all: use reflect.StringHeader and reflect.SliceHeader for pointer conversions

[smasher164]

As @mdempsky states in #19367 (comment),

It's not guaranteed that you can convert a pointer to a slice to a pointer to any other struct type except reflect.SliceHeader.

Therefore, the following conversions in the golang subrepos (except runtime and reflect) should be modified to use reflect.StringHeader and reflect.SliceHeader when converting between an unsafe.Pointer and a string/slice.

• syscall/syscall_unix.go
• x/sys/unix/syscall_unix.go
• x/sys/unix/syscall_darwin.1_13.go
• cmd/internal/goobj2/objfile.go
• cmd/oldlink/internal/objfile/objfile.go

[mdempsky]

Thanks. Confirmed, those are all misuses of unsafe.Pointer.

[twmb]

Don't most of these fall under unsafe rule (1), and the uintptr ones fall under managing syscall stuff directly?

[mdempsky]

Yes, the last three examples are safe today (on gc and gccgo, at least) under rule 1, but might not be safe in the future. As reflect.SliceHeader's docs warn: "its representation may change in a later release." Using reflect.SliceHeader is the future proof solution.

The first two don't involve direct uses of syscall.Syscall, so rule 4 is inapplicable.

But that does make me note that the first example is within package syscall, which isn't allowed to depend on package reflect. :/

[twmb]

I thought I remembered a proposal about adding equivalent unsafe.StringHeader and unsafe.SliceHeader types, but I can't find it. But, that idea may be worthwhile to address the syscall issue, as well as to allow users of just StringHeader/SliceHeader to avoid importing the reflect package.

[smasher164]

Can't syscall access runtime.slice instead of reflect.SliceHeader? It already linknames in runtime functions.

Edit: @twmb You are referring to #19367 which forked off into this issue to fix pre-existing conversions.

[mdempsky]

@smasher164 linkname only works for functions and variables; not types.

[smasher164]

@smasher164 linkname only works for functions and variables; not types.

Right, so we would need a runtime function similar to func newslice() unsafe.Pointer that syscall would call to turn it into *[]byte. Although, that might be too heavyweight of an addition to support one package.

[bcmills]

Seems like syscall could use the usual “convert to *[reallyBig]byte and then slice down to size” pattern, but for portability we'll presumably need build constraints for the definition of reallyBig (see #13656 (comment)).