On Fri, Jul 16, 2021 at 2:44 PM Alan Maguire <alan.maguire@xxxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Alan Maguire <alan.maguire@xxxxxxxxxx>
> ---
> 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
>
