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

[Juergen Gross <jgross@suse.com>]

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On 21.08.26 15:39, 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 :)
> 
> 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@suse.com>
> 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@suse.com>
> Signed-off-by: Jiri Kosina <jkosina@suse.com>

Tested-by: Jürgen Groß <jgross@suse.com>


Jürgen

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