Merge branch 'bpf: Implement two type cast kfuncs'

Yonghong Song says:

====================

Currenty, a non-tracing bpf program typically has a single 'context' argument
with predefined uapi struct type. Following these uapi struct, user is able
to access other fields defined in uapi header. Inside the kernel, the
user-seen 'context' argument is replaced with 'kernel context' (or 'kctx'
in short) which can access more information than what uapi header provides.
To access other info not in uapi header, people typically do two things:
  (1). extend uapi to access more fields rooted from 'context'.
  (2). use bpf_probe_read_kernl() helper to read particular field based on
    kctx.
Using (1) needs uapi change and using (2) makes code more complex since
direct memory access is not allowed.

There are already a few instances trying to access more information from
kctx:
  . trying to access some fields from perf_event kctx ([1]).
  . trying to access some fields from xdp kctx ([2]).

This patch set tried to allow direct memory access for kctx fields
by introducing bpf_cast_to_kern_ctx() kfunc.

Martin mentioned a use case like type casting below:
  #define skb_shinfo(SKB) ((struct skb_shared_info *)(skb_end_pointer(SKB)))
basically a 'unsigned char *" casted to 'struct skb_shared_info *'. This patch
set tries to support such a use case as well with bpf_rdonly_cast().

For the patch series, Patch 1 added support for a kfunc available to all
prog types. Patch 2 added bpf_cast_to_kern_ctx() kfunc. Patch 3 added
bpf_rdonly_cast() kfunc. Patch 4 added a few positive and negative tests.

  [1] https://lore.kernel.org/bpf/ad15b398-9069-4a0e-48cb-4bb651ec3088@meta.com/
  [2] https://lore.kernel.org/bpf/20221109215242.1279993-1-john.fastabend@gmail.com/

Changelog:
  v3 -> v4:
    - remove unnecessary bpf_ctx_convert.t error checking
    - add and use meta.ret_btf_id instead of meta.arg_constant.value for
      bpf_cast_to_kern_ctx().
    - add PTR_TRUSTED to the return PTR_TO_BTF_ID type for bpf_cast_to_kern_ctx().
  v2 -> v3:
    - rebase on top of bpf-next (for merging conflicts)
    - add the selftest to s390x deny list
  rfcv1 -> v2:
    - break original one kfunc into two.
    - add missing error checks and error logs.
    - adapt to the new conventions in
      https://lore.kernel.org/all/20221118015614.2013203-1-memxor@gmail.com/
      for example, with __ign and __k suffix.
    - added support in fixup_kfunc_call() to replace kfunc calls with a single mov.
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/btf.h b/include/linux/btf.h
index d38aa4251c28..9ed00077db6e 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -487,6 +487,7 @@ const struct btf_member *
 btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
 		      const struct btf_type *t, enum bpf_prog_type prog_type,
 		      int arg);
+int get_kern_ctx_btf_id(struct bpf_verifier_log *log, enum bpf_prog_type prog_type);
 bool btf_types_are_same(const struct btf *btf1, u32 id1,
 			const struct btf *btf2, u32 id2);
 #else
@@ -531,6 +532,10 @@ btf_get_prog_ctx_type(struct bpf_verifier_log *log, const struct btf *btf,
 {
 	return NULL;
 }
+static inline int get_kern_ctx_btf_id(struct bpf_verifier_log *log,
+				      enum bpf_prog_type prog_type) {
+	return -EINVAL;
+}
 static inline bool btf_types_are_same(const struct btf *btf1, u32 id1,
 				      const struct btf *btf2, u32 id2)
 {