Re: [PATCH v2 bpf 0/5] Tighten up arg:ctx type enforcement

[Eduard Zingerman <eddyz87@gmail.com>]

On Wed, 2024-01-17 at 14:33 -0800, Andrii Nakryiko wrote:
> Follow up fixes for kernel-side and libbpf-side logic around handling arg:ctx
> (__arg_ctx) tagged arguments of BPF global subprogs.
> 
> Patch #1 adds libbpf feature detection of kernel-side __arg_ctx support to
> avoid unnecessary rewriting BTF types. With stricter kernel-side type
> enforcement this is now mandatory to avoid problems with using `struct
> bpf_user_pt_regs_t` instead of actual typedef. For __arg_ctx tagged arguments
> verifier is now ignoring superficial `bpf_user_pt_regs_t` typedef and resolves
> it down to the actual struct (pt_regs/user_pt_regs/etc, depending on
> architecture), but for old kernels without __arg_ctx support it's more
> backwards compatible for libbpf to use `struct bpf_user_pt_regs_t` rewrite
> which will work on wider range of kernels. So feature detection prevent libbpf
> accidentally breaking global subprogs on new kernels.
> 
> We also adjust selftests to do similar feature detection (much simpler, but
> potentially breaking due to kernel source code refactoring, which is fine for
> selftests), and skip tests expecting libbpf's BTF type rewrites.
> 
> Patch #2 is preparatory refactoring for patch #3 which adds type enforcement
> for arg:ctx tagged global subprog args. See the patch for specifics.
> 
> Patch #4 adds many new cases to ensure type logic works as expected.
> 
> Finally, patch #5 adds a relevant subset of kernel-side type checks to
> __arg_ctx cases that libbpf supports rewrite of. In libbpf's case, type
> violations are reported as warnings and BTF rewrite is not performed, which
> will eventually lead to BPF verifier complaining at program verification time.
> 
> Good care was taken to avoid conflicts between bpf and bpf-next tree (which
> has few follow up refactorings in the same code area). Once trees converge
> some of the code will be moved around a bit (and some will be deleted), but
> with no change to functionality or general shape of the code.
> 
> v1->v2:
>   - add user_pt_regs and user_regs_struct support for PERF_EVENT (CI);
>   - drop FEAT_ARG_CTX_TAG enum leftover from patch #1;
>   - fix warning about default: without break in the switch (CI).

I've read through patch-set and it seem to be ok,
checks match behavior described in patch #3 description.

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>

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