Re: [PATCH bpf-next 3/6] bpf: fetch the vmlinux BTF where kernel types enter a program

[Jay Wang]

Addressed since v2; the current version is v3 (patch 3/9):
https://lore.kernel.org/bpf/20260925224229.1850-1-wanjay@xxxxxxxxxx/

Two of these are real gaps in the set of fetch sites and are
fixed:

- bpf_ctx_convert.t: the program context types are kernel types too, and
  btf_prepare_func_args() (global subprogs taking the context) and
  btf_translate_to_vmlinux() (ctx access of tracing/EXT programs) read
  the table without anything having fetched the BTF.  It is now fetched
  in the table's readers.  Tested with a socket filter whose global
  subprogram takes struct __sk_buff * as the first user on a fresh boot.

- bpf_snprintf_btf()/bpf_seq_printf_btf(): the type id travels inside
  struct btf_ptr, so the arg/ret type check does not see it.  The two
  helpers are now named explicitly.  Tested with a raw_tp program
  calling bpf_snprintf_btf() as the first user.

Also addressed:

- The IS_ERR(btf_vmlinux) check in bpf_check() now reads through
  bpf_peek_btf_vmlinux().  With =m a failed parse is not stored (below),
  so it only fires for =y, where the BTF is parsed at boot as before;
  with =m each fetch site reports the failure itself.
- btf_prepare_func_args() and btf_check_kfunc_name() check the result
  of bpf_get_btf_vmlinux() (pre-existing, but cheap).
- A failed parse is no longer cached with =m; the payload was verified
  against the kernel when the module loaded, so a failure there is a
  resource problem and the next caller retries.  =y keeps caching, as
  before: a parse failure there means the build is broken.

Not a bug:

- bpf_verifier_lock vs request_module(): the lock is only taken for
  unprivileged programs, and none of the fetch sites is reachable from
  one: kfuncs, ksyms, map pointer access and the BTF helpers all need
  privileged program types, and the context-type path needs a global
  subprogram, i.e. program BTF, which BPF_BTF_LOAD only accepts with
  CAP_BPF.  modprobe does not load BPF programs.

Jay