[PATCH] arm64/gcs: Fix error handling in arch_set_shadow_stack_status()

[Breno Leitao]

On Mon, Feb 02, 2026 at 02:44:43PM +0000, Will Deacon wrote:
> On Fri, Jan 30, 2026 at 01:43:09AM -0800, Breno Leitao wrote:
> > alloc_gcs() returns an error-encoded pointer on failure, which comes
> > from do_mmap(), not NULL.
> > 
> > The current NULL check fails to detect errors, which could lead to using
> > an invalid GCS address.
> > 
> > Use IS_ERR_VALUE() to properly detect errors, consistent with the
> > check in gcs_alloc_thread_stack().
> > 
> > Fixes: b57180c75c7eb ("arm64/gcs: Implement shadow stack prctl() interface")
> > Signed-off-by: Breno Leitao <leitao at debian.org>
> > ---
> > PS: This was compiled-tested only, given I unfortunately don't have
> > a hardware to test on _yet_.
> > ---
> >  arch/arm64/mm/gcs.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c
> > index 6e93f78de79b1..efce7642b1d7b 100644
> > --- a/arch/arm64/mm/gcs.c
> > +++ b/arch/arm64/mm/gcs.c
> > @@ -199,8 +199,8 @@ int arch_set_shadow_stack_status(struct task_struct *task, unsigned long arg)
> >  
> >  		size = gcs_size(0);
> >  		gcs = alloc_gcs(0, size);
> > -		if (!gcs)
> > -			return -ENOMEM;
> > +		if (IS_ERR_VALUE(gcs))
> > +			return PTR_ERR((void *)gcs);
> 
> Why do you need to go via PTR_ERR() here? 'gcs' is an 'unsigned long' so
> can't we just return that directly?

yea, PTR_ERR() is not helping here. We can definitely return gcs
directly.

I will update it,
--breno