• Ian Kent's avatar
    autofs: work around unhappy compat problem on x86-64 · a32744d4
    Ian Kent authored
    When the autofs protocol version 5 packet type was added in commit
    5c0a32fc
    
     ("autofs4: add new packet type for v5 communications"), it
    obvously tried quite hard to be word-size agnostic, and uses explicitly
    sized fields that are all correctly aligned.
    
    However, with the final "char name[NAME_MAX+1]" array at the end, the
    actual size of the structure ends up being not very well defined:
    because the struct isn't marked 'packed', doing a "sizeof()" on it will
    align the size of the struct up to the biggest alignment of the members
    it has.
    
    And despite all the members being the same, the alignment of them is
    different: a "__u64" has 4-byte alignment on x86-32, but native 8-byte
    alignment on x86-64.  And while 'NAME_MAX+1' ends up being a nice round
    number (256), the name[] array starts out a 4-byte aligned.
    
    End result: the "packed" size of the structure is 300 bytes: 4-byte, but
    not 8-byte aligned.
    
    As a result, despite all the fields being in the same place on all
    architectures, sizeof() will round up that size to 304 bytes on
    architectures that have 8-byte alignment for u64.
    
    Note that this is *not* a problem for 32-bit compat mode on POWER, since
    there __u64 is 8-byte aligned even in 32-bit mode.  But on x86, 32-bit
    and 64-bit alignment is different for 64-bit entities, and as a result
    the structure that has exactly the same layout has different sizes.
    
    So on x86-64, but no other architecture, we will just subtract 4 from
    the size of the structure when running in a compat task.  That way we
    will write the properly sized packet that user mode expects.
    
    Not pretty.  Sadly, this very subtle, and unnecessary, size difference
    has been encoded in user space that wants to read packets of *exactly*
    the right size, and will refuse to touch anything else.
    Reported-and-tested-by: default avatarThomas Meyer <thomas@m3y3r.de>
    Signed-off-by: default avatarIan Kent <raven@themaw.net>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    a32744d4