Em Mon, Oct 18, 2021 at 02:16:21PM +0100, Douglas RAILLARD escreveu:
> From: Douglas Raillard <douglas.raillard@xxxxxxx>
>
> BTF does not carry alignment information, but it carries the offset in
> structs. This allows inferring the original alignment, yielding a C
> header dump that is not identical to the original C code, but is
> guaranteed to lead to the same memory layout.
>
> This allows using the output of pahole in another program to poke at
> memory, with the assurance that we will not read garbage.
>
> Note: Since the alignment is inferred from the offset, it sometimes
> happens that the offset was already correctly aligned, which means the
> inferred alignment will be smaller than in the original source. This
> does not impact the ability to read existing structs, but it could
> impact creating such struct if other client code expects higher
> alignment than the one exposed in the generated header.
this one makes btfdiff fail, example:
@@ -125578,7 +125640,7 @@ struct xt_entry_match {
struct xt_match * match; /* 8 8 */
} kernel; /* 0 16 */
__u16 match_size; /* 0 2 */
- } u; /* 0 32 */
+ } u; /* 0 32 */
unsigned char data[]; /* 32 0 */
/* size: 32, cachelines: 1, members: 2 */
Why the change in the generated source code comment alignment?
There are some other differences, I'll check tomorrow.
- Arnaldo
> Signed-off-by: Douglas Raillard <douglas.raillard@xxxxxxx>
> ---
> btf_loader.c | 36 ++++++++++++++++++++++++++++++++++++
> dwarves.c | 2 +-
> dwarves.h | 2 ++
> 3 files changed, 39 insertions(+), 1 deletion(-)
>
> diff --git a/btf_loader.c b/btf_loader.c
> index 9c2daee..2885252 100644
> --- a/btf_loader.c
> +++ b/btf_loader.c
> @@ -471,10 +471,37 @@ static int btf__load_sections(struct btf *btf, struct cu *cu)
> return btf__load_types(btf, cu);
> }
>
> +static uint32_t class__infer_alignment(uint32_t byte_offset,
> + uint32_t natural_alignment,
> + uint32_t smallest_offset)
> +{
> + uint32_t alignment = 0;
> + uint32_t offset_delta = byte_offset - smallest_offset;
> +
> + if (offset_delta) {
> + if (byte_offset % 2 == 0) {
> + /* Find the power of 2 immediately higher than
> + * offset_delta
> + */
> + alignment = 1 << (8 * sizeof(offset_delta) -
> + __builtin_clz(offset_delta));
> + } else {
> + alignment = 0;
> + }
> + }
> +
> + /* Natural alignment, nothing to do */
> + if (alignment <= natural_alignment || alignment == 1)
> + alignment = 0;
> +
> + return alignment;
> +}
> +
> static int class__fixup_btf_bitfields(struct tag *tag, struct cu *cu)
> {
> struct class_member *pos;
> struct type *tag_type = tag__type(tag);
> + uint32_t smallest_offset = 0;
>
> type__for_each_data_member(tag_type, pos) {
> struct tag *type = tag__strip_typedefs_and_modifiers(&pos->tag, cu);
> @@ -508,8 +535,17 @@ static int class__fixup_btf_bitfields(struct tag *tag, struct cu *cu)
> pos->byte_offset = pos->bit_offset / 8;
> }
> }
> +
> + pos->alignment = class__infer_alignment(pos->byte_offset,
> + tag__natural_alignment(type, cu),
> + smallest_offset);
> + smallest_offset = pos->byte_offset + pos->byte_size;
> }
>
> + tag_type->alignment = class__infer_alignment(tag_type->size,
> + tag__natural_alignment(tag, cu),
> + smallest_offset);
> +
> return 0;
> }
>
> diff --git a/dwarves.c b/dwarves.c
> index b6f2489..bb8af5b 100644
> --- a/dwarves.c
> +++ b/dwarves.c
> @@ -1515,7 +1515,7 @@ void class__find_holes(struct class *class)
>
> static size_t type__natural_alignment(struct type *type, const struct cu *cu);
>
> -static size_t tag__natural_alignment(struct tag *tag, const struct cu *cu)
> +size_t tag__natural_alignment(struct tag *tag, const struct cu *cu)
> {
> size_t natural_alignment = 1;
>
> diff --git a/dwarves.h b/dwarves.h
> index 30d33fa..c2fea0a 100644
> --- a/dwarves.h
> +++ b/dwarves.h
> @@ -1002,6 +1002,8 @@ struct type {
>
> void __type__init(struct type *type);
>
> +size_t tag__natural_alignment(struct tag *tag, const struct cu *cu);
> +
> static inline struct class *type__class(const struct type *type)
> {
> return (struct class *)type;
> --
> 2.25.1
--
- Arnaldo
