is_tar.c revision 186690 
1/*
2 * Copyright (c) Ian F. Darwin 1986-1995.
3 * Software written by Ian F. Darwin and others;
4 * maintained 1995-present by Christos Zoulas and others.
5 *
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
8 * are met:
9 * 1. Redistributions of source code must retain the above copyright
10 *    notice immediately at the beginning of the file, without modification,
11 *    this list of conditions, and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 *    notice, this list of conditions and the following disclaimer in the
14 *    documentation and/or other materials provided with the distribution.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE FOR
20 * ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 */
28/*
29 * is_tar() -- figure out whether file is a tar archive.
30 *
31 * Stolen (by the author!) from the public domain tar program:
32 * Public Domain version written 26 Aug 1985 John Gilmore (ihnp4!hoptoad!gnu).
33 *
34 * @(#)list.c 1.18 9/23/86 Public Domain - gnu
35 *
36 * Comments changed and some code/comments reformatted
37 * for file command by Ian Darwin.
38 */
39
40#include "file.h"
41#include "magic.h"
42#include <string.h>
43#include <ctype.h>
44#include <sys/types.h>
45#include "tar.h"
46
47#ifndef lint
48FILE_RCSID("@(#)$File: is_tar.c,v 1.31 2008/02/04 20:51:17 christos Exp $")
49#endif
50
51#define	isodigit(c)	( ((c) >= '0') && ((c) <= '7') )
52
53private int is_tar(const unsigned char *, size_t);
54private int from_oct(int, const char *);	/* Decode octal number */
55
56static const char tartype[][32] = {
57	"tar archive",
58	"POSIX tar archive",
59	"POSIX tar archive (GNU)",
60};
61
62protected int
63file_is_tar(struct magic_set *ms, const unsigned char *buf, size_t nbytes)
64{
65	/*
66	 * Do the tar test first, because if the first file in the tar
67	 * archive starts with a dot, we can confuse it with an nroff file.
68	 */
69	int tar = is_tar(buf, nbytes);
70	int mime = ms->flags & MAGIC_MIME;
71
72	if (tar < 1 || tar > 3)
73		return 0;
74
75	if (mime == MAGIC_MIME_ENCODING)
76		return 0;
77
78	if (file_printf(ms, mime ? "application/x-tar" :
79	    tartype[tar - 1]) == -1)
80		return -1;
81	return 1;
82}
83
84/*
85 * Return
86 *	0 if the checksum is bad (i.e., probably not a tar archive),
87 *	1 for old UNIX tar file,
88 *	2 for Unix Std (POSIX) tar file,
89 *	3 for GNU tar file.
90 */
91private int
92is_tar(const unsigned char *buf, size_t nbytes)
93{
94	const union record *header = (const union record *)(const void *)buf;
95	int	i;
96	int	sum, recsum;
97	const char	*p;
98
99	if (nbytes < sizeof(union record))
100		return 0;
101
102	recsum = from_oct(8,  header->header.chksum);
103
104	sum = 0;
105	p = header->charptr;
106	for (i = sizeof(union record); --i >= 0;) {
107		/*
108		 * We cannot use unsigned char here because of old compilers,
109		 * e.g. V7.
110		 */
111		sum += 0xFF & *p++;
112	}
113
114	/* Adjust checksum to count the "chksum" field as blanks. */
115	for (i = sizeof(header->header.chksum); --i >= 0;)
116		sum -= 0xFF & header->header.chksum[i];
117	sum += ' '* sizeof header->header.chksum;
118
119	if (sum != recsum)
120		return 0;	/* Not a tar archive */
121
122	if (strcmp(header->header.magic, GNUTMAGIC) == 0)
123		return 3;		/* GNU Unix Standard tar archive */
124	if (strcmp(header->header.magic, TMAGIC) == 0)
125		return 2;		/* Unix Standard tar archive */
126
127	return 1;			/* Old fashioned tar archive */
128}
129
130
131/*
132 * Quick and dirty octal conversion.
133 *
134 * Result is -1 if the field is invalid (all blank, or nonoctal).
135 */
136private int
137from_oct(int digs, const char *where)
138{
139	int	value;
140
141	while (isspace((unsigned char)*where)) {	/* Skip spaces */
142		where++;
143		if (--digs <= 0)
144			return -1;		/* All blank field */
145	}
146	value = 0;
147	while (digs > 0 && isodigit(*where)) {	/* Scan til nonoctal */
148		value = (value << 3) | (*where++ - '0');
149		--digs;
150	}
151
152	if (digs > 0 && *where && !isspace((unsigned char)*where))
153		return -1;			/* Ended on non-space/nul */
154
155	return value;
156}
157