binfmt_misc: expand the register format limit to 1920 bytes

binfmt_misc: expand the register format limit to 1920 bytes

The current code places a 256 byte limit on the registration format.
This ends up being fairly limited when you try to do matching against a
binary format like ELF:

 - the magic & mask formats cannot have any embedded NUL chars
   (string_unescape_inplace halts at the first NUL)
 - each escape sequence quadruples the size: \x00 is needed for NUL
 - trying to match bytes at the start of the file as well as further
   on leads to a lot of \x00 sequences in the mask
 - magic & mask have to be the same length (when decoded)
 - still need bytes for the other fields
 - impossible!

Let's look at a concrete (and common) example: using QEMU to run MIPS
ELFs.  The name field uses 11 bytes "qemu-mipsel".  The interp uses 20
bytes "/usr/bin/qemu-mipsel".  The type & flags takes up 4 bytes.  We
need 7 bytes for the delimiter (usually ":").  We can skip offset.  So
already we're down to 107 bytes to use with the magic/mask instead of
the real limit of 128 (BINPRM_BUF_SIZE).  If people use shell code to
register (which they do the majority of the time), they're down to ~26
possible bytes since the escape sequence must be \x##.

The ELF format looks like (both 32 & 64 bit):

	e_ident: 16 bytes
	e_type: 2 bytes
	e_machine: 2 bytes

Those 20 bytes are enough for most architectures because they have so few
formats in the first place, thus they can be uniquely identified.  That
also means for shell users, since 20 is smaller than 26, they can sanely
register a handler.

But for some targets (like MIPS), we need to poke further.  The ELF fields
continue on:

	e_entry: 4 or 8 bytes
	e_phoff: 4 or 8 bytes
	e_shoff: 4 or 8 bytes
	e_flags: 4 bytes

We only care about e_flags here as that includes the bits to identify
whether the ELF is O32/N32/N64.  But now we have to consume another 16
bytes (for 32 bit ELFs) or 28 bytes (for 64 bit ELFs) just to match the
flags.  If every byte is escaped, we send 288 more bytes to the kernel
((20 {e_ident,e_type,e_machine} + 12 {e_entry,e_phoff,e_shoff} + 4
{e_flags}) * 2 {mask,magic} * 4 {escape}) and we've clearly blown our
budget.

Even if we try to be clever and do the decoding ourselves (rather than
relying on the kernel to process \x##), we still can't hit the mark --
string_unescape_inplace treats mask & magic as C strings so NUL cannot
be embedded.  That leaves us with having to pass \x00 for the 12/24
entry/phoff/shoff bytes (as those will be completely random addresses),
and that is a minimum requirement of 48/96 bytes for the mask alone.
Add up the rest and we blow through it (this is for 64 bit ELFs):
magic: 20 {e_ident,e_type,e_machine} + 24 {e_entry,e_phoff,e_shoff} +
       4 {e_flags} = 48              # ^^ See note below.
mask: 20 {e_ident,e_type,e_machine} + 96 {e_entry,e_phoff,e_shoff} +
       4 {e_flags} = 120
Remember above we had 107 left over, and now we're at 168.  This is of
course the *best* case scenario -- you'll also want to have NUL bytes
in the magic & mask too to match literal zeros.

Note: the reason we can use 24 in the magic is that we can work off of the
fact that for bytes the mask would clobber, we can stuff any value into
magic that we want.  So when mask is \x00, we don't need the magic to also
be \x00, it can be an unescaped raw byte like '!'.  This lets us handle
more formats (barely) under the current 256 limit, but that's a pretty
tall hoop to force people to jump through.

With all that said, let's bump the limit from 256 bytes to 1920.  This way
we support escaping every byte of the mask & magic field (which is 1024
bytes by themselves -- 128 * 4 * 2), and we leave plenty of room for other
fields.  Like long paths to the interpreter (when you have source in your
/really/long/homedir/qemu/foo).  Since the current code stuffs more than
one structure into the same buffer, we leave a bit of space to easily
round up to 2k.  1920 is just as arbitrary as 256 ;).

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>