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Diffstat (limited to 'lib/areadlinkat-with-size.c')
| -rw-r--r-- | lib/areadlinkat-with-size.c | 150 |
1 files changed, 150 insertions, 0 deletions
diff --git a/lib/areadlinkat-with-size.c b/lib/areadlinkat-with-size.c new file mode 100644 index 0000000..cca69da --- /dev/null +++ b/lib/areadlinkat-with-size.c @@ -0,0 +1,150 @@ +/* readlinkat wrapper to return the link name in malloc'd storage. + Unlike xreadlinkat, only call exit on failure to change directory. + + Copyright (C) 2001, 2003-2007, 2009-2023 Free Software Foundation, Inc. + + This program is free software: you can redistribute it and/or modify + it under the terms of the GNU General Public License as published by + the Free Software Foundation, either version 3 of the License, or + (at your option) any later version. + + This program 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 General Public License for more details. + + You should have received a copy of the GNU General Public License + along with this program. If not, see <https://www.gnu.org/licenses/>. */ + +/* Written by Jim Meyering <jim@meyering.net> + and Eric Blake <ebb9@byu.net>. */ + +#include <config.h> + +#include "areadlink.h" + +#include <errno.h> +#include <limits.h> +#include <stdint.h> +#include <stdlib.h> +#include <string.h> +#include <unistd.h> + +#if HAVE_READLINKAT + +/* SYMLINK_MAX is used only for an initial memory-allocation sanity + check, so it's OK to guess too small on hosts where there is no + arbitrary limit to symbolic link length. */ +# ifndef SYMLINK_MAX +# define SYMLINK_MAX 1024 +# endif + +# define MAXSIZE (SIZE_MAX < SSIZE_MAX ? SIZE_MAX : SSIZE_MAX) + +/* Call readlinkat to get the symbolic link value of FILE, relative to FD. + SIZE is a hint as to how long the link is expected to be; + typically it is taken from st_size. It need not be correct. + Return a pointer to that NUL-terminated string in malloc'd storage. + If readlinkat fails, malloc fails, or if the link value is longer + than SSIZE_MAX, return NULL (caller may use errno to diagnose). + However, failure to change directory during readlinkat will issue + a diagnostic and exit. */ + +char * +areadlinkat_with_size (int fd, char const *file, size_t size) +{ + /* Some buggy file systems report garbage in st_size. Defend + against them by ignoring outlandish st_size values in the initial + memory allocation. */ + size_t symlink_max = SYMLINK_MAX; + size_t INITIAL_LIMIT_BOUND = 8 * 1024; + size_t initial_limit = (symlink_max < INITIAL_LIMIT_BOUND + ? symlink_max + 1 + : INITIAL_LIMIT_BOUND); + + enum { stackbuf_size = 128 }; + + /* The initial buffer size for the link value. */ + size_t buf_size = (size == 0 ? stackbuf_size + : size < initial_limit ? size + 1 : initial_limit); + + while (1) + { + ssize_t r; + size_t link_length; + char stackbuf[stackbuf_size]; + char *buf = stackbuf; + char *buffer = NULL; + + if (! (size == 0 && buf_size == stackbuf_size)) + { + buf = buffer = malloc (buf_size); + if (!buffer) + /* We can assume errno == ENOMEM here, since all platforms that have + readlinkat() have a POSIX compliant malloc(). */ + return NULL; + } + + r = readlinkat (fd, file, buf, buf_size); + link_length = r; + + if (r < 0) + { + free (buffer); + return NULL; + } + + if (link_length < buf_size) + { + buf[link_length] = 0; + if (!buffer) + { + buffer = malloc (link_length + 1); + if (buffer) + return memcpy (buffer, buf, link_length + 1); + } + else if (link_length + 1 < buf_size) + { + /* Shrink BUFFER before returning it. */ + char *shrinked_buffer = realloc (buffer, link_length + 1); + if (shrinked_buffer != NULL) + buffer = shrinked_buffer; + } + return buffer; + } + + free (buffer); + if (buf_size <= MAXSIZE / 2) + buf_size *= 2; + else if (buf_size < MAXSIZE) + buf_size = MAXSIZE; + else + { + errno = ENOMEM; + return NULL; + } + } +} + +#else /* !HAVE_READLINKAT */ + + +/* It is more efficient to change directories only once and call + areadlink_with_size, rather than repeatedly call the replacement + readlinkat. */ + +# define AT_FUNC_NAME areadlinkat_with_size +# define AT_FUNC_F1 areadlink_with_size +# define AT_FUNC_POST_FILE_PARAM_DECLS , size_t size +# define AT_FUNC_POST_FILE_ARGS , size +# define AT_FUNC_RESULT char * +# define AT_FUNC_FAIL NULL +# include "at-func.c" +# undef AT_FUNC_NAME +# undef AT_FUNC_F1 +# undef AT_FUNC_POST_FILE_PARAM_DECLS +# undef AT_FUNC_POST_FILE_ARGS +# undef AT_FUNC_RESULT +# undef AT_FUNC_FAIL + +#endif /* !HAVE_READLINKAT */ |