Re: [PATCH 2/7] perf dwarf-aux: Add die_same_file() and die_get_type_die()

[Namhyung Kim]

+Cc Masami

On Sat, Sep 19, 2026 at 05:40:22PM -0300, Arnaldo Carvalho de Melo wrote:
> From: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> 
> dwarf_dieoffset() is relative to the file the DIE is in, so an offset
> saved by die_collect_vars()/die_collect_global_vars() is only
> meaningful in that file: add die_same_file(), comparing the Dwarf each
> DIE's CU belongs to, to record which file that is, and
> die_get_type_die() to resolve the offset in it, with the saved tag as a
> sanity check.  Resolving the offset in the main file instead parses
> whatever is at it when it came from the dwz common file.
> 
> dwarf_cu_getdwarf() is new in elfutils 0.160, so the libdw feature test
> probes for it and Makefile.config says 0.160.
> 
> Assisted-by: LLM
> Signed-off-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> ---
>  tools/build/feature/test-libdw.c | 13 ++++++-
>  tools/perf/Makefile.config       |  2 +-
>  tools/perf/util/dwarf-aux.c      | 62 ++++++++++++++++++++++++++++++--
>  tools/perf/util/dwarf-aux.h      | 17 +++++++++
>  4 files changed, 90 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/build/feature/test-libdw.c b/tools/build/feature/test-libdw.c
> index aabd63ca76b4d7e6..0ebe7bb4adf9d0bc 100644
> --- a/tools/build/feature/test-libdw.c
> +++ b/tools/build/feature/test-libdw.c
> @@ -49,8 +49,19 @@ int test_elfutils(void)
>  	return 0;
>  }
>  
> +/*
> + * dwarf_cu_getdwarf() needs elfutils 0.160: take its address only, so
> + * that older versions fail the probe instead of the link.
> + */
> +int test_libdw_cu_getdwarf(void)
> +{
> +	void *sym = (void *)dwarf_cu_getdwarf;
> +
> +	return sym == NULL;
> +}
> +
>  int main(void)
>  {
>  	return test_libdw() + test_libdw_unwind() + test_libdw_getlocations() +
> -	       test_libdw_getcfi() + test_elfutils();
> +	       test_libdw_getcfi() + test_libdw_cu_getdwarf() + test_elfutils();
>  }
> diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
> index 4ee7393a39f91d85..0585d71e3182c6dd 100644
> --- a/tools/perf/Makefile.config
> +++ b/tools/perf/Makefile.config
> @@ -470,7 +470,7 @@ else
>    else
>      ifneq ($(feature-libdw), 1)
>        ifndef NO_LIBDW
> -        $(warning No libdw.h found or old libdw.h found or elfutils is older than 0.157, disables dwarf support. Please install new elfutils-devel/libdw-dev)
> +        $(warning No libdw.h found or old libdw.h found or elfutils is older than 0.160, disables dwarf support. Please install new elfutils-devel/libdw-dev)
>          NO_LIBDW := 1
>        endif
>      endif # Dwarf support
> diff --git a/tools/perf/util/dwarf-aux.c b/tools/perf/util/dwarf-aux.c
> index b5ffeea54446408d..de14d18a9d2f87d8 100644
> --- a/tools/perf/util/dwarf-aux.c
> +++ b/tools/perf/util/dwarf-aux.c
> @@ -1680,6 +1680,16 @@ Dwarf_Die *die_find_variable_by_addr(Dwarf_Die *sc_die, Dwarf_Addr addr,
>  	return result;
>  }
>  
> +/*
> + * Whether two DIEs live in the same debug file, i.e. whether their
> + * offsets have to be resolved in the same file: dwarf_dieoffset() is
> + * relative to the file the DIE is in.
> + */
> +bool die_same_file(Dwarf_Die *die_a, Dwarf_Die *die_b)
> +{
> +	return dwarf_cu_getdwarf(die_a->cu) == dwarf_cu_getdwarf(die_b->cu);
> +}
> +
>  static int __die_collect_vars_cb(Dwarf_Die *die_mem, void *arg)
>  {
>  	struct die_var_type **var_types = arg;
> @@ -1724,6 +1734,8 @@ static int __die_collect_vars_cb(Dwarf_Die *die_mem, void *arg)
>  			vt->is_reg_var_addr = true;
>  
>  		vt->die_off = dwarf_dieoffset(&type_die);
> +		vt->die_tag = dwarf_tag(&type_die);
> +		vt->from_alt = !die_same_file(die_mem, &type_die);
>  		vt->addr = start;
>  		vt->end = end;
>  		vt->has_range = (end != 0 || start != 0);
> @@ -1743,7 +1755,8 @@ static int __die_collect_vars_cb(Dwarf_Die *die_mem, void *arg)
>   *
>   * Save all variables and parameters in the @sc_die and save them to @var_types.
>   * The @var_types is a singly-linked list containing type and location info.
> - * Actual type can be retrieved using dwarf_offdie() with 'die_off' later.
> + * Actual type can be retrieved using die_get_type_die() with 'die_off',
> + * 'die_tag' and 'from_alt' later.
>   *
>   * Callers should free @var_types.
>   */
> @@ -1789,6 +1802,8 @@ static int __die_collect_global_vars_cb(Dwarf_Die *die_mem, void *arg)
>  		return DIE_FIND_CB_END;
>  
>  	vt->die_off = dwarf_dieoffset(&type_die);
> +	vt->die_tag = dwarf_tag(&type_die);
> +	vt->from_alt = !die_same_file(die_mem, &type_die);
>  	vt->addr = ops->number;
>  	vt->end = 0;
>  	vt->has_range = false;
> @@ -1800,6 +1815,48 @@ static int __die_collect_global_vars_cb(Dwarf_Die *die_mem, void *arg)
>  	return DIE_FIND_CB_SIBLING;
>  }
>  
> +/**
> + * die_get_type_die - Get a type DIE saved by die_collect_vars()
> + * @dbg: the main debug info
> + * @die_off: offset of the type DIE, from dwarf_dieoffset()
> + * @die_tag: tag that DIE had when the offset was saved
> + * @from_alt: whether the type DIE is in the dwz alt file
> + * @die_mem: where to store the resulting DIE
> + *
> + * Resolve @die_off in the file it was recorded as belonging to: the
> + * offset is only meaningful there, and there is deliberately no
> + * fallback, as resolving an alt file offset in the main file parses
> + * whatever is at it.  @die_tag is a sanity check.
> + */
> +Dwarf_Die *die_get_type_die(Dwarf *dbg, u64 die_off, int die_tag, bool from_alt,
> +			    Dwarf_Die *die_mem)
> +{
> +	Dwarf *target = dbg;
> +	Dwarf_Die die;
> +
> +	if (from_alt) {
> +		/*
> +		 * No fallback to the main file: resolving an alt file offset in it
> +		 * does not fail, it parses whatever is there as a DIE.
> +		 */
> +		target = dwarf_getalt(dbg);
> +		if (target == NULL) {
> +			pr_debug("DWARF: no alt (dwz) debug file to resolve the type DIE at offset 0x%lx in\n",
> +				 (unsigned long)die_off);
> +			return NULL;
> +		}
> +	}
> +
> +	if (dwarf_offdie(target, die_off, &die) && dwarf_tag(&die) == die_tag) {
> +		*die_mem = die;
> +		return die_mem;
> +	}
> +
> +	pr_debug("DWARF: no DIE with tag %d at offset 0x%lx in the %s debug file\n",
> +		 die_tag, (unsigned long)die_off, from_alt ? "alt" : "main");
> +	return NULL;
> +}
> +
>  /**
>   * die_collect_global_vars - Save all global variables
>   * @cu_die: a CU DIE
> @@ -1807,7 +1864,8 @@ static int __die_collect_global_vars_cb(Dwarf_Die *die_mem, void *arg)
>   *
>   * Save all global variables in the @cu_die and save them to @var_types.
>   * The @var_types is a singly-linked list containing type and location info.
> - * Actual type can be retrieved using dwarf_offdie() with 'die_off' later.
> + * Actual type can be retrieved using die_get_type_die() with 'die_off',
> + * 'die_tag' and 'from_alt' later.
>   *
>   * Callers should free @var_types.
>   */
> diff --git a/tools/perf/util/dwarf-aux.h b/tools/perf/util/dwarf-aux.h
> index 161f0bf980b6ee6a..299ffddab0358baa 100644
> --- a/tools/perf/util/dwarf-aux.h
> +++ b/tools/perf/util/dwarf-aux.h
> @@ -152,6 +152,8 @@ int die_get_scopes(Dwarf_Die *cu_die, Dwarf_Addr pc, Dwarf_Die **scopes);
>  struct die_var_type {
>  	struct die_var_type *next;
>  	u64 die_off;
> +	int die_tag;
> +	bool from_alt;	/* die_off is relative to the alt (dwz) file */
>  	u64 addr;
>  	u64 end;        /* end address of location range */
>  	int reg;
> @@ -183,6 +185,21 @@ Dwarf_Die *die_find_variable_by_addr(Dwarf_Die *sc_die, Dwarf_Addr addr,
>  /* Save all variables and parameters in this scope */
>  void die_collect_vars(Dwarf_Die *sc_die, struct die_var_type **var_types);
>  
> +/*
> + * Get the type DIE saved by die_collect_vars()/die_collect_global_vars().
> + *
> + * Those save the dwarf_dieoffset() of the type DIE, which is relative
> + * to the file it lives in, so @from_alt, recorded when the offset was
> + * saved, says which file to resolve it in; @die_tag is a sanity check.
> + * Resolving an alt file offset in the main file parses whatever is at
> + * it, which is what hung 'perf report -s type'.
> + */
> +Dwarf_Die *die_get_type_die(Dwarf *dbg, u64 die_off, int die_tag, bool from_alt,
> +			    Dwarf_Die *die_mem);
> +
> +/* Whether two DIEs live in the same debug file */
> +bool die_same_file(Dwarf_Die *die_a, Dwarf_Die *die_b);
> +
>  /* Save all global variables in this CU */
>  void die_collect_global_vars(Dwarf_Die *cu_die, struct die_var_type **var_types);
>  
> -- 
> 2.53.0
>