Re: [PATCH v2 3/3] btf_loader.c: Infer alignment info

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Mon, Oct 18, 2021 at 02:16:21PM +0100, Douglas RAILLARD escreveu:
> From: Douglas Raillard <douglas.raillard@arm.com>
> 
> 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@arm.com>
> ---
>  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