Re: [PATCH] arm64/sve: fix genksyms generation

[Arnd Bergmann]

On Mon, Jun 17, 2019 at 1:26 PM Will Deacon <will.deacon@xxxxxxx> wrote:
>
> Hi Arnd,
>
> On Mon, Jun 17, 2019 at 12:42:11PM +0200, Arnd Bergmann wrote:
> > genksyms does not understand __uint128_t, so we get a build failure
> > in the fpsimd module when it cannot export a symbol right:
>
> The fpsimd code is builtin, so which module is actually failing? My
> allmodconfig build succeeds, so I must be missing something.

It happened for me on randconfig builds, you can find one such configuration
at https://pastebin.com/cU8iQ4ta now. I was building this with clang
rather than gcc, which may affect the issue, but I assumed not.

> > WARNING: EXPORT symbol "kernel_neon_begin" [vmlinux] version generation failed, symbol will not be versioned.
> > /home/arnd/cross/x86_64/gcc-8.1.0-nolibc/aarch64-linux/bin/aarch64-linux-ld: arch/arm64/kernel/fpsimd.o: relocation R_AARCH64_ABS32 against `__crc_kernel_neon_begin' can not be used when making a shared object
> > arch/arm64/kernel/fpsimd.o:(.data+0x0): dangerous relocation: unsupported relocation
> > arch/arm64/kernel/fpsimd.o:(".discard.addressable"+0x0): dangerous relocation: unsupported relocation
> > arch/arm64/kernel/fpsimd.o:(".discard.addressable"+0x8): dangerous relocation: unsupported relocation
> >
> > We could teach genksyms about the type, but it's easier to just
> > work around it by defining that type locally in a way that genksyms
> > understands.
> >
> > Fixes: 41040cf7c5f0 ("arm64/sve: Fix missing SVE/FPSIMD endianness conversions")
>
> I can't see which part of that patch causes the problem, so I'm a bit wary
> of the fix. We've been using __uint128_t for a while now, and I see there's
> one in the x86 kvm code as well, so it would be nice to understand what's
> happening here so that we can avoid running into it in future as well.

The problem is only in files that export a symbol. This is also the
case in arch/x86/kernel/kvm.c, but it may be lucky because the
type only appears /after/ the last export in that file.

> > Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> > ---
> >  arch/arm64/kernel/fpsimd.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> > index 07f238ef47ae..2aba07cccf50 100644
> > --- a/arch/arm64/kernel/fpsimd.c
> > +++ b/arch/arm64/kernel/fpsimd.c
> > @@ -400,6 +400,9 @@ static int __init sve_sysctl_init(void) { return 0; }
> >  #define ZREG(sve_state, vq, n) ((char *)(sve_state) +                \
> >       (SVE_SIG_ZREG_OFFSET(vq, n) - SVE_SIG_REGS_OFFSET))
> >
> > +#ifdef __GENKSYMS__
> > +typedef __u64 __uint128_t[2];
> > +#endif
>
> I suspect I need to figure out what genksyms is doing, but I'm nervous
> about exposing this as an array type without understanding whether or
> not that has consequences for its operation.

The entire point is genksyms is to ensure that types of exported symbols
are compatible. To do this, it has a limited parser for C source code that
understands the basic types (char, int, long, _Bool, etc) and how to
aggregate them into structs and function arguments. This process has
always been fragile, and it clearly breaks when it fails to understand a
particular type.

          Arnd