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

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

[Alan Maguire]

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@kernel.org>
Signed-off-by: Alan Maguire <alan.maguire@oracle.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

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

[Andrii Nakryiko]

On Fri, Jul 16, 2021 at 2:44 PM Alan Maguire <alan.maguire@oracle.com> wrote:
>
> 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@kernel.org>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.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)
> +

there is no need for macro, please use static function. And
ptr_is_aligned() is probably a better form:

if (!ptr_is_aligned(data, sz)) {
    /* handle uncommon case */
}

ptr_is_aligned() can be probably reused more readily in some other places later.

>  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;

sizeof(void *) could be 4 on the host system and 8 in BTF. If you want
to preserve the speed, I'd do something like:

if (ptr_is_aligned(data, sizeof(void *)) && sizeof(void *) == d->ptr_sz) {
    btf_dump_type_values(d, "%p", *(void **)data);
} else {
    /* fetch pointer value as unaligned integer */
    if (d->ptr_sz == 4)
        printf("0x%x")
    else
        printf("0x%llx")
}

Maybe there is some cleaner way. But that should work, no?

> +
> +       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
>