On Wed, 11 Oct 2023 20:50:58 -0700
Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> We want to track type states as instructions are executed. Each
> instruction can access compound types like struct or union and load/
> store its members to a different location.
>
> The die_deref_ptr_type() is to find a type of memory access with a
> pointer variable. If it points to a compound type like struct, the
> target memory is a member in the struct. The access will happen
> with an offset indicating which member it refers. Let's follow the
> DWARF info to figure out the type of the pointer target.
>
> For example, say we have the following code.
>
> struct foo {
> int a;
> int b;
> };
>
> struct foo *p = malloc(sizeof(*p));
> p->b = 0;
>
> The last pointer access should produce x86 asm like below:
>
> mov 0x0, 4(%rbx)
>
> And we know %rbx register has a pointer to struct foo. Then offset 4
> should return the debug info of member 'b'.
>
> Also variables of compound types can be accessed directly without a
> pointer. The die_get_member_type() is to handle a such case.
Looks good to me. perf probe search the member by name, but these
APIs searches it by offset.
Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
Thanks!
>
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
> ---
> tools/perf/util/dwarf-aux.c | 110 ++++++++++++++++++++++++++++++++++++
> tools/perf/util/dwarf-aux.h | 6 ++
> 2 files changed, 116 insertions(+)
>
> diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> index 16e63d8caf83..5ec895e0a069 100644
> --- a/tools/perf/util/dwarf-aux.c
> +++ b/tools/perf/util/dwarf-aux.c
> @@ -1838,3 +1838,113 @@ int die_get_scopes(Dwarf_Die *cu_die, Dwarf_Addr pc, Dwarf_Die **scopes)
> *scopes = data.scopes;
> return data.nr;
> }
> +
> +static int __die_find_member_offset_cb(Dwarf_Die *die_mem, void *arg)
> +{
> + Dwarf_Die type_die;
> + Dwarf_Word size, loc;
> + Dwarf_Word offset = (long)arg;
> + int tag = dwarf_tag(die_mem);
> +
> + if (tag != DW_TAG_member)
> + return DIE_FIND_CB_SIBLING;
> +
> + /* Unions might not have location */
> + if (die_get_data_member_location(die_mem, &loc) < 0)
> + loc = 0;
> +
> + if (offset == loc)
> + return DIE_FIND_CB_END;
> +
> + die_get_real_type(die_mem, &type_die);
> +
> + if (dwarf_aggregate_size(&type_die, &size) < 0)
> + size = 0;
> +
> + if (loc < offset && offset < (loc + size))
> + return DIE_FIND_CB_END;
> +
> + return DIE_FIND_CB_SIBLING;
> +}
> +
> +/**
> + * die_get_member_type - Return type info of struct member
> + * @type_die: a type DIE
> + * @offset: offset in the type
> + * @die_mem: a buffer to save the resulting DIE
> + *
> + * This function returns a type of a member in @type_die where it's located at
> + * @offset if it's a struct. For now, it just returns the first matching
> + * member in a union. For other types, it'd return the given type directly
> + * if it's within the size of the type or NULL otherwise.
> + */
> +Dwarf_Die *die_get_member_type(Dwarf_Die *type_die, int offset,
> + Dwarf_Die *die_mem)
> +{
> + Dwarf_Die *member;
> + Dwarf_Die mb_type;
> + int tag;
> +
> + tag = dwarf_tag(type_die);
> + /* If it's not a compound type, return the type directly */
> + if (tag != DW_TAG_structure_type && tag != DW_TAG_union_type) {
> + Dwarf_Word size;
> +
> + if (dwarf_aggregate_size(type_die, &size) < 0)
> + size = 0;
> +
> + if ((unsigned)offset >= size)
> + return NULL;
> +
> + *die_mem = *type_die;
> + return die_mem;
> + }
> +
> + mb_type = *type_die;
> + /* TODO: Handle union types better? */
> + while (tag == DW_TAG_structure_type || tag == DW_TAG_union_type) {
> + member = die_find_child(&mb_type, __die_find_member_offset_cb,
> + (void *)(long)offset, die_mem);
> + if (member == NULL)
> + return NULL;
> +
> + if (die_get_real_type(member, &mb_type) == NULL)
> + return NULL;
> +
> + tag = dwarf_tag(&mb_type);
> +
> + if (tag == DW_TAG_structure_type || tag == DW_TAG_union_type) {
> + Dwarf_Word loc;
> +
> + /* Update offset for the start of the member struct */
> + if (die_get_data_member_location(member, &loc) == 0)
> + offset -= loc;
> + }
> + }
> + *die_mem = mb_type;
> + return die_mem;
> +}
> +
> +/**
> + * die_deref_ptr_type - Return type info for pointer access
> + * @ptr_die: a pointer type DIE
> + * @offset: access offset for the pointer
> + * @die_mem: a buffer to save the resulting DIE
> + *
> + * This function follows the pointer in @ptr_die with given @offset
> + * and saves the resulting type in @die_mem. If the pointer points
> + * a struct type, actual member at the offset would be returned.
> + */
> +Dwarf_Die *die_deref_ptr_type(Dwarf_Die *ptr_die, int offset,
> + Dwarf_Die *die_mem)
> +{
> + Dwarf_Die type_die;
> +
> + if (dwarf_tag(ptr_die) != DW_TAG_pointer_type)
> + return NULL;
> +
> + if (die_get_real_type(ptr_die, &type_die) == NULL)
> + return NULL;
> +
> + return die_get_member_type(&type_die, offset, die_mem);
> +}
> diff --git a/tools/perf/util/dwarf-aux.h b/tools/perf/util/dwarf-aux.h
> index d0ef41738abd..df846bd30134 100644
> --- a/tools/perf/util/dwarf-aux.h
> +++ b/tools/perf/util/dwarf-aux.h
> @@ -144,6 +144,12 @@ struct die_var_type {
> int offset;
> };
>
> +/* Return type info of a member at offset */
> +Dwarf_Die *die_get_member_type(Dwarf_Die *type_die, int offset, Dwarf_Die *die_mem);
> +
> +/* Return type info where the pointer and offset point to */
> +Dwarf_Die *die_deref_ptr_type(Dwarf_Die *ptr_die, int offset, Dwarf_Die *die_mem);
> +
> #ifdef HAVE_DWARF_GETLOCATIONS_SUPPORT
>
> /* Get byte offset range of given variable DIE */
> --
> 2.42.0.655.g421f12c284-goog
>
--
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
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