Re: binfmt_misc broken

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Jun 10, 2013 at 09:52:44PM +0800, Jeff Chua wrote:
> 
> 
> According to Documentation/binfmt_misc.txt, the 'magic' and 'mask'
> can be set by echoing it to /proc/sys/fs/binfmt_misc/register.
> 
> Here's the problem I can across while working on ARM.
> 
> # echo ':arm:M::\x7fELF\x01\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x28\x00:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/usr/bin/qemu-arm-static:'
> >/proc/sys/fs/binfmt_misc/register
> 
> # cat /proc/sys/fs/binfmt_misc/arm
> wrong ...
> magic 7f454c46010101
> mask ffffffffffffff
> 
> right ...
> magic 7f454c4601010100000000000000000002002800
> mask ffffffffffffff00fffffffffffffffffeffffff
> 
> 
> binfmt_misc is truncating e->size, so once ARM's magic is loaded,
> 32-bit x86 can no longer run.
> 
> Here's a patch for it. It's looking for the delimiter ":" instead of
> \0. Now 32-bit x86 can run concurrent while qemu-arm is handling
> ARM's magic.

Patch is complete BS and I really wonder what kernel have you observed that bug on -
with mainline on amd64 your example yields
root@kvm-amd64:~# cat /proc/sys/fs/binfmt_misc/arm 
enabled
interpreter /usr/bin/qemu-arm-static
flags: 
offset 0
magic 7f454c4601010100000000000000000002002800
mask ffffffffffffff00fffffffffffffffffeffffff

A reproducer, please...  As for the memcmp() Linus has suggested - it's !Magic case, i.e.
what we are comparing there is not the file contents, it's the extension.  IOW, strcmp()
is the right thing to use there - pathnames do not contain NULs in the middle...