path: root/Documentation/binfmt_misc.txt
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authorMike Frysinger <vapier@gentoo.org>2014-10-13 15:52:03 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-10-14 02:18:15 +0200
commitbbaecc088245e840e59a5abe23d69cf7748b3c88 (patch)
treed68ea1953d0c608cd0f655ea03c7d4810e5ae37c /Documentation/binfmt_misc.txt
parentf78d98f6ce66fc7cc0be714d56b0240923a8b4f4 (diff)
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>
Diffstat (limited to 'Documentation/binfmt_misc.txt')
1 files changed, 1 insertions, 1 deletions
diff --git a/Documentation/binfmt_misc.txt b/Documentation/binfmt_misc.txt
index c1ed6948ba80..f64372b284e8 100644
--- a/Documentation/binfmt_misc.txt
+++ b/Documentation/binfmt_misc.txt
@@ -58,7 +58,7 @@ Here is what the fields mean:
There are some restrictions:
- - the whole register string may not exceed 255 characters
+ - the whole register string may not exceed 1920 characters
- the magic must reside in the first 128 bytes of the file, i.e.
offset+size(magic) has to be less than 128
- the interpreter string may not exceed 127 characters

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