[PATCH] rust: fix bindgen build error with fstrict-flex-arrays

[PATCH] rust: fix bindgen build error with fstrict-flex-arrays

[Andrea Righi]

Commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") enabled
'-fstrict-flex-arrays=3' globally, but bindgen does not recognized this
compiler option, triggering the following build error:

 error: unknown argument: '-fstrict-flex-arrays=3', err: true

Add '-fstrict-flex-arrays' to the list of cflags that should be ignored
by bindgen.

Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
---
 rust/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/rust/Makefile b/rust/Makefile
index 4124bfa01798..ae2f5421da25 100644
--- a/rust/Makefile
+++ b/rust/Makefile
@@ -258,6 +258,7 @@ bindgen_skip_c_flags := -mno-fp-ret-in-387 -mpreferred-stack-boundary=% \
 	-fno-reorder-blocks -fno-allow-store-data-races -fasan-shadow-offset=% \
 	-fzero-call-used-regs=% -fno-stack-clash-protection \
 	-fno-inline-functions-called-once -fsanitize=bounds-strict \
+	-fstrict-flex-arrays=% \
 	--param=% --param asan-%
 
 # Derived from `scripts/Makefile.clang`.
-- 
2.40.1

Re: [PATCH] rust: fix bindgen build error with fstrict-flex-arrays

[Miguel Ojeda]

On Tue, Aug 15, 2023 at 8:54 AM Andrea Righi <andrea.righi@canonical.com> wrote:
>
> Commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") enabled
> '-fstrict-flex-arrays=3' globally, but bindgen does not recognized this

It may be more accurate to say libclang here (bindgen forwards the options).

Also, df8fc4e934c1 did it so only conditionally (if the C compiler
supports it). This explains what you are seeing: if I am right, you
are compiling with a modern enough GCC, which enables the option, but
with an old enough Clang.

> compiler option, triggering the following build error:
>
>  error: unknown argument: '-fstrict-flex-arrays=3', err: true

This should only be true with libclang < 16, since Clang 16
implemented the option, right?

In fact, Clang 15 seems to work too -- it seems the compiler does not
error if the option is not within [0,3] (unlike GCC).

Kees: this should only affect `__builtin_object_size` and not `sizeof`, right?

From a quick test across Clang 14/15/16 and different levels for the
flag (0/1/2/3/4 and no flag), bindgen seems to generate the same
output in all cases: `__IncompleteArrayField` is always generated for
`[]` and `[0]`; and never for `[1]`, i.e. regardless of the flag. The
`sizeof`s agree with the C side, and they are all the same (as
expected because this only changes BOS).

So I think the patch contents (i.e. ignoring the flag for bindgen)
should be fine. Except we could have somewhere a
`__builtin_object_size` influencing a layout. Since GCC seems to
always treat it as not a constexpr (unlike Clang), I assume nobody is
using it like that (since we compile the kernel with and without the
flag). But it is still possible in theory -- it would require
something like:

    struct X {
        int x[__is_constexpr(__builtin_object_size(s->c, 1)) ? 1 : 2];
    };

to make GCC and Clang (as well as Clang `=3` vs. Clang without the
flag) to compile but disagree on the size:
https://godbolt.org/z/6TqPjGK3d

Cheers,
Miguel

Re: [PATCH] rust: fix bindgen build error with fstrict-flex-arrays

[Miguel Ojeda]

On Tue, Aug 15, 2023 at 2:06 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> to make GCC and Clang (as well as Clang `=3` vs. Clang without the
> flag) to compile but disagree on the size:

To be clear: Clang `=3` vs. Clang without the flag should not be a
problem for us because under `CC_IS_CLANG` we pass the flags unaltered
to bindgen.

But for GCC-built kernels with Rust enabled, the disagreement above
could be a problem.

Cheers,
Miguel

Re: [PATCH] rust: fix bindgen build error with fstrict-flex-arrays

[Andrea Righi]

On Tue, Aug 15, 2023 at 02:06:36PM +0200, Miguel Ojeda wrote:
> On Tue, Aug 15, 2023 at 8:54 AM Andrea Righi <andrea.righi@canonical.com> wrote:
> >
> > Commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") enabled
> > '-fstrict-flex-arrays=3' globally, but bindgen does not recognized this
> 
> It may be more accurate to say libclang here (bindgen forwards the options).
> 
> Also, df8fc4e934c1 did it so only conditionally (if the C compiler
> supports it). This explains what you are seeing: if I am right, you
> are compiling with a modern enough GCC, which enables the option, but
> with an old enough Clang.

Oh yes, indeed! I'm using clang/libclang-14 with gcc-13, if I switch to
clang-15 everything seems fine, so I can simply move to this version.

> 
> > compiler option, triggering the following build error:
> >
> >  error: unknown argument: '-fstrict-flex-arrays=3', err: true
> 
> This should only be true with libclang < 16, since Clang 16
> implemented the option, right?
> 
> In fact, Clang 15 seems to work too -- it seems the compiler does not
> error if the option is not within [0,3] (unlike GCC).

The combo gcc-13 + libclang-15 seems to work in my case, instead with
libclang-16 I get this:

  BINDGEN rust/bindings/bindings_generated.rs
  thread 'main' panicked at '"ftrace_branch_data_union_(anonymous_at__/_/include/linux/compiler_types_h_146_2)" is not a valid Ident', /build/rust-bindgen-0.56-DgAMvF/rust-bindgen-0.56-0.56.0/debian/vendor/proc-macro2-1.0.24/src/fallback.rs:693:9

Thanks,
-Andrea

Re: [PATCH] rust: fix bindgen build error with fstrict-flex-arrays

[Miguel Ojeda]

On Tue, Aug 15, 2023 at 2:44 PM Andrea Righi <andrea.righi@canonical.com> wrote:
>
> Oh yes, indeed! I'm using clang/libclang-14 with gcc-13, if I switch to
> clang-15 everything seems fine, so I can simply move to this version.

Thanks for confirming!

> The combo gcc-13 + libclang-15 seems to work in my case, instead with
> libclang-16 I get this:
>
>   BINDGEN rust/bindings/bindings_generated.rs
>   thread 'main' panicked at '"ftrace_branch_data_union_(anonymous_at__/_/include/linux/compiler_types_h_146_2)" is not a valid Ident', /build/rust-bindgen-0.56-DgAMvF/rust-bindgen-0.56-0.56.0/debian/vendor/proc-macro2-1.0.24/src/fallback.rs:693:9

Yeah, this is fixed in `bindgen` 0.62.0. We have upgraded in
`rust-next`, commit 08ab786556ff ("rust: bindgen: upgrade to 0.65.1").

Cheers,
Miguel

Re: [PATCH] rust: fix bindgen build error with fstrict-flex-arrays

[Kees Cook]

On Tue, Aug 15, 2023 at 02:06:36PM +0200, Miguel Ojeda wrote:
> On Tue, Aug 15, 2023 at 8:54 AM Andrea Righi <andrea.righi@canonical.com> wrote:
> >
> > Commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") enabled
> > '-fstrict-flex-arrays=3' globally, but bindgen does not recognized this
> 
> It may be more accurate to say libclang here (bindgen forwards the options).
> 
> Also, df8fc4e934c1 did it so only conditionally (if the C compiler
> supports it). This explains what you are seeing: if I am right, you
> are compiling with a modern enough GCC, which enables the option, but
> with an old enough Clang.
> 
> > compiler option, triggering the following build error:
> >
> >  error: unknown argument: '-fstrict-flex-arrays=3', err: true
> 
> This should only be true with libclang < 16, since Clang 16
> implemented the option, right?
> 
> In fact, Clang 15 seems to work too -- it seems the compiler does not
> error if the option is not within [0,3] (unlike GCC).
> 
> Kees: this should only affect `__builtin_object_size` and not `sizeof`, right?

That's correct. It would affect __builtin_object_size,
__builtin_dynamic_object_size, and -fsantize=bounds instrumentation.

-- 
Kees Cook

Re: [PATCH] rust: fix bindgen build error with fstrict-flex-arrays

[Gary Guo]

On Tue, 15 Aug 2023 08:53:46 +0200
Andrea Righi <andrea.righi@canonical.com> wrote:

> Commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") enabled
> '-fstrict-flex-arrays=3' globally, but bindgen does not recognized this
> compiler option, triggering the following build error:
> 
>  error: unknown argument: '-fstrict-flex-arrays=3', err: true
> 
> Add '-fstrict-flex-arrays' to the list of cflags that should be ignored
> by bindgen.
> 
> Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
> Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

Tested-by: Gary Guo <gary@garyguo.net>

> ---
>  rust/Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/rust/Makefile b/rust/Makefile
> index 4124bfa01798..ae2f5421da25 100644
> --- a/rust/Makefile
> +++ b/rust/Makefile
> @@ -258,6 +258,7 @@ bindgen_skip_c_flags := -mno-fp-ret-in-387 -mpreferred-stack-boundary=% \
>  	-fno-reorder-blocks -fno-allow-store-data-races -fasan-shadow-offset=% \
>  	-fzero-call-used-regs=% -fno-stack-clash-protection \
>  	-fno-inline-functions-called-once -fsanitize=bounds-strict \
> +	-fstrict-flex-arrays=% \
>  	--param=% --param asan-%
>  
>  # Derived from `scripts/Makefile.clang`.

Re: [PATCH] rust: fix bindgen build error with fstrict-flex-arrays

[Miguel Ojeda]

On Tue, Aug 15, 2023 at 8:54 AM Andrea Righi <andrea.righi@canonical.com> wrote:
>
> Commit df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3") enabled
> '-fstrict-flex-arrays=3' globally, but bindgen does not recognized this
> compiler option, triggering the following build error:
>
>  error: unknown argument: '-fstrict-flex-arrays=3', err: true
>
> Add '-fstrict-flex-arrays' to the list of cflags that should be ignored
> by bindgen.
>
> Fixes: df8fc4e934c1 ("kbuild: Enable -fstrict-flex-arrays=3")
> Signed-off-by: Andrea Righi <andrea.righi@canonical.com>

Applied to `rust-fixes` (with an extra summary of the discussion in
the list), thanks everyone!

Cheers,
Miguel