thodg/libgit2/src/mwindow.h

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• Hash : 322c15ee
  Author :
  Date : 2020-08-01T18:24:41
  

  Make the pack and mwindow implementations data-race-free
  
  This change fixes a packfile heap corruption that can happen when
  interacting with multiple packfiles concurrently across multiple
  threads. This is exacerbated by setting a lower mwindow open file limit.
  
  This change:
  
  * Renames most of the internal methods in pack.c to clearly indicate
    that they expect to be called with a certain lock held, making
    reasoning about the state of locks a bit easier.
  * Splits the `git_pack_file` lock in two: the one in `git_pack_file`
    only protects the `index_map`. The protection to `git_mwindow_file` is
    now in that struct.
  * Explicitly checks for freshness of the `git_pack_file` in
    `git_packfile_unpack_header`: this allows the mwindow implementation
    to close files whenever there is enough cache pressure, and
    `git_packfile_unpack_header` will reopen the packfile if needed.
  * After a call to `p_munmap()`, the `data` and `len` fields are poisoned
    with `NULL` to make use-after-frees more evident and crash rather than
    being open to the possibility of heap corruption.
  * Adds a test case to prevent this from regressing in the future.
  
  Fixes: #5591
  

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• src/mwindow.h

• /*
   * Copyright (C) the libgit2 contributors. All rights reserved.
   *
   * This file is part of libgit2, distributed under the GNU GPL v2 with
   * a Linking Exception. For full terms see the included COPYING file.
   */
  
  #ifndef INCLUDE_mwindow__
  #define INCLUDE_mwindow__
  
  #include "common.h"
  
  #include "map.h"
  #include "vector.h"
  
  typedef struct git_mwindow {
  	struct git_mwindow *next;
  	git_map window_map;
  	off64_t offset;
  	size_t last_used;
  	size_t inuse_cnt;
  } git_mwindow;
  
  typedef struct git_mwindow_file {
  	git_mutex lock; /* protects updates to fd */
  	git_mwindow *windows;
  	int fd;
  	off64_t size;
  } git_mwindow_file;
  
  typedef struct git_mwindow_ctl {
  	size_t mapped;
  	unsigned int open_windows;
  	unsigned int mmap_calls;
  	unsigned int peak_open_windows;
  	size_t peak_mapped;
  	size_t used_ctr;
  	git_vector windowfiles;
  } git_mwindow_ctl;
  
  int git_mwindow_contains(git_mwindow *win, off64_t offset);
  int git_mwindow_free_all(git_mwindow_file *mwf); /* locks */
  unsigned char *git_mwindow_open(git_mwindow_file *mwf, git_mwindow **cursor, off64_t offset, size_t extra, unsigned int *left);
  int git_mwindow_file_register(git_mwindow_file *mwf);
  void git_mwindow_file_deregister(git_mwindow_file *mwf);
  void git_mwindow_close(git_mwindow **w_cursor);
  
  extern int git_mwindow_global_init(void);
  
  struct git_pack_file; /* just declaration to avoid cyclical includes */
  int git_mwindow_get_pack(struct git_pack_file **out, const char *path);
  int git_mwindow_put_pack(struct git_pack_file *pack);
  
  #endif
  

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