Re: [PATCH v4 28/39] unwind_user/deferred: Add deferred unwinding interface

[Peter Zijlstra]

On Wed, Jan 22, 2025 at 02:51:27PM -0800, Josh Poimboeuf wrote:
> On Wed, Jan 22, 2025 at 03:16:16PM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 22, 2025 at 02:37:30PM +0100, Peter Zijlstra wrote:
> > > On Tue, Jan 21, 2025 at 06:31:20PM -0800, Josh Poimboeuf wrote:
> > > > +/*
> > > > + * The context cookie is a unique identifier which allows post-processing to
> > > > + * correlate kernel trace(s) with user unwinds.  The high 12 bits are the CPU
> > > 
> > > s/12/16/ ?
> > > 
> > > > + * id; the lower 48 bits are a per-CPU entry counter.
> > > > + */
> > > > +static u64 ctx_to_cookie(u64 cpu, u64 ctx)
> > > > +{
> > > > +	BUILD_BUG_ON(NR_CPUS > 65535);
> > > > +	return (ctx & ((1UL << 48) - 1)) | (cpu << 48);
> > > > +}
> > 
> > Also, I have to note that 0 is a valid return value here, which will
> > give a ton of fun.
> 
> The ctx_ctr is always incremented before calling this, so 0 isn't a
> valid cookie.

Right, so that's the problem. You're considering 0 an invalid cookie,
but ctx_to_cookie(0, 1<<48) will be a 0 cookie.

That thing *will* wrap.