[Aboriginal] [musl] Re: musl and kernel headers [was Re: system-images 1.4.2: od is broken; bzip2 is missing]

[Szabolcs Nagy]

* Denys Vlasenko <vda.linux at googlemail.com> [2015-10-13 14:10:24 +0200]:
> On Fri, Oct 9, 2015 at 9:46 PM, Rich Felker <dalias at aerifal.cx> wrote:
> >> Looking at kernel's libc-compat.h, it looks like you can get away
> >> with using __UAPI_DEF_foo's like this?
> >>
> >>
> >> #if  defined(__UAPI_DEF_SOCKADDR_IN) && __UAPI_DEF_SOCKADDR_IN == 1
> >> /* kernel already defined the struct, do nothing */
> >> #else
> >> struct sockaddr_in {
> >>         ...
> >> };
> >
> > This would address the case where the kernel header is included first,
> > but it's not a case I or most of the musl community wants to support,
> > because there's no guarantee that the kernel's definitions of these
> > structures will actually be compatible with use elsewhere in the libc
> > headers, etc.
> 
> If kernel's definition does not match yours, there is a much
> bigger problem than "includes do not compile":
> kernel and userspace definitions of these structs *must* match
> (modulo harmless things like different typedef names for field types).
> 
> So in this case either kernel or libc would need to be fixed.
> 

why?

in practice most types are c abi compatible with the kernel
because translating the types at the syscall boundary is
painful/impossible.

but even with compatible binary representation there is
plenty space for disagreement between kernel and libc on
the source level. (of course code that includes both libc
and kernel headers might not care about posix namespace
violations or undefined behaviour in kernel headers..)

and libc-compat does not cover all conflicting cases
(i assume they just add workarouds when somebody hits
a conflict), e.g. sys/inotify.h and linux/inotify.h are
in conflict (and linux/inotify.h is not even standard c).