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magic - file command's magic number file
Each line of the file specifies a test to be performed. A test compares the data starting at a particular offset in the file with a 1-byte, 2-byte, or 4-byte numeric value or a string. If the test succeeds, a message is printed. The line consists of the following fields:
The numeric types may optionally be followed by Am] and a numeric value, to specify that the value is to be AND'ed with the numeric value before any comparisons are done. Prepending a u to the type indicates that ordered comparisons should be unsigned.
Numeric values may be preceded by a character indicating the operation to be performed. It may be = to specify that the value from the file must equal the specified value, Lt] to specify that the value from the file must be less than the specified value, Gt] to specify that the value from the file must be greater than the specified value, Am] to specify that the value from the file must have set all of the bits that are set in the specified value, ^ to specify that the value from the file must have clear any of the bits that are set in the specified value, or ~ the value specified after is negated before tested. x to specify that any value will match. If the character is omitted, it is assumed to be = Operators Am] ^ and ~ don't work with floats and doubles. For all tests except string and regex operation ! specifies that the line matches if the test does not succeed.
Numeric values are specified in C form; e.g. 13 is decimal, 013 is octal, and 0x13 is hexadecimal.
For string values, the byte string from the file must match the specified byte string. The operators = Lt] and Gt] (but not Am] can be applied to strings. The length used for matching is that of the string argument in the magic file. This means that a line can match any string, and then presumably print that string, by doing Gt]\0 (because all strings are greater than the null string).
The special test x always evaluates to true. message The message to be printed if the comparison succeeds. If the string contains a printf(3) format specification, the value from the file (with any specified masking performed) is printed using the message as the format string. If the string begins with ``\b'', the message printed is the remainder of the string with no whitespace added before it: multiple matches are normally separated by a single space.
Some file formats contain additional information which is to be printed along with the file type or need additional tests to determine the true file type. These additional tests are introduced by one or more Gt] characters preceding the offset. The number of Gt] on the line indicates the level of the test; a line with no Gt] at the beginning is considered to be at level 0. Tests are arranged in a tree-like hierarchy: If a the test on a line at level n succeeds, all following tests at level n+1 are performed, and the messages printed if the tests succeed, untile a line with level n (or less) appears. For more complex files, one can use empty messages to get just the "if/then" effect, in the following way:
0 string MZ Gt]0x18 leshort Lt]0x40 MS-DOS executable Gt]0x18 leshort Gt]0x3f extended PC executable (e.g., MS Windows)
Offsets do not need to be constant, but can also be read from the file being examined. If the first character following the last Gt] is a ( then the string after the parenthesis is interpreted as an indirect offset. That means that the number after the parenthesis is used as an offset in the file. The value at that offset is read, and is used again as an offset in the file. Indirect offsets are of the form: (( x [.[bslBSL]][+-][ y ]) The value of x is used as an offset in the file. A byte, short or long is read at that offset depending on the [bslBSLm] type specifier. The capitalized types interpret the number as a big endian value, whereas the small letter versions interpret the number as a little endian value; the m type interprets the number as a middle endian (PDP-11) value. To that number the value of y is added and the result is used as an offset in the file. The default type if one is not specified is long.
That way variable length structures can be examined:
# MS Windows executables are also valid MS-DOS executables 0 string MZ Gt]0x18 leshort Lt]0x40 MZ executable (MS-DOS) # skip the whole block below if it is not an extended executable Gt]0x18 leshort Gt]0x3f Gt]Gt](0x3c.l) string PE\0\0 PE executable (MS-Windows) Gt]Gt](0x3c.l) string LX\0\0 LX executable (OS/2)
This strategy of examining has one drawback: You must make sure that you eventually print something, or users may get empty output (like, when there is neither PE\0\0 nor LE\0\0 in the above example)
If this indirect offset cannot be used as-is, there are simple calculations possible: appending [+-*/%Am]|^]Lt]numberGt] inside parentheses allows one to modify the value read from the file before it is used as an offset:
# MS Windows executables are also valid MS-DOS executables 0 string MZ # sometimes, the value at 0x18 is less that 0x40 but there's still an # extended executable, simply appended to the file Gt]0x18 leshort Lt]0x40 Gt]Gt](4.s*512) leshort 0x014c COFF executable (MS-DOS, DJGPP) Gt]Gt](4.s*512) leshort !0x014c MZ executable (MS-DOS)
Sometimes you do not know the exact offset as this depends on the length or position (when indirection was used before) of preceding fields. You can specify an offset relative to the end of the last up-level field using `Am]' as a prefix to the offset:
0 string MZ Gt]0x18 leshort Gt]0x3f Gt]Gt](0x3c.l) string PE\0\0 PE executable (MS-Windows) # immediately following the PE signature is the CPU type Gt]Gt]Gt]Am]0 leshort 0x14c for Intel 80386 Gt]Gt]Gt]Am]0 leshort 0x184 for DEC Alpha
Indirect and relative offsets can be combined:
0 string MZ Gt]0x18 leshort Lt]0x40 Gt]Gt](4.s*512) leshort !0x014c MZ executable (MS-DOS) # if it's not COFF, go back 512 bytes and add the offset taken # from byte 2/3, which is yet another way of finding the start # of the extended executable Gt]Gt]Gt]Am](2.s-514) string LE LE executable (MS Windows VxD driver)
Or the other way around:
0 string MZ Gt]0x18 leshort Gt]0x3f Gt]Gt](0x3c.l) string LE\0\0 LE executable (MS-Windows) # at offset 0x80 (-4, since relative offsets start at the end # of the up-level match) inside the LE header, we find the absolute # offset to the code area, where we look for a specific signature Gt]Gt]Gt](Am]0x7c.l+0x26) string UPX \b, UPX compressed
Or even both!
0 string MZ Gt]0x18 leshort Gt]0x3f Gt]Gt](0x3c.l) string LE\0\0 LE executable (MS-Windows) # at offset 0x58 inside the LE header, we find the relative offset # to a data area where we look for a specific signature Gt]Gt]Gt]Am](Am]0x54.l-3) string UNACE \b, ACE self-extracting archive
Finally, if you have to deal with offset/length pairs in your file, even the second value in a parenthesized expression can be taken from the file itself, using another set of parentheses. Note that this additional indirect offset is always relative to the start of the main indirect offset.
0 string MZ Gt]0x18 leshort Gt]0x3f Gt]Gt](0x3c.l) string PE\0\0 PE executable (MS-Windows) # search for the PE section called ".idata"... Gt]Gt]Gt]Am]0xf4 search/0x140 .idata # ...and go to the end of it, calculated from start+length; # these are located 14 and 10 bytes after the section name Gt]Gt]Gt]Gt](Am]0xe.l+(-4)) string PK\3\4 \b, ZIP self-extracting archive
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