Re: [PATCH] HID: simplify code in fetch_item()

[Nathan Chancellor]

Hi Dmitry,

On Tue, Oct 01, 2024 at 08:42:36AM -0700, Dmitry Torokhov wrote:
> We can easily calculate the size of the item using arithmetic (shifts).
> This allows to pull duplicated code out of the switch statement, making
> it cleaner.
> 
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
> ---
>  drivers/hid/hid-core.c | 31 ++++++++++++++-----------------
>  1 file changed, 14 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
> index 988d0acbdf04..00942d40fe08 100644
> --- a/drivers/hid/hid-core.c
> +++ b/drivers/hid/hid-core.c
> @@ -754,35 +754,32 @@ static u8 *fetch_item(__u8 *start, __u8 *end, struct hid_item *item)
>  	}
>  
>  	item->format = HID_ITEM_FORMAT_SHORT;
> -	item->size = b & 3;
> +	item->size = BIT(b & 3) >> 1; /* 0, 1, 2, 3 -> 0, 1, 2, 4 */
> +
> +	if (end - start < item->size)
> +		return NULL;
>  
>  	switch (item->size) {
>  	case 0:
> -		return start;
> +		break;
>  
>  	case 1:
> -		if ((end - start) < 1)
> -			return NULL;
> -		item->data.u8 = *start++;
> -		return start;
> +		item->data.u8 = *start;
> +		break;
>  
>  	case 2:
> -		if ((end - start) < 2)
> -			return NULL;
>  		item->data.u16 = get_unaligned_le16(start);
> -		start = (__u8 *)((__le16 *)start + 1);
> -		return start;
> +		break;
>  
> -	case 3:
> -		item->size++;
> -		if ((end - start) < 4)
> -			return NULL;
> +	case 4:
>  		item->data.u32 = get_unaligned_le32(start);
> -		start = (__u8 *)((__le32 *)start + 1);
> -		return start;
> +		break;
> +
> +	default:
> +		unreachable();
>  	}
>  
> -	return NULL;
> +	return start + item->size;
>  }

I am noticing some interesting behavior when building with clang, namely
some objtool warnings and a failed boot when LTO is enabled, which I
bisected to this change as commit 61595012f280 ("HID: simplify code in
fetch_item()"), such as:

  $ make -skj"$(nproc)" ARCH=x86_64 LLVM=1 mrproper defconfig vmlinux
  vmlinux.o: warning: objtool: hid_open_report() falls through to next function hid_parser_main()
  vmlinux.o: warning: objtool: hid_scan_report() falls through to next function hid_allocate_device()

With LTO enabled, the warning becomes:

  vmlinux.o: warning: objtool: hid_open_report+0x21b: can't find jump dest instruction at .text.hid_open_report+0x40f

A bare unreachable(), especially in the default case of a switch
statement, is generally considered harmful in my experience, as it can
introduce undefined behavior, which can mess up how a compiler might
optimize a function. Commit d652d5f1eeeb ("drm/edid: fix objtool warning
in drm_cvt_modes()") and commit 3764647b255a ("bcachefs: Remove
undefined behavior in bch2_dev_buckets_reserved()") have some good
commit messages talking about it.

Getting rid of the unreachable() in some way resolves the issue. I
tested using BUG() in lieu of unreachable() like the second change I
mentioned above, which resolves the issue cleanly, as the default case
clearly cannot happen. Another option I tested was some sort of printk
statement and returning NULL, which some maintainers prefer, even in
spite of impossible conditions. I am happy to send a patch with one of
those changes or open to other suggestions.

Cheers,
Nathan