linux-kernel - [PATCH bpf-next] libbpf: clarify/fix unaligned data issues for btf typed dump

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Date:   Fri, 16 Jul 2021 22:43:33 +0100
From:   Alan Maguire <alan.maguire@...cle.com>
To:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org
Cc:     kafai@...com, songliubraving@...com, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, morbo@...gle.com,
        shuah@...nel.org, bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alan Maguire <alan.maguire@...cle.com>
Subject: [PATCH bpf-next] libbpf: clarify/fix unaligned data issues for btf typed dump

If data is packed, data structures can store it outside of usual
boundaries.  For example a 4-byte int can be stored on a unaligned
boundary in a case like this:

struct s {
	char f1;
	int f2;
} __attribute((packed));

...the int is stored at an offset of one byte.  Some platforms have
problems dereferencing data that is not aligned with its size, and
code exists to handle most cases of this for BTF typed data display.
However pointer display was missed, and a simple macro to test if
"data_is_unaligned(data, data_sz)" would help clarify this code.

Suggested-by: Andrii Nakryiko <andrii@...nel.org>
Signed-off-by: Alan Maguire <alan.maguire@...cle.com>
---
 tools/lib/bpf/btf_dump.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/tools/lib/bpf/btf_dump.c b/tools/lib/bpf/btf_dump.c
index 929cf93..9dfe9c1 100644
--- a/tools/lib/bpf/btf_dump.c
+++ b/tools/lib/bpf/btf_dump.c
@@ -1654,6 +1654,8 @@ static int btf_dump_base_type_check_zero(struct btf_dump *d,
 	return 0;
 }
 
+#define data_is_unaligned(data, data_sz)	(((uintptr_t)data) % data_sz)
+
 static int btf_dump_int_data(struct btf_dump *d,
 			     const struct btf_type *t,
 			     __u32 type_id,
@@ -1672,7 +1674,7 @@ static int btf_dump_int_data(struct btf_dump *d,
 	/* handle packed int data - accesses of integers not aligned on
 	 * int boundaries can cause problems on some platforms.
 	 */
-	if (((uintptr_t)data) % sz)
+	if (data_is_unaligned(data, sz))
 		return btf_dump_bitfield_data(d, t, data, 0, 0);
 
 	switch (sz) {
@@ -1739,7 +1741,7 @@ static int btf_dump_float_data(struct btf_dump *d,
 	int sz = t->size;
 
 	/* handle unaligned data; copy to local union */
-	if (((uintptr_t)data) % sz) {
+	if (data_is_unaligned(data, sz)) {
 		memcpy(&fl, data, sz);
 		flp = &fl;
 	}
@@ -1897,7 +1899,10 @@ static int btf_dump_ptr_data(struct btf_dump *d,
 			      __u32 id,
 			      const void *data)
 {
-	btf_dump_type_values(d, "%p", *(void **)data);
+	void *ptrval;
+
+	memcpy(&ptrval, data, d->ptr_sz);
+	btf_dump_type_values(d, "%p", ptrval);
 	return 0;
 }
 
@@ -1910,7 +1915,7 @@ static int btf_dump_get_enum_value(struct btf_dump *d,
 	int sz = t->size;
 
 	/* handle unaligned enum value */
-	if (((uintptr_t)data) % sz) {
+	if (data_is_unaligned(data, sz)) {
 		*value = (__s64)btf_dump_bitfield_get_data(d, t, data, 0, 0);
 		return 0;
 	}
-- 
1.8.3.1