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/* Read symbolic links into a buffer without size limitation, relative to fd.
Copyright (C) 2001, 2003-2004, 2007, 2009-2023 Free Software Foundation,
Inc.
This file is free software: you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 2.1 of the
License, or (at your option) any later version.
This file 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 Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
/* Written by Paul Eggert, Bruno Haible, and Jim Meyering. */
#include <config.h>
#include "careadlinkat.h"
#include "idx.h"
#include "minmax.h"
#include <errno.h>
#include <limits.h>
#include <string.h>
#include <unistd.h>
/* Define this independently so that stdint.h is not a prerequisite. */
#ifndef SIZE_MAX
# define SIZE_MAX ((size_t) -1)
#endif
#ifndef SSIZE_MAX
# define SSIZE_MAX ((ssize_t) (SIZE_MAX / 2))
#endif
#include "allocator.h"
enum { STACK_BUF_SIZE = 1024 };
/* Act like careadlinkat (see below), with an additional argument
STACK_BUF that can be used as temporary storage.
If GCC_LINT is defined, do not inline this function with GCC 10.1
and later, to avoid creating a pointer to the stack that GCC
-Wreturn-local-addr incorrectly complains about. See:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93644
Although the noinline attribute can hurt performance a bit, no better way
to pacify GCC is known; even an explicit #pragma does not pacify GCC.
When the GCC bug is fixed this workaround should be limited to the
broken GCC versions. */
#if _GL_GNUC_PREREQ (10, 1)
# if defined GCC_LINT || defined lint
__attribute__ ((__noinline__))
# elif __OPTIMIZE__ && !__NO_INLINE__
# define GCC_BOGUS_WRETURN_LOCAL_ADDR
# endif
#endif
static char *
readlink_stk (int fd, char const *filename,
char *buffer, size_t buffer_size,
struct allocator const *alloc,
ssize_t (*preadlinkat) (int, char const *, char *, size_t),
char stack_buf[STACK_BUF_SIZE])
{
if (! alloc)
alloc = &stdlib_allocator;
if (!buffer)
{
buffer = stack_buf;
buffer_size = STACK_BUF_SIZE;
}
char *buf = buffer;
idx_t buf_size_max = MIN (IDX_MAX, MIN (SSIZE_MAX, SIZE_MAX));
idx_t buf_size = MIN (buffer_size, buf_size_max);
while (buf)
{
/* Attempt to read the link into the current buffer. */
idx_t link_length = preadlinkat (fd, filename, buf, buf_size);
if (link_length < 0)
{
if (buf != buffer)
{
int readlinkat_errno = errno;
alloc->free (buf);
errno = readlinkat_errno;
}
return NULL;
}
idx_t link_size = link_length;
if (link_size < buf_size)
{
buf[link_size++] = '\0';
if (buf == stack_buf)
{
char *b = alloc->allocate (link_size);
buf_size = link_size;
if (! b)
break;
return memcpy (b, buf, link_size);
}
if (link_size < buf_size && buf != buffer && alloc->reallocate)
{
/* Shrink BUF before returning it. */
char *b = alloc->reallocate (buf, link_size);
if (b)
return b;
}
return buf;
}
if (buf != buffer)
alloc->free (buf);
if (buf_size_max / 2 <= buf_size)
{
errno = ENAMETOOLONG;
return NULL;
}
buf_size = 2 * buf_size + 1;
buf = alloc->allocate (buf_size);
}
if (alloc->die)
alloc->die (buf_size);
errno = ENOMEM;
return NULL;
}
/* Assuming the current directory is FD, get the symbolic link value
of FILENAME as a null-terminated string and put it into a buffer.
If FD is AT_FDCWD, FILENAME is interpreted relative to the current
working directory, as in openat.
If the link is small enough to fit into BUFFER put it there.
BUFFER's size is BUFFER_SIZE, and BUFFER can be null
if BUFFER_SIZE is zero.
If the link is not small, put it into a dynamically allocated
buffer managed by ALLOC. It is the caller's responsibility to free
the returned value if it is nonnull and is not BUFFER. A null
ALLOC stands for the standard allocator.
The PREADLINKAT function specifies how to read links. It operates
like POSIX readlinkat()
<https://pubs.opengroup.org/onlinepubs/9699919799/functions/readlink.html>
but can assume that its first argument is the same as FD.
If successful, return the buffer address; otherwise return NULL and
set errno. */
char *
careadlinkat (int fd, char const *filename,
char *buffer, size_t buffer_size,
struct allocator const *alloc,
ssize_t (*preadlinkat) (int, char const *, char *, size_t))
{
/* Allocate the initial buffer on the stack. This way, in the
common case of a symlink of small size, we get away with a
single small malloc instead of a big malloc followed by a
shrinking realloc. */
#ifdef GCC_BOGUS_WRETURN_LOCAL_ADDR
#warning "GCC might issue a bogus -Wreturn-local-addr warning here."
#warning "See <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93644>."
#endif
char stack_buf[STACK_BUF_SIZE];
return readlink_stk (fd, filename, buffer, buffer_size, alloc,
preadlinkat, stack_buf);
}
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