Re: [PATCH] x86/asm: Don't use rbp as temp register in csum_partial_copy_generic()

[Josh Poimboeuf]

On Thu, May 04, 2017 at 03:56:49PM +0000, David Laight wrote:
> From: Josh Poimboeuf
> > Sent: 04 May 2017 15:52
> > Andrey Konovalov reported the following warning while fuzzing the kernel
> > with syzkaller:
> > 
> >   WARNING: kernel stack regs at ffff8800686869f8 in a.out:4933 has bad 'bp' value c3fc855a10167ec0
> > 
> > The unwinder dump revealed that rbp had a bad value when an interrupt
> > occurred in csum_partial_copy_generic().
> > 
> > That function saves rbp on the stack and then overwrites it, using it as
> > a scratch register.  That's problematic because it breaks stack traces
> > if an interrupt occurs in the middle of the function.
> 
> Does gcc guarantee not to use bp as a scratch register in leaf functions?

At least in practice, gcc doesn't touch rbp in leaf functions.  (I don't
know about guarantees.)

-- 
Josh