5.1.0/src/path/glue.c

/* -*- c-set-style: "K&R"; c-basic-offset: 8 -*-
 *
 * This file is part of PRoot.
 *
 * Copyright (C) 2014 STMicroelectronics
 *
 * 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 2 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, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
 * 02110-1301 USA.
 */

#include <sys/types.h> /* mkdir(2), lstat(2), */
#include <sys/stat.h> /* mkdir(2), lstat(2), */
#include <fcntl.h>    /* mknod(2), */
#include <unistd.h>   /* mknod(2), lstat(2), unlink(2), rmdir(2), */
#include <string.h>   /* string(3),  */
#include <assert.h>   /* assert(3), */
#include <limits.h>   /* PATH_MAX, */
#include <errno.h>    /* errno, E* */
#include <talloc.h>   /* talloc_*, */

#include "path/binding.h"
#include "path/path.h"
#include "path/temp.h"
#include "cli/note.h"

#include "compat.h"

/**
 * Remove @path if it is empty only.
 *
 * Note: this is a Talloc destructor.
 */
static int remove_placeholder(char *path)
{
	struct stat statl;
	int status;

	status = lstat(path, &statl);
	if (status)
		return 0; /* Not fatal.  */

	if (!S_ISDIR(statl.st_mode)) {
		if (statl.st_size != 0)
			return 0; /* Not fatal.  */
		status = unlink(path);
	}
	else
		status = rmdir(path);
	if (status)
		return 0; /* Not fatal.  */

	return 0;
}

/**
 * Attach a copy of @path to the autofree context, and set its
 * destructor to remove_placeholder().
 */
static void set_placeholder_destructor(const char *path)
{
	TALLOC_CTX *autofreed;
	char *placeholder;

	autofreed = talloc_autofree_context();
	if (autofreed == NULL)
		return;

	placeholder = talloc_strdup(autofreed, path);
	if (placeholder == NULL)
		return;

	talloc_set_destructor(placeholder, remove_placeholder);
}

/**
 * Build in a temporary filesystem the glue between the guest part and
 * the host part of the @binding_path.  This function returns the type
 * of the bound path, otherwise 0 if an error occured.
 *
 * For example, assuming the host path "/opt" is mounted/bound to the
 * guest path "/black/holes/and/revelations", and assuming this path
 * can't be created in the guest rootfs (eg. permission denied), then
 * it is created in a temporary rootfs and all these paths are glued
 * that way:
 *
 *   $GUEST/black/ --> $GLUE/black/
 *                               ./holes
 *                               ./holes/and
 *                               ./holes/and/revelations --> $HOST/opt/
 *
 * This glue allows operations on paths that do not exist in the guest
 * rootfs but that were specified as the guest part of a binding.
 */
mode_t build_glue(Tracee *tracee, const char *guest_path, char host_path[PATH_MAX],
		Finality finality)
{
	bool belongs_to_gluefs;
	Comparison comparison;
	Binding *binding;
	mode_t type;
	mode_t mode;
	int status;

	assert(tracee->glue_type != 0);

	/* Create the temporary directory where the "glue" rootfs will
	 * lie.  */
	if (tracee->glue == NULL) {
		tracee->glue = create_temp_directory(NULL, tracee->tool_name);
		if (tracee->glue == NULL) {
			note(tracee, ERROR, INTERNAL, "can't create glue rootfs");
			return 0;
		}
		talloc_set_name_const(tracee->glue, "$glue");
	}

	comparison = compare_paths(tracee->glue, host_path);
	belongs_to_gluefs = (comparison == PATHS_ARE_EQUAL || comparison == PATH1_IS_PREFIX);

	/* If it's not a final component then it is a directory.  I definitely
	 * hate how the potential type of the final component is propagated
	 * from initialize_binding() down to here, sadly there's no elegant way
	 * to know its type at this stage.  */
	if (IS_FINAL(finality)) {
		type = tracee->glue_type;
		mode = (belongs_to_gluefs ? 0777 : 0);
	}
	else {
		type = S_IFDIR;
		mode = 0777;
	}

	if (getenv("PROOT_DONT_POLLUTE_ROOTFS") != NULL && !belongs_to_gluefs)
		goto create_binding;

	/* Try to create this component into the "guest" or "glue"
	 * rootfs (depending if there were a glue previously).  */
	if (S_ISDIR(type))
		status = mkdir(host_path, mode);
	else /* S_IFREG, S_IFCHR, S_IFBLK, S_IFIFO or S_IFSOCK.  */
		status = mknod(host_path, mode | type, 0);

	/* Remove placeholders from the guest rootfs once PRoot is
	 * terminated.  */
	if (status >= 0 && !belongs_to_gluefs)
		set_placeholder_destructor(host_path);

	/* Nothing else to do if the path already exists or if it is
	 * the final component since it will be pointed to by the
	 * binding being initialized (from the example,
	 * "$GUEST/black/holes/and/revelations" -> "$HOST/opt").  */
	if (status >= 0 || errno == EEXIST || IS_FINAL(finality))
		return type;

	/* mkdir/mknod are supposed to always succeed in
	 * tracee->glue.  */
	if (belongs_to_gluefs) {
		note(tracee, WARNING, SYSTEM, "mkdir/mknod");
		return 0;
	}

create_binding:
	/* Sanity checks.  */
	if (   strnlen(tracee->glue, PATH_MAX) >= PATH_MAX
	    || strnlen(guest_path, PATH_MAX) >= PATH_MAX) {
		note(tracee, WARNING, INTERNAL, "installing the binding: guest path too long");
		return 0;
	}

	/* From the example, create the binding "/black" ->
	 * "$GLUE/black".  */
	binding = insort_binding3(tracee, tracee->glue, tracee->glue, guest_path);
	if (binding == NULL)
		return 0;

	/* TODO: emulation of getdents(parent(guest_path)) to finalize
	 * the glue, "black" in getdents("/") from the example.  */

	return type;
}