[OE-core] pseudo: host user contamination

[Enrico Scholz]

Seebs <seebs at seebs.net> writes:

>> There are no 64 bit arguments (on 32 bit platforms) which might
>> require a special treatment as described in [1]
>> "Architecture-specific requirements".
>
> Okay, ignore the pointer case, and pretend it's the 64-bit value case,
> since we have specific-ish documentation for that.
>
> Look at the example for SYS_readahead, stating that the caller must
> pass an extra value.

SYS_readahead is one of a few syscalls which pass 64 bit arguments on 32
bit architectures.  Without the manual splitting, the ABI will cause the
compiler to insert a dummy argument so that registers are aligned for 64
bit values.

The caller of syscall(2) has to split such arguments. When the wrapper
does not handle the syscall itself, it just need to pass the arguments
as-is.


> At that point, if you have a series of va_arg calls corresponding to
> the values that would have been arguments had they not passed the extra
> value, I don't think you get the expected arguments. So far as I can
> tell, if the caller actually wrote
> 	varargsfunc(SYS_readahead, 0, uint64_t_value, ...)
> and the function did
> 	va_arg(ap, uint64_t);

That's true and must be checked when writing a wrapper for a syscall
which takes a 64 bit argument on a 32 bit architecture.

But for renameat2() it does not matter; every of its arguments fits into
a single register.



Enrico