Re: [PATCH 2/3] vmsplice: make vmsplice a trivial wrapper for preadv2/pwritev2

[Linus Torvalds]

On Thu, 4 Jun 2026 at 10:38, David Laight <david.laight.linux@xxxxxxxxx> wrote:
>
> Bool is another matter entirely, (IIRC from a couple of weeks ago)
> gcc will assume that the low 8 bits of the parameter register are
> either 0 or 1 and clang assumes that the low 32 bits are 0 or 1.
> You can't even check with 'if ((u32)bool_param > 1) error()' because
> the compiler 'knows' it can't be false.

Nobody should ever use 'bool' as a system call argument. Anything that
takes a boolean should take a 'flags' field with bits.

But this is basically what a lot of the SYSCALL_DEFINEx() macros are
all about - sorting out ABI assumptions.

For example, on powerpc (iirc - maybe it was 390), a 32-bit argument
is always sign-extended by the ABI, and the compiler *depends* on
that. But at system call boundaries we can't trust that the user side
actually follows the ABI, so SYSCALL_DEFINEx() will actually take a
'unsigned long' and turn it into a 32-bit argument so that things like
this are well-defined and you can't fool the kernel by not following
the ABI rules.

The same would be the case if some system call actually takes bool
(but I don't think such garbage exists). The SYSCALL_DEFINE() macro
magic would take the full register content and *force* it to follow
the ABI conventions, whatever they are on that platform.

              Linus