Re: eBPF CO-RE cross-compilation for 32-bit ARM platforms

On Fri, Aug 07, 2020 at 01:54:02PM -0700, Alexei Starovoitov wrote:
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typedef __Poly8_t poly8x16_t[16];

AFAICT these are ARM NEON intrinsic definitions which are GCC-specific. We
have opted to comment out this line as there was no additional `poly8x16_t`
usage in the header file.

It looks like this "__Poly8_t" type is internal to GCC (provided in
arm_neon.h) and clang has its own internals. I managed to reproduce this
with an arm64 allyesconfig kernel (+BTF), but don't know how to fix it at
the moment. Maybe libbpf should generate defines to translate these
intrinsics between clang and gcc? Not very elegant. I'll take another
look next week.

libbpf is already blacklisting __builtin_va_list for GCC, so we can
just add __Poly8_t to the list. See [0].
Are there any other types like that? If you guys can provide me this,
I'll gladly update libbpf to take those compiler-provided
types/built-ins into account.

Shouldn't __Int8x16_t and friends cause the same trouble?

I think these do get properly defined, for example in my vmlinux.h:

	typedef signed char int8x16_t[16];

From a cursory reading of the "ARM C Language Extension" doc (IHI0053D) it
looks like only the poly8/16/64/128_t types are unspecified. It's safe to
drop them as long as they're not used in structs or function parameters,
but I sent a more generic fix [1] that copies the clang defintions. When
building the kernel with clang, the polyX_t types do get typedefs.

Thanks,
Jean

[1] https://lore.kernel.org/bpf/20200810122835.2309026-1-jean-philippe@linaro.org/ (local)

There is a bunch more in gcc/config/arm/arm-simd-builtin-types.def.
May be there is a way to detect compiler builtin types by pattern matching
their dwarf/btf shape and skip them automatically?
The simplest, of course, is to only add a few that caused this known
trouble to blocklist.

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