Merge branch 'bpf: Support calling kernel function'

Martin KaFai says:

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

This series adds support to allow bpf program calling kernel function.

The use case included in this set is to allow bpf-tcp-cc to directly
call some tcp-cc helper functions (e.g. "tcp_cong_avoid_ai()").  Those
functions have already been used by some kernel tcp-cc implementations.

This set will also allow the bpf-tcp-cc program to directly call the
kernel tcp-cc implementation,  For example, a bpf_dctcp may only want to
implement its own dctcp_cwnd_event() and reuse other dctcp_*() directly
from the kernel tcp_dctcp.c instead of reimplementing (or
copy-and-pasting) them.

The tcp-cc kernel functions mentioned above will be white listed
for the struct_ops bpf-tcp-cc programs to use in a later patch.
The white listed functions are not bounded to a fixed ABI contract.
Those functions have already been used by the existing kernel tcp-cc.
If any of them has changed, both in-tree and out-of-tree kernel tcp-cc
implementations have to be changed.  The same goes for the struct_ops
bpf-tcp-cc programs which have to be adjusted accordingly.

Please see individual patch for details.

v2:
- Patch 2 in v1 is removed.  No need to support extern func in kernel.
  Changed libbpf to adjust the .ksyms datasec for extern func
  in patch 11. (Andrii)
- Name change: btf_check_func_arg_match() and btf_check_subprog_arg_match()
  in patch 2. (Andrii)
- Always set unreliable on any error in patch 2 since it does not
  matter. (Andrii)
- s/kern_func/kfunc/ and s/descriptor/desc/ in this set. (Andrii)
- Remove some unnecessary changes in disasm.h and disasm.c
  in patch 3.  In particular, no need to change the function
  signature in bpf_insn_revmap_call_t.  Also, removed the changes
  in print_bpf_insn().
- Fixed an issue in check_kfunc_call() when the calling kernel function
  returns a pointer in patch 3.  Added a selftest.
- Adjusted the verifier selftests due to the changes in the verifier log
  in patch 3.
- Fixed a comparison issue in kfunc_desc_cmp_by_imm() in patch 3. (Andrii)
- Name change: is_ldimm64_insn(),
  new helper: is_call_insn() in patch 10 (Andrii)
- Move btf_func_linkage() from btf.h to libbpf.c in patch 11. (Andrii)
- Fixed the linker error when CONFIG_BPF_SYSCALL is not defined.
  Moved the check_kfunc_call from filter.c to test_run.c in patch 14.
  (kernel test robot)
====================

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

1 files changed, 10 insertions, 3 deletions

diff --git a/kernel/bpf/disasm.c b/kernel/bpf/disasm.c
index 3acc7e0b6916..dad821c8ecd0 100644
--- a/kernel/bpf/disasm.c
+++ b/kernel/bpf/disasm.c
@@ -19,16 +19,23 @@ static const char *__func_get_name(const struct bpf_insn_cbs *cbs,
 {
 	BUILD_BUG_ON(ARRAY_SIZE(func_id_str) != __BPF_FUNC_MAX_ID);
 
-	if (insn->src_reg != BPF_PSEUDO_CALL &&
+	if (!insn->src_reg &&
 	    insn->imm >= 0 && insn->imm < __BPF_FUNC_MAX_ID &&
 	    func_id_str[insn->imm])
 		return func_id_str[insn->imm];
 
-	if (cbs && cbs->cb_call)
-		return cbs->cb_call(cbs->private_data, insn);
+	if (cbs && cbs->cb_call) {
+		const char *res;
+
+		res = cbs->cb_call(cbs->private_data, insn);
+		if (res)
+			return res;
+	}
 
 	if (insn->src_reg == BPF_PSEUDO_CALL)
 		snprintf(buff, len, "%+d", insn->imm);
+	else if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL)
+		snprintf(buff, len, "kernel-function");
 
 	return buff;
 }