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

[Paul E. McKenney]

On Tue, Oct 15, 2024 at 11:28:26AM -0700, Dmitry Torokhov wrote:
> Hi Nathan,
> 
> On Thu, Oct 10, 2024 at 03:24:51PM -0700, Nathan Chancellor wrote:
> > 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.
> 
> Oh well, if our toolchain does not like "unreachable()" then we can
> simply remove it - the switch does cover all possible values and the
> "return" statement should be valid even if compiler somehow decides that
> "switch" statement can be skipped.
> 
> If you can send a patch that would be great.
> 
> I'm adding Paul and a few others to CC who apparently seeing the same
> issue.

Commenting out the unreachable() fixes things for me, as does
replacing the unreachable() with BUG().  So, for either solution:

Tested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

							Thanx, Paul