From: Markos Chandras > On 06/23/2014 10:44 AM, David Laight wrote: > > From: Markos Chandras > >> Remove BUG_ON() if the shift immediate is >=32 to avoid > >> kernel crashes due to malicious user input. Since the micro-assembler > >> will not allow an immediate greater or equal to 32, we will use the > >> maximum value which is 31. This will do the correct thing on either 32- > >> or 64-bit cores since no 64-bit instructions are being used in JIT. > > > > I'm not sure that bounding the shift to 31 bits 'is the correct thing'. > > I'd have thought that emulating the large shift or masking the shift > > to 5 bits are equally 'correct'. > > > > ... > Hi David, > > Since we use 32-bit registers (or rather, we ignore the top 32bits on > MIPS64), shifting >= 32 will always result to 0. > Alexei suggested [1] to allow large shifts and emulate them, so this > patch aims to do that by treating >=32 shift values as 31. Please tell > me if I got this wrong. Shifting by 31 converts 0xffffffff to 1, not 0. David
