From: Jean-Philippe Brucker <jean-philippe@linaro.org>
Date: Fri, 19 Mar 2021 12:25:54 +0100
Subject: libbpf: Fix BTF dump of pointer-to-array-of-struct
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Patch-mainline: v5.12-rc5
Git-commit: 901ee1d750f29a335423eeb9463c3ca461ca18c2
References: bsc#1155518

The vmlinux.h generated from BTF is invalid when building
drivers/phy/ti/phy-gmii-sel.c with clang:

vmlinux.h:61702:27: error: array type has incomplete element type ‘struct reg_field’
61702 |  const struct reg_field (*regfields)[3];
      |                           ^~~~~~~~~

bpftool generates a forward declaration for this struct regfield, which
compilers aren't happy about. Here's a simplified reproducer:

	struct inner {
		int val;
	};
	struct outer {
		struct inner (*ptr_to_array)[2];
	} A;

After build with clang -> bpftool btf dump c -> clang/gcc:
./def-clang.h:11:23: error: array has incomplete element type 'struct inner'
        struct inner (*ptr_to_array)[2];

Member ptr_to_array of struct outer is a pointer to an array of struct
inner. In the DWARF generated by clang, struct outer appears before
struct inner, so when converting BTF of struct outer into C, bpftool
issues a forward declaration to struct inner. With GCC the DWARF info is
reversed so struct inner gets fully defined.

That forward declaration is not sufficient when compilers handle an
array of the struct, even when it's only used through a pointer. Note
that we can trigger the same issue with an intermediate typedef:

	struct inner {
	        int val;
	};
	typedef struct inner inner2_t[2];
	struct outer {
	        inner2_t *ptr_to_array;
	} A;

Becomes:

	struct inner;
	typedef struct inner inner2_t[2];

And causes:

./def-clang.h:10:30: error: array has incomplete element type 'struct inner'
	typedef struct inner inner2_t[2];

To fix this, clear through_ptr whenever we encounter an intermediate
array, to make the inner struct part of a strong link and force full
declaration.

Fixes: 351131b51c7a ("libbpf: add btf_dump API for BTF-to-C conversion")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210319112554.794552-2-jean-philippe@linaro.org
Acked-by: Gary Lin <glin@suse.com>
---
 tools/lib/bpf/btf_dump.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -443,7 +443,7 @@ static int btf_dump_order_type(struct bt
 		return err;
 
 	case BTF_KIND_ARRAY:
-		return btf_dump_order_type(d, btf_array(t)->type, through_ptr);
+		return btf_dump_order_type(d, btf_array(t)->type, false);
 
 	case BTF_KIND_STRUCT:
 	case BTF_KIND_UNION: {