Re: [PATCH 28/48] perf dwarf-aux: Add die_find_variable_by_addr()

[Google]

On Wed, 11 Oct 2023 20:50:51 -0700
Namhyung Kim <namhyung@xxxxxxxxxx> wrote:

> The die_find_variable_by_addr() is to find a variables in the given DIE
> using given (PC-relative) address.  Global variables will have a
> location expression with DW_OP_addr which has an address so can simply
> compare it with the address.
> 
>   <1><143a7>: Abbrev Number: 2 (DW_TAG_variable)
>       <143a8>   DW_AT_name        : loops_per_jiffy
>       <143ac>   DW_AT_type        : <0x1cca>
>       <143b0>   DW_AT_external    : 1
>       <143b0>   DW_AT_decl_file   : 193
>       <143b1>   DW_AT_decl_line   : 213
>       <143b2>   DW_AT_location    : 9 byte block: 3 b0 46 41 82 ff ff ff ff
>                                      (DW_OP_addr: ffffffff824146b0)
> 
> Note that the type-offset should be calculated from the base address of
> the global variable.
> 
> Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>

Looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

BTW, for the global variable, you can also find it via maps. Can't you?

Thanks,

> ---
>  tools/perf/util/dwarf-aux.c | 80 +++++++++++++++++++++++++++++++++++++
>  tools/perf/util/dwarf-aux.h | 14 +++++++
>  2 files changed, 94 insertions(+)
> 
> diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> index 5bb05c84d249..97d9ae56350e 100644
> --- a/tools/perf/util/dwarf-aux.c
> +++ b/tools/perf/util/dwarf-aux.c
> @@ -1266,8 +1266,12 @@ int die_get_var_range(Dwarf_Die *sp_die, Dwarf_Die *vr_die, struct strbuf *buf)
>  struct find_var_data {
>  	/* Target instruction address */
>  	Dwarf_Addr pc;
> +	/* Target memory address (for global data) */
> +	Dwarf_Addr addr;
>  	/* Target register */
>  	unsigned reg;
> +	/* Access offset, set for global data */
> +	int offset;
>  };
>  
>  /* Max number of registers DW_OP_regN supports */
> @@ -1328,6 +1332,82 @@ Dwarf_Die *die_find_variable_by_reg(Dwarf_Die *sc_die, Dwarf_Addr pc, int reg,
>  	};
>  	return die_find_child(sc_die, __die_find_var_reg_cb, &data, die_mem);
>  }
> +
> +/* Only checks direct child DIEs in the given scope */
> +static int __die_find_var_addr_cb(Dwarf_Die *die_mem, void *arg)
> +{
> +	struct find_var_data *data = arg;
> +	int tag = dwarf_tag(die_mem);
> +	ptrdiff_t off = 0;
> +	Dwarf_Attribute attr;
> +	Dwarf_Addr base, start, end;
> +	Dwarf_Word size;
> +	Dwarf_Die type_die;
> +	Dwarf_Op *ops;
> +	size_t nops;
> +
> +	if (tag != DW_TAG_variable)
> +		return DIE_FIND_CB_SIBLING;
> +
> +	if (dwarf_attr(die_mem, DW_AT_location, &attr) == NULL)
> +		return DIE_FIND_CB_SIBLING;
> +
> +	while ((off = dwarf_getlocations(&attr, off, &base, &start, &end, &ops, &nops)) > 0) {
> +		if (ops->atom != DW_OP_addr)
> +			continue;
> +
> +		if (data->addr < ops->number)
> +			continue;
> +
> +		if (data->addr == ops->number) {
> +			/* Update offset relative to the start of the variable */
> +			data->offset = 0;
> +			return DIE_FIND_CB_END;
> +		}
> +
> +		if (die_get_real_type(die_mem, &type_die) == NULL)
> +			continue;
> +
> +		if (dwarf_aggregate_size(&type_die, &size) < 0)
> +			continue;
> +
> +		if (data->addr >= ops->number + size)
> +			continue;
> +
> +		/* Update offset relative to the start of the variable */
> +		data->offset = data->addr - ops->number;
> +		return DIE_FIND_CB_END;
> +	}
> +	return DIE_FIND_CB_SIBLING;
> +}
> +
> +/**
> + * die_find_variable_by_addr - Find variable located at given address
> + * @sc_die: a scope DIE
> + * @pc: the program address to find
> + * @addr: the data address to find
> + * @die_mem: a buffer to save the resulting DIE
> + * @offset: the offset in the resulting type
> + *
> + * Find the variable DIE located at the given address (in PC-relative mode).
> + * This is usually for global variables.
> + */
> +Dwarf_Die *die_find_variable_by_addr(Dwarf_Die *sc_die, Dwarf_Addr pc,
> +				     Dwarf_Addr addr, Dwarf_Die *die_mem,
> +				     int *offset)
> +{
> +	struct find_var_data data = {
> +		.pc = pc,
> +		.addr = addr,
> +	};
> +	Dwarf_Die *result;
> +
> +	result = die_find_child(sc_die, __die_find_var_addr_cb, &data, die_mem);
> +	if (result)
> +		*offset = data.offset;
> +	return result;
> +}
> +
>  #endif
>  
>  /*
> diff --git a/tools/perf/util/dwarf-aux.h b/tools/perf/util/dwarf-aux.h
> index 574405c57d3b..742098e3ee7e 100644
> --- a/tools/perf/util/dwarf-aux.h
> +++ b/tools/perf/util/dwarf-aux.h
> @@ -144,6 +144,11 @@ int die_get_var_range(Dwarf_Die *sp_die, Dwarf_Die *vr_die, struct strbuf *buf);
>  Dwarf_Die *die_find_variable_by_reg(Dwarf_Die *sc_die, Dwarf_Addr pc, int reg,
>  				    Dwarf_Die *die_mem);
>  
> +/* Find a (global) variable located in the 'addr' */
> +Dwarf_Die *die_find_variable_by_addr(Dwarf_Die *sc_die, Dwarf_Addr pc,
> +				     Dwarf_Addr addr, Dwarf_Die *die_mem,
> +				     int *offset);
> +
>  #else /*  HAVE_DWARF_GETLOCATIONS_SUPPORT */
>  
>  static inline int die_get_var_range(Dwarf_Die *sp_die __maybe_unused,
> @@ -161,6 +166,15 @@ static inline Dwarf_Die *die_find_variable_by_reg(Dwarf_Die *sc_die __maybe_unus
>  	return NULL;
>  }
>  
> +static inline Dwarf_Die *die_find_variable_by_addr(Dwarf_Die *sc_die __maybe_unused,
> +						   Dwarf_Addr pc __maybe_unused,
> +						   Dwarf_Addr addr __maybe_unused,
> +						   Dwarf_Die *die_mem __maybe_unused,
> +						   int *offset __maybe_unused)
> +{
> +	return NULL;
> +}
> +
>  #endif /* HAVE_DWARF_GETLOCATIONS_SUPPORT */
>  
>  #endif /* _DWARF_AUX_H */
> -- 
> 2.42.0.655.g421f12c284-goog
> 
> 


-- 
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>