On Sat, Feb 27, 2021 at 5:12 PM 'Axel Wagner' via golang-nuts
<[email protected]> wrote:
>
> On Sat, Feb 27, 2021 at 11:19 PM Reto <[email protected]> wrote:
>>
>> Now, as far as I can tell this forces non stdlib packages to adhere to
>> exactly that.
>> As far as I can tell x/sys is just a common namespace for the go authors, but
>> as far as the compiler itself is concerned, that's a normal module not the
>> stdlib.
>>
>>
>> Or is this a wrong assumption?
>
>
> I think you are correct in that the compiler does not give `x/sys` special
> treatment.
>
> However, `x/sys` is still somewhat special, in that, as its maintained by the
> Go team, they can make sure that `x/sys` stays up to date with regards to
> implementation details. That is, the documentation of the `unsafe` rules is
> more strict than it needs to be currently, to reserve the right to change the
> implementation in the future. And `x/sys` can use a more lenient
> interpretation, because if that implementation changes, it will be changed in
> lockstep.
>
> Furthermore, I'm not sure that the compiler gives `syscall` special treatment
> either. At least a cursory grep through `cmd/compile` for "syscall" seems to
> give few hits. The most relevant seems
> https://github.com/golang/go/blob/release-branch.go1.16/src/cmd/compile/internal/gc/esc.go#L386
> which notably isn't restricted to `syscall` - it applies to any function,
> AIUI. This could basically mean that while the rule *states* "passing to
> syscall.Syscall", it actually exempts any call, because the special casing
> has not been done yet. But a future version *might*, so the rule still is
> more specific.
Although rule 4 in the documentation of the unsafe package mentions
syscall.Syscall, the implementation isn't restricted to the syscall
package. It applies to any function that is written in assembly: that
is, a function that is declared in Go without a body.
We should perhaps tweak the docs to make that the rule.
Or, we could observe that golang.org/x/sys.Syscall actually just
forwards to syscall.Syscall, so they are in effect the same function.
>> Reason I ask is because the code in x/sys clearly violates that rule.
>>
>> in unix/ioctl.go there's
>>
>> ```
>> // IoctlSetPointerInt performs an ioctl operation which sets an
>> // integer value on fd, using the specified request number. The ioctl
>> // argument is called with a pointer to the integer value, rather than
>> // passing the integer value directly.
>> func IoctlSetPointerInt(fd int, req uint, value int) error {
>> v := int32(value)
>> return ioctl(fd, req, uintptr(unsafe.Pointer(&v)))
>> }
>> ```
>>
>> and the declaration of ioctl in zsyskall_linux.go:
>>
>> ```
>> func ioctl(fd int, req uint, arg uintptr) (err error) {
>> _, _, e1 := Syscall(SYS_IOCTL, uintptr(fd), uintptr(req),
>> uintptr(arg))
>> if e1 != 0 {
>> err = errnoErr(e1)
>> }
>> return
>> }
>> ```
Now that you point this out, I'm actually not sure that this is OK. I
don't know what keeps v pinned during this call.
>> Now, for starters ioctl includes a pointless conversion of a uintptr to a
>> uintptr,
>> for the arg parameter can anyone tell me why?
The functions in z*.go in x/sys are generated (by mksyscall.go). It's
easier to always write a conversion to uintptr then to try to decide
whether it is required or not.
>> Second (and this is my actual question), isn't that in violation of the
>> unsafe
>> constraints cited above?
>>
>> IoctlSetPointerInt clearly converts a unsafe.Pointer to a uintptr and
>> *doesn't*
>> directly call syscall.Syscall.
>>
>> Why is this valid?
I'm not sure it is.
Ian
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