[PATCH 1/3] arm64: ptrace: Fix VFP register dumping in compat coredumps

[Dave Martin]

On Wed, Jun 21, 2017 at 04:23:20PM +0100, Mark Rutland wrote:
> On Wed, Jun 21, 2017 at 04:00:42PM +0100, Dave Martin wrote:
> > Currently, VFP registers are omitted from coredumps for compat
> > processes, due to a bug in the REGSET_COMPAT_VFP regset
> > implementation.
> > 
> > compat_vfp_get() needs to transfer non-contiguous data from
> > thread_struct.fpsimd_state, and uses put_user() to handle the
> > offending trailing word (FPSCR).  This fails when copying to a
> > kernel address (i.e., kbuf && !ubuf), which is what happens when
> > dumping core.  As a result, the ELF coredump core code silently
> > omits the NT_ARM_VFP note from the dump.
> > 
> > It would be possible to work around this with additional special
> > case code for the put_user(), but since user_regset_copyout() is
> > explcltly designed to handle this scenario it is cleaner to port
> 
> Nit: explicitly

Fixed for respin (if there is one).

> > the put_user() to a user_regset_copyout() call, which this patch
> > does.
> 
> Given, 32-bit arm also uses user_regset_copyout(), it seems like the
> all-round right thing to do.

Agreed.  There may be cases where user_regset_copyout() doesn't cut it,
but it seems OK here.

> > 
> > Signed-off-by: Dave Martin <Dave.Martin at arm.com>
> > ---
> >  arch/arm64/kernel/ptrace.c | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
> > index c142459..0e5aaec 100644
> > --- a/arch/arm64/kernel/ptrace.c
> > +++ b/arch/arm64/kernel/ptrace.c
> > @@ -894,7 +894,7 @@ static int compat_vfp_get(struct task_struct *target,
> >  {
> >  	struct user_fpsimd_state *uregs;
> >  	compat_ulong_t fpscr;
> > -	int ret;
> > +	int ret, vregs_end_pos;
> >  
> >  	uregs = &target->thread.fpsimd_state.user_fpsimd;
> >  
> > @@ -902,13 +902,16 @@ static int compat_vfp_get(struct task_struct *target,
> >  	 * The VFP registers are packed into the fpsimd_state, so they all sit
> >  	 * nicely together for us. We just need to create the fpscr separately.
> >  	 */
> > -	ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, uregs, 0,
> > -				  VFP_STATE_SIZE - sizeof(compat_ulong_t));
> > +	vregs_end_pos = VFP_STATE_SIZE - sizeof(compat_ulong_t);
> > +	ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, uregs,
> > +				  0, vregs_end_pos);
> >  
> >  	if (count && !ret) {
> >  		fpscr = (uregs->fpsr & VFP_FPSCR_STAT_MASK) |
> >  			(uregs->fpcr & VFP_FPSCR_CTRL_MASK);
> > -		ret = put_user(fpscr, (compat_ulong_t *)ubuf);
> > +
> > +		ret = user_regset_copyout(&pos, &count, &kbuf, &ubuf, &fpscr,
> > +					  vregs_end_pos, VFP_STATE_SIZE);
> >  	}
> 
> It's a shame compat_user_vfp is defined in signal32.c, otherwise we
> could've used offsetof(struct compat-user_vfp, fpscr) here (and also for
> the fpregs), mirroring the structure of 32-bit's vfp_get().

Could be nicer -- I was trying to make the minimum change here.

> Otherwise, this looks sane to me.

Thanks for looking it over.

Cheers
---Dave