* The address to which the file was mapped. */
  char *memory;
 
  /* The length of the file (and therefore the mapped region). */
  size_t length;
 
  /* Platform-specific handle for the mapping. */
  void *handle;
 
  /* Flag to indicate IO errors. Needs to be volatile since it's being set from
   * within the signal handler, which is not part of the normal execution flow. */
  volatile bool io_error;
};
 
#ifndef WIN32
/* When using memory-mapped files, any IO errors will result in a SIGBUS signal.
 * Therefore, we need to catch that signal and stop reading the file in question.
 * To do so, we keep a list of all current FileDatas that use memory-mapped files,
 * and if a SIGBUS is caught, we check if the failed address is inside one of the
 * mapped regions.
 * If it is, we set a flag to indicate a failed read and remap the memory in
 * question to a zero-backed region in order to avoid additional signals.
 * The code that actually reads the memory area has to check whether the flag was
 * set after it's done reading.
 * If the error occurred outside of a memory-mapped region, we call the previous
 * handler if one was configured and abort the process otherwise.
 */
 
static struct error_handler_data {
  ListBase open_mmaps;
  char configured;
  void (*next_handler)(int, siginfo_t *, void *);
} error_handler = {0};
 
static void sigbus_handler(int sig, siginfo_t *siginfo, void *ptr)
{
  /* We only handle SIGBUS here for now. */
  BLI_assert(sig == SIGBUS);
 
  const char *error_addr = (const char *)siginfo->si_addr;
  /* Find the file that this error belongs to. */
  LISTBASE_FOREACH (LinkData *, link, &error_handler.open_mmaps) {
    BLI_mmap_file *file = link->data;
 
    /* Is the address where the error occurred in this file's mapped range? */
    if (error_addr >= file->memory && error_addr < file->memory + file->length) {
      file->io_error = true;
 
      /* Replace the mapped memory with zeroes. */
      const void *mapped_memory = mmap(
          file->memory, file->length, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
      if (mapped_memory == MAP_FAILED) {
        fprintf(stderr, "SIGBUS handler: Error replacing mapped file with zeros\n");
      }
 
      return;
    }
  }
 
  /* Fall back to other handler if there was one. */
  if (error_handler.next_handler) {
    error_handler.next_handler(sig, siginfo, ptr);
  }
  else {
    fprintf(stderr, "Unhandled SIGBUS caught\n");
    abort();
  }
}
 
/* Ensures that the error handler is set up and ready. */
static bool sigbus_handler_setup(void)
{
  if (!error_handler.configured) {
    struct sigaction newact = {0}, oldact = {0};
 
    newact.sa_sigaction = sigbus_handler;
    newact.sa_flags = SA_SIGINFO;
 
    if (sigaction(SIGBUS, &newact, &oldact)) {
      return false;
    }
 
    /* Remember the previously configured handler to fall back to it if the error
     * does not belong to any of the mapped files. */
    error_handler.next_handler = oldact.sa_sigaction;
    error_handler.configured = 1;
  }
 
  return true;
}
 
/* Adds a file to the list that the error handler checks. */
static void sigbus_handler_add(BLI_mmap_file *file)
{
  BLI_addtail(&error_handler.open_mmaps, BLI_genericNodeN(file));
}
 
/* Removes a file from the list that the error handler checks. */
static void sigbus_handler_remove(BLI_mmap_file *file)
{
  LinkData *link = BLI_findptr(&error_handler.open_mmaps, file, offsetof(LinkData, data));
  BLI_freelinkN(&error_handler.open_mmaps, link);
}
#endif
 
BLI_mmap_file *BLI_mmap_open(int fd)
{
  void *memory, *handle = NULL;
  const size_t length = BLI_lseek(fd, 0, SEEK_END);
  if (UNLIKELY(length == (size_t)-1)) {
    return NULL;
  }
 
#ifndef WIN32
  /* Ensure that the SIGBUS handler is configured. */
  if (!sigbus_handler_setup()) {
    return NULL;
  }
 
  /* Map the given file to memory. */
  memory = mmap(NULL, length, PROT_READ, MAP_PRIVATE, fd, 0);
  if (memory == MAP_FAILED) {
    return NULL;
  }
#else
  /* Convert the POSIX-style file descriptor to a Windows handle. */
  void *file_handle = (void *)_get_osfhandle(fd);
  /* Memory mapping on Windows is a two-step process - first we create a mapping,
   * then we create a view into that mapping.
   * In our case, one view that spans the entire file is enough. */
  handle = CreateFileMapping(file_handle, NULL, PAGE_READONLY, 0, 0, NULL);
  if (handle == NULL) {
    return NULL;
  }
  memory = MapViewOfFile(handle, FILE_MAP_READ, 0, 0, 0);
  if (memory == NULL) {
    CloseHandle(handle);
    return NULL;
  }
#endif
 
  /* Now that the mapping was successful, allocate memory and set up the BLI_mmap_file. */
  BLI_mmap_file *file = MEM_callocN(sizeof(BLI_mmap_file), __func__);
  file->memory = memory;
  file->handle = handle;
  file->length = length;
 
#ifndef WIN32
  /* Register the file with the error handler. */
  sigbus_handler_add(file);
#endif
 
  return file;
}
 
bool BLI_mmap_read(BLI_mmap_file *file, void *dest, size_t offset, size_t length)
{
  /* If a previous read has already failed or we try to read past the end,
   * don't even attempt to read any further. */
  if (file->io_error || (offset + length > file->length)) {
    return false;
  }
 
#ifndef WIN32
  /* If an error occurs in this call, sigbus_handler will be called and will set
   * file->io_error to true. */
  memcpy(dest, file->memory + offset, length);
#else
  /* On Windows, we use exception handling to be notified of errors. */
  __try
  {
    memcpy(dest, file->memory + offset, length);
  }
  __except (GetExceptionCode() == EXCEPTION_IN_PAGE_ERROR ? EXCEPTION_EXECUTE_HANDLER :
                                                            EXCEPTION_CONTINUE_SEARCH)
  {
    file->io_error = true;
    return false;
  }
#endif
 
  return !file->io_error;
}
 
void *BLI_mmap_get_pointer(BLI_mmap_file *file)
{
  return file->memory;
}
 
size_t BLI_mmap_get_length(const BLI_mmap_file *file)
{
  return file->length;
}
 
void BLI_mmap_free(BLI_mmap_file *file)
{
#ifndef WIN32
  munmap((void *)file->memory, file->length);
  sigbus_handler_remove(file);
#else
  UnmapViewOfFile(file->memory);
  CloseHandle(file->handle);
#endif
 
  MEM_freeN(file);
}