Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH 0/1] Slightly relax the type checking done by min() and max(). | Date | Fri, 25 Nov 2022 16:14:58 +0000 |
| |
From: 'Andy Shevchenko' > Sent: 25 November 2022 15:58 > > On Fri, Nov 25, 2022 at 03:27:07PM +0000, David Laight wrote: > > From: Andy Shevchenko > > > Sent: 25 November 2022 15:21 > > > On Fri, Nov 25, 2022 at 03:00:40PM +0000, David Laight wrote: > > > > The min() and max() defines include a type check to avoid the unexpected > > > > behaviour when a negative value is compared against and unsigned value. > > > > However a lot of code hits this check and uses min_t() to avoid the error. > > > > Many of these are just plain wrong. > > > > > > > > Those casting to u8 or u16 are particularly suspect, eg: > > > > drivers/usb/misc/usb251xb.c:528: > > > > hub->max_current_sp = min_t(u8, property_u32 / 2000, 50); > > > > > > I don't buy this. What's exactly wrong with this code? > > > > Consider what happens if propery_u32 is 512000. > > The returned value is 0 not 50. > > I considered that and there are two things to consider on your side: > 1) it's coming from device property; > 2) device property is validated using YAML schema. > > On top of that, the wrong property is on the user. We have a lot of stuff that > user may put wrongly, but it's user's choice. > > Any better example, please?
How about:
data_size = min_t(u16, buf_size, len);
https://elixir.bootlin.com/linux/v6.1-rc6/source/kernel/printk/printk_ringbuffer.c#L1738
Now, maybe, you could claim that buf_size > 64k never happens. But the correct cast here is u32 to match buf_size. len (being u16) will be promoted to int before the compare.
Just search the kernel for "min_t(u8," or "min_t(u16," while some might be ok, I really wouldn't want to verify each case.
If you look hard enough there are also some: u32_var = min_t(u32, u32_val, u64_val); where the intent is to limit values that might be invalid for u32.
I did try compiling a kernel with min_t() defined to be min() (no casts) but there were too many false positives without allowing all unsigned v unsigned compares.
David
- Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
| |