binfmt_elf: partially sanitize PRSTATUS_SIZE and SET_PR_FPVALID

On 64bit architectures that support 32bit processes there are
two possible layouts for NT_PRSTATUS note in ELF coredumps.
For one thing, several fields are 64bit for native processes
and 32bit for compat ones (pr_sigpend, etc.).  For another,
the register dump is obviously different - the size and number
of registers are not going to be the same for 32bit and 64bit
variants of processor.

Usually that's handled by having two structures - elf_prstatus
for native layout and compat_elf_prstatus for 32bit one.
32bit processes are handled by fs/compat_binfmt_elf.c, which
defines a macro called 'elf_prstatus' that expands to compat_elf_prstatus.
Then it includes fs/binfmt_elf.c, which makes all references to
struct elf_prstatus to be textually replaced with struct
compat_elf_prstatus.  Ugly and somewhat brittle, but it works.

However, amd64 is worse - there are _three_ possible layouts.
One for native 64bit processes, another for i386 (32bit) processes
and yet another for x32 (32bit address space with full 64bit
registers).

Both i386 and x32 processes are handled by fs/compat_binfmt_elf.c,
with usual compat_binfmt_elf.c trickery.  However, the layouts
for i386 and x32 are not identical - they have the common beginning,
but the register dump part (pr_reg) is bigger on x32.  Worse, pr_reg
is not the last field - it's followed by int pr_fpvalid, so that
field ends up at different offsets for i386 and x32 layouts.

Fortunately, there's not much code that cares about any of that -
it's all encapsulated in fill_thread_core_info().  Since x32
variant is bigger, we define compat_elf_prstatus to match that
layout.  That way i386 processes have enough space to fit
their layout into.

Moreover, since these layouts are identical prior to pr_reg,
we don't need to distinguish x32 and i386 cases when we are
setting the fields prior to pr_reg.

Filling pr_reg itself is done by calling ->get() method of
appropriate regset, and that method knows what layout (and size)
to use.

We do need to distinguish x32 and i386 cases only for two
things: setting ->pr_fpvalid (offset differs for x32 and
i386) and choosing the right size for our note.

The way it's done is Not Nice, for the lack of more accurate
printable description.  There are two macros (PRSTATUS_SIZE and
SET_PR_FPVALID), that default essentially to sizeof(struct elf_prstatus)
and (S)->pr_fpvalid = 1.  On x86 asm/compat.h provides its own
variants.

Unfortunately, quite a few things go wrong there:
	* PRSTATUS_SIZE doesn't use the normal test for process
being an x32 one; it compares the size reported by regset with
the size of pr_reg.
	* it hardcodes the sizes of x32 and i386 variants (296 and 144
resp.), so if some change in includes leads to asm/compat.h pulled
in by fs/binfmt_elf.c we are in trouble - it will end up using
the size of x32 variant for 64bit processes.
	* it's in the wrong place; asm/compat.h couldn't define
the structure for i386 layout, since it lacks quite a few types
needed for it.  Hardcoded sizes are largely due to that.

The proper fix would be to have an explicitly defined i386 variant
of structure and have PRSTATUS_SIZE/SET_PR_FPVALID check for
TIF_X32 to choose the variant that should be used.  Unfortunately,
that requires some manipulations of headers; we'll do that later
in the series, but for now let's go with the minimal variant -
rename PRSTATUS_SIZE in asm/compat.h to COMPAT_PRSTATUS_SIZE,
have fs/compat_binfmt_elf.c define PRSTATUS_SIZE to COMPAT_PRSTATUS_SIZE
and use the normal TIF_X32 check in that macro.  The size of i386 variant
is kept hardcoded for now.  Similar story for SET_PR_FPVALID.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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