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/*
* Copyright (C) 2019 The Android Open Source Project
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* * Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in
* the documentation and/or other materials provided with the
* distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
* FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
* COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
* OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
* AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
* OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
* OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#include <stdlib.h>
#include <errno.h>
#include <fcntl.h>
#include <string.h>
#include <sys/stat.h>
#include "private/FdPath.h"
#include "private/ScopedFd.h"
// This function needs a 4KiB (PATH_MAX) buffer.
// The alternative is to heap allocate and then trim, but that's 2x the code.
// (Remember that readlink(2) won't tell you the needed size, so the multi-pass
// algorithm isn't even an option unless you want to just guess, in which case
// you're back needing to trim again.)
#pragma GCC diagnostic ignored "-Wframe-larger-than="
char* realpath(const char* path, char* result) {
// Weird special case.
if (!path) {
errno = EINVAL;
return nullptr;
}
// Get an O_PATH fd, and...
ScopedFd fd(open(path, O_PATH | O_CLOEXEC));
if (fd.get() == -1) return nullptr;
// (...remember the device/inode that we're talking about and...)
struct stat sb;
if (fstat(fd.get(), &sb) == -1) return nullptr;
dev_t st_dev = sb.st_dev;
ino_t st_ino = sb.st_ino;
// ...ask the kernel to do the hard work for us.
FdPath fd_path(fd.get());
char dst[PATH_MAX];
ssize_t l = readlink(fd_path.c_str(), dst, sizeof(dst) - 1);
if (l == -1) return nullptr;
dst[l] = '\0';
// What if the file was removed in the meantime? readlink(2) will have
// returned "/a/b/c (deleted)", and we want to return ENOENT instead.
if (stat(dst, &sb) == -1 || st_dev != sb.st_dev || st_ino != sb.st_ino) {
errno = ENOENT;
return nullptr;
}
return result ? strcpy(result, dst) : strdup(dst);
}
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