Re: [PATCH v2] HID: hyperv: make pointer arithmetics understandable for FORTIFY_SOURCE

[Benjamin Tissoires]

On Aug 21 2026, Jiri Kosina wrote:
> On Fri, 21 Aug 2026, Benjamin Tissoires wrote:
> 
> > > -	report = ((u8 *)&info->hid_descriptor) + info->hid_descriptor.bLength;
> > > +	report = (u8 *)info + offsetof(struct synthhid_device_info, hid_descriptor) + info->hid_descriptor.bLength;
> > 
> > Isn't that overcomplicated?:
> > 
> > Above we have:
> > struct synthhid_device_info {
> > 	struct synthhid_msg_hdr header;
> > 	struct hv_input_dev_info hid_dev_info;
> > 	struct hid_descriptor hid_descriptor;
> > };
> >   ...
> > 	struct synthhid_device_info *info;
> > 	...
> > 	info = kunit_kzalloc(test, sizeof(*info) + 4, GFP_KERNEL);
> > 
> > so info is 0 allocated with the sizeof(struct synthhid_device_info) plus
> > 4 for the report (immediately after).
> > 
> > with the bLength being set a couple of lines above, we basically have:
> > + report = (u8 *)info + offsetof(struct synthhid_device_info, hid_descriptor) + sizeof(struct hid_descriptor);
> > 
> > So pointer address + offset of the last field + size of the last field.
> > 
> > Isn't that equivalent to:
> >   report = (u8 *)info + sizeof(*info);
> > 
> > or even: `(u8 *)(info + 1)`?
> 
> I considered that one as well, but then I decided to keep what the 
> original author intended as perhaps it's more self-explanatory what we're 
> actually doing. But the argument that whenever a new field is added after 
> hid_descriptor, this will silently break, is convicing enough :)

Yep, that's a killer argument :)

Acked-by: Benjamin Tissoires <bentiss@xxxxxxxxxx>

Cheers,
Benjamin

> 
> Thanks. v2 below.
> 
> Juergen, could you please re-Ack your Tested-by: on this? For some reason, 
> my allyesconfig with gcc-15 doesn't trigger it.
> 
> 
> 
> 
> From: Jiri Kosina <jkosina@xxxxxxxx>
> Subject: [PATCH] HID: hyperv: make pointer arithmetics understandable for FORTIFY_SOURCE
> 
> Commit 83df7b5fa6735b5084ecd2 ("HID: hyperv: add KUnit coverage for device info
> bounds") introduced this piece of code
> 
> 	report = ((u8 *)&info->hid_descriptor) + info->hid_descriptor.bLength;
> 	memset(report, 0x42, 4);
> 
> to populate the report descriptor, making use of the fact that the report
> &info->hid_descriptor points to a struct hid_descriptor (which is a fixed-size struct).
> GCC's FORTIFY_SOURCE infer the object size from that specific struct field rather than the
> outer dynamically allocated info buffer. As a result, writing past sizeof(struct hid_descriptor)
> triggers the __write_overflow_field warning.
> 
> Calculate the pointer offset using info directly, so the compiler evaluates the
> memory bounds against the allocated flexible layout of struct
> synthhid_device_info instead of the nested struct.
> 
> Fixes: 83df7b5fa6735b5084ecd2 ("HID: hyperv: add KUnit coverage for device info bounds")
> Reported-by: Jürgen Groß <jgross@xxxxxxxx>
> Signed-off-by: Jiri Kosina <jkosina@xxxxxxxx>
> ---
>  drivers/hid/hid-hyperv.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hid/hid-hyperv.c b/drivers/hid/hid-hyperv.c
> index 6579bd19da13..cfc061dbdd24 100644
> --- a/drivers/hid/hid-hyperv.c
> +++ b/drivers/hid/hid-hyperv.c
> @@ -687,7 +687,7 @@ static void mousevsc_device_info_valid_descriptor(struct kunit *test)
>  
>  	info->hid_descriptor.bLength = sizeof(struct hid_descriptor);
>  	info->hid_descriptor.rpt_desc.wDescriptorLength = cpu_to_le16(4);
> -	report = ((u8 *)&info->hid_descriptor) + info->hid_descriptor.bLength;
> +	report = (u8 *)(info + 1);
>  	memset(report, 0x42, 4);
>  
>  	mousevsc_on_receive_device_info(input_dev, info, sizeof(*info) + 4);
> @@ -713,7 +713,7 @@ static void mousevsc_device_info_report_desc_oob(struct kunit *test)
>  
>  	info->hid_descriptor.bLength = sizeof(struct hid_descriptor);
>  	info->hid_descriptor.rpt_desc.wDescriptorLength = cpu_to_le16(64);
> -	report = ((u8 *)&info->hid_descriptor) + info->hid_descriptor.bLength;
> +	report = (u8 *)(info + 1);
>  	memset(report, 0x42, 8);
>  
>  	mousevsc_on_receive_device_info(input_dev, info, sizeof(*info) + 8);
> 
> -- 
> Jiri Kosina
> SUSE Labs
> 
>