Re: [PATCH RFC bpf-next 2/8] bpf: mark instructions accessing program stack

On Thu, Jun 4, 2026 at 10:14=E2=80=AFAM Alexis Lothor=C3=A9
<[email protected]> wrote:
>
> On Thu Jun 4, 2026 at 6:24 PM CEST, Alexei Starovoitov wrote:
> > On Thu, Jun 4, 2026 at 5:23=E2=80=AFAM Alexis Lothor=C3=A9
> > <[email protected]> wrote:
> >>
> >> On Sat Apr 25, 2026 at 7:05 AM CEST, sashiko-bot wrote:
> >>
> >> [...]
> >>
> >> >>  #define LR_FRAMENO_BITS     3
> >> >>  #define LR_SPI_BITS 6
> >> > [ ... ]
> >> >> @@ -6493,6 +6498,8 @@ static int check_mem_access(struct bpf_verifi=
er_env *env, int insn_idx, u32 regn
> >> >>              else
> >> >>                      err =3D check_stack_write(env, regno, off, siz=
e,
> >> >>                                              value_regno, insn_idx)=
;
> >> >> +
> >> >> +            mark_insn_accesses_stack(env, insn_idx);
> >> >
> >> > Global subprograms are verified independently of their call sites, a=
nd pointer
> >> > arguments are tracked as PTR_TO_MEM rather than PTR_TO_STACK. Inside=
 the
> >> > subprogram, check_mem_access() evaluates loads and stores under the
> >> > PTR_TO_MEM branch, which does not invoke mark_insn_accesses_stack().
> >> >
> >> > If a BPF caller dynamically passes a pointer to its stack as an argu=
ment to
> >> > the global subprogram, will the JIT-compiled global subprogram execu=
te with
> >> > KASAN checks enabled for those accesses?
> >> >
> >> > Does this erroneously monitor the BPF stack, bypassing the intended
> >> > suppression mechanism?
> >>
> >> After having thought about this one for quite some time, I see no
> >> simple way to correctly distinguish stack accesses hidden behing a
> >> PTR_TO_MEM passed to a global subprog, as those are verified separatel=
y.
> >>
> >> I can either:
> >> 1. skip PTR_TO_MEM memory instrumentation when we are in global
> >> subprograms, at the risk of missing instrumentation on passed memory
> >> accesses that _need_ to be instrumented
> >> 2. systematically instrument passed PTR_TO_MEM memory accesses in glob=
al
> >> subprog, at the risk of inserting unneeded instrumentation when the
> >> passed memory comes in fact from caller memory.
> >>
> >> I then propose to be conservative and apply 2:  the only downside I ca=
n
> >> think of is that a few alwasy-passing kasan checks will be inserted, b=
ut
> >> we are at least sure not to miss any instrumentation.
> >
> > Isn't it the other way around? Replacing LDX with asan_load() will
> > check stack access, but it doesn't have shadow memory behind it?
> > Or you mean to assume kasan_vmalloc and vmalloced_stack kconfig?
>
> Ah yes, my bad, I did not state it explicitely, but my point above
> indeed assumes that we are with CONFIG_KASAN_VMALLOC and
> CONFIG_VMAP_STACK

ok, then it's an ok path forward. Better than option 1,
since skipping all of PTR_TO_MEM in global subprogs would leave a big gap.