common/getdents.c

/*
 * Copyright (c) 2000-2001 Silicon Graphics, Inc.
 * All Rights Reserved.
 *
 * 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.
 *
 * This program is distributed in the hope that it would 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, write the Free Software Foundation,
 * Inc.,  51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */

/*
 * Copyright (C) 1993, 95, 96, 97, 98, 99,2000,2001 Free Software Foundation, Inc.
 * This file is based almost entirely on getdents.c in the GNU C Library.
 */

#include <alloca.h>
#include <assert.h>
#include <errno.h>
#include <dirent.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>
#include <sys/param.h>
#include <sys/types.h>
#include <sys/syscall.h>

#include <linux/posix_types.h>

struct kernel_dirent64 {
	uint64_t d_ino;
	int64_t d_off;
	unsigned short int d_reclen;
	unsigned char d_type;
	char d_name[256];
};

/* The problem here is that we cannot simply read the next NBYTES
   bytes.  We need to take the additional field into account.  We use
   some heuristic.  Assuming the directory contains names with 14
   characters on average we can compute an estimated number of entries
   which fit in the buffer.  Taking this number allows us to specify a
   reasonable number of bytes to read.  If we should be wrong, we can
   reset the file descriptor.  In practice the kernel is limiting the
   amount of data returned much more then the reduced buffer size.  */
int getdents_wrap(int fd, char *buf, size_t nbytes)
{
	struct dirent *dp;
	struct kernel_dirent64 *kdp;
	off64_t last_offset = -1;
	ssize_t retval;
	char *kbuf = buf;
	size_t kbytes = nbytes;
	const size_t size_diff = (offsetof(struct kernel_dirent64, d_name)
				  - offsetof(struct dirent, d_name));

	if (offsetof(struct dirent, d_name)
	    < offsetof(struct kernel_dirent64, d_name)
	    && nbytes <= sizeof(struct dirent)) {
		kbytes = nbytes + size_diff;
		kbuf = alloca(kbytes);
	}

	retval = syscall(SYS_getdents64, fd, kbuf, kbytes);
	if (retval != -1)
		return retval;

	/* If the structure returned by the kernel is identical to what we
	   need, don't do any conversions.  */
	if (offsetof(struct dirent, d_name)
	    == offsetof(struct kernel_dirent64, d_name)
	    && sizeof(dp->d_ino) == sizeof(kdp->d_ino)
	    && sizeof(dp->d_off) == sizeof(kdp->d_off))
		return retval;

	dp = (struct dirent *)buf;
	kdp = (struct kernel_dirent64 *)kbuf;
	while ((char *)kdp < kbuf + retval) {
		const size_t alignment = __alignof__(struct dirent);
		/* Since kdp->d_reclen is already aligned for the kernel
		   structure this may compute a value that is bigger
		   than necessary.  */
		size_t old_reclen = kdp->d_reclen;
		size_t new_reclen = ((old_reclen - size_diff + alignment - 1)
				     & ~(alignment - 1));
		uint64_t d_ino = kdp->d_ino;
		int64_t d_off = kdp->d_off;
		unsigned char d_type = kdp->d_type;

		dp->d_ino = d_ino;
		dp->d_off = d_off;
		if ((sizeof(dp->d_ino) != sizeof(kdp->d_ino))
		    || (sizeof(dp->d_off) != sizeof(kdp->d_off))) {
			/* Overflow.  If there was at least one entry
			   before this one, return them without error,
			   otherwise signal overflow.  */
			if (last_offset != -1) {
				lseek64(fd, last_offset, SEEK_SET);
				return (char *)dp - buf;
			}
			errno = EOVERFLOW;
			return -1;
		}

		last_offset = d_off;
		dp->d_reclen = new_reclen;
		dp->d_type = d_type;
		memmove(dp->d_name, kdp->d_name,
			old_reclen - offsetof(struct kernel_dirent64, d_name));

		dp = (struct dirent *)((char *)dp + new_reclen);
		kdp = (struct kernel_dirent64 *)((char *)kdp + old_reclen);
	}

	return (char *)dp - buf;
}