On Tue, Jan 07, 2020 at 01:13:23PM +0100, Jiri Olsa wrote:

On Mon, Jan 06, 2020 at 09:36:17PM +0000, Yonghong Song wrote:

quoted

On 12/29/19 6:37 AM, Jiri Olsa wrote:

quoted

I'm not sure why the restriction was added,
but I can't access pointers to POD types like
const char * when probing vfs_read function.

Removing the check and allow non struct type
access in context.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
  kernel/bpf/btf.c | 6 ------
  1 file changed, 6 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index ed2075884724..ae90f60ac1b8 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c

@@ -3712,12 +3712,6 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
  	/* skip modifiers */
  	while (btf_type_is_modifier(t))
  		t = btf_type_by_id(btf, t->type);
-	if (!btf_type_is_struct(t)) {
-		bpf_log(log,
-			"func '%s' arg%d type %s is not a struct\n",
-			tname, arg, btf_kind_str[BTF_INFO_KIND(t->info)]);
-		return false;
-	}

Hi, Jiri, the RFC looks great! Especially, you also referenced this will
give great performance boost for bcc scripts.

Could you provide more context on why the above change is needed?
The function btf_ctx_access is used to check validity of accessing
function parameters which are wrapped inside a structure, I am wondering
what kinds of accesses you tried to address here.

when I was transforming opensnoop.py to use this I got fail in
there when I tried to access filename arg in do_sys_open

but actualy it seems this should get recognized earlier by:

          if (btf_type_is_int(t))
                /* accessing a scalar */
                return true;

I'm not sure why it did not pass for const char*, I'll check

it seems we don't check for pointer to scalar (just void),
which is the case in my example 'const char *filename'

I'll post this in v2 with other changes

jirka


---

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index ed2075884724..650df4ed346e 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c

@@ -3633,7 +3633,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		    const struct bpf_prog *prog,
 		    struct bpf_insn_access_aux *info)
 {
-	const struct btf_type *t = prog->aux->attach_func_proto;
+	const struct btf_type *tp, *t = prog->aux->attach_func_proto;
 	struct bpf_prog *tgt_prog = prog->aux->linked_prog;
 	struct btf *btf = bpf_prog_get_target_btf(prog);
 	const char *tname = prog->aux->attach_func_name;

@@ -3695,6 +3695,17 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		 */
 		return true;
 
+	tp = btf_type_by_id(btf, t->type);
+	/* skip modifiers */
+	while (btf_type_is_modifier(tp))
+		tp = btf_type_by_id(btf, tp->type);
+
+	if (btf_type_is_int(tp))
+		/* This is a pointer scalar.
+		 * It is the same as scalar from the verifier safety pov.
+		 */
+		return true;
+
 	/* this is a pointer to another type */
 	info->reg_type = PTR_TO_BTF_ID;
 	info->btf_id = t->type;