Re: [PATCH bpf-next 1/8] bpf: Add __srcptr kfunc argument suffix

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Oct 29, 2026 at 02:49:22PM -0700, Andrii Nakryiko wrote:
> On Thu, Oct 29, 2026 at 2:26 PM Jiri Olsa <jolsa@kernel.org> wrote:
> >
> > Generic sized pointer arguments are treated as writable memory. This
> > prevents kfuncs that only read from a source buffer from accepting
> > read-only BPF memory, such as const data.
> >
> > Adding __srcptr to mark pointers the kfunc only reads.
> 
> why can't we infer read-only from constness of a pointer? what's the
> advantage of __srcptr (which, honestly, says absolutely nothing,
> "__readonly" or "__ro" would probably make more sense if we have to go
> with suffix)

the reason was that current pointer arguments are marked as MEM_WRITE,
so passing readonly pointer to bpf_copy_to_user caused verifier error

but I think we can use const, did not think of that.. will change

thanks,
jirka


> 
> >
> > The get_kfunc_arg_type change sets the initial memory access mode
> > and the resolve_func_arg_type preserves the suffix’s access mode
> > when it converts a BTF pointer classification into a memory argument.
> >
> > Assisted-by: Codex:GPT-6
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  Documentation/bpf/kfuncs.rst | 24 ++++++++++++++++++++++++
> >  kernel/bpf/verifier.c        | 20 +++++++++++++++++---
> >  2 files changed, 41 insertions(+), 3 deletions(-)
> >
> 
> [...]