E0587 error on packed and aligned structures from C

[ahomescu]

I ran into the following issue when converting some kernel headers with bindgen 0.48.1:

When converting C structures that are both packed and aligned using either C2Rust or bindgen, such as:

struct xregs_state {
	struct fxregs_state		i387;
	struct xstate_header		header;
	u8				extended_state_area[0];
} __attribute__ ((packed, aligned (64)));

the code that either tool produces looks something like:

#[repr(C, packed(64))]
#[repr(align(64))]
pub struct xregs_state {
    pub i387: fxregs_state,
    pub header: xstate_header,
    pub extended_state_area: __IncompleteArrayField<u8>,
}

This Rust code fails to compile due to error E0587:

error[E0587]: type has conflicting packed and align representation hints
    --> .../out/bindings.rs:3894:1
     |
3894 | / pub struct xregs_state {
3895 | |     pub i387: fxregs_state,
3896 | |     pub header: xstate_header,
3897 | |     pub extended_state_area: __IncompleteArrayField<u8>,
3898 | | }
     | |_^

We can work around this in C2Rust by emitting an aligned outer/packed inner structure pair (I think bindgen could do the same), but I'm wondering if it would be better to fix this on the Rust language/compiler side (I also opened rust-lang/rust#59154 for a discussion on this).

[emilio]

I ran into the following issue when converting some kernel headers with bindgen 0.48.3:

I assume you mean 0.47.3?

We can work around this in C2Rust by emitting an aligned outer/packed inner structure pair (I think bindgen could do the same), but I'm wondering if it would be better to fix this on the Rust language/compiler side (I also opened rust-lang/rust#59154 for a discussion on this).

Can you put an example? I assume the workaround would look like:

#[repr(align(64))]
pub struct xregs_state(pub xreg_state_inner);

#[repr(packed)]
pub struct xreg_state_inner {
    // fields
}

Is that right? If so, that's feasible. But it only solves this very specific case, which is slightly unfortunate.

I haven't dug deep into how to properly generate rust code for aligned structs inside packed structs and such. That's something that rustc errors on, but C compilers happily allow it without issues.

Actually, for this test-case, it seems we're being naive, and we could just generate #[repr(packed(64)], it's the whole purpose of that. So that's an even better fix for this that I'm happy to write.

[emilio]

Err, wrong button of course.

[emilio]

Ah, I was wrong. #[repr(packed(64)] doesn't generate the expected output, since it doesn't affect the size of the struct.

[emilio]

There's a further complication, which is that there's no good way I know of distinguishing this case from #pragma packed(N) using libclang...

[emilio]

I submitted https://reviews.llvm.org/D59299 to be able to distinguish that case, eventually.

[alex]

Woo, thanks for jumping on this.

[ahomescu]

I assume you mean 0.47.3?

Sorry, I meant 0.48.1 (the latest version).

[ahomescu]

Can you put an example? I assume the workaround would look like:

#[repr(align(64))]
pub struct xregs_state(pub xreg_state_inner);

#[repr(packed)]
pub struct xreg_state_inner {
    // fields
}

Yes, that's basically what we had in mind, but I'm hoping that this can be fixed in rustc instead.

[ahomescu]

Worth mentioning: 0.47.3 generates code that compiles just fine because the definition for that structure is:

#[repr(C)]
pub struct xregs_state {
    pub _bindgen_opaque_blob: [u8; 576usize],
}

This is only a problem on newer bindgen because it now generates the full definition for the structure.

[emilio]

Well, that's not fine (as in, it doesn't have the right alignment), but as I said you can go back to that (and with the right alignment thanks to #1530) using the opaque_type / --opaque-type feature.