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Message-ID: <CAP-5=fXp4nbnMdgbXogxzQaeo3G-vVWbiTRcFi3PO+vLsB67JA@mail.gmail.com>
Date: Fri, 28 Nov 2025 00:42:15 -0800
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Tony Jones <tonyj@...e.de>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
Arnaldo Carvalho de Melo <acme@...nel.org>, Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
Jiri Olsa <jolsa@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>,
James Clark <james.clark@...aro.org>, Howard Chu <howardchu95@...il.com>,
Stephen Brennan <stephen.s.brennan@...cle.com>, linux-kernel@...r.kernel.org,
linux-perf-users@...r.kernel.org
Subject: Re: [PATCH v1] perf addr2line: Add a libdw implementation
On Thu, Nov 27, 2025 at 12:53 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Thu, Nov 27, 2025 at 03:43:34AM -0800, Ian Rogers wrote:
> > On Wed, Nov 26, 2025 at 10:27 AM Namhyung Kim <namhyung@...nel.org> wrote:
> > >
> > > On Sat, Nov 22, 2025 at 01:39:34AM -0800, Ian Rogers wrote:
> > > > Add an implementation of addr2line that uses libdw. Other addr2line
> > > > implementations or, in the case of forking addr2line, slow. Add an
> > > > implementation that caches the libdw information in the dso and uses
> > > > it to find the file and line number information.
> >
> > Thanks James and Namhyung for the reviews! I agree with James' comment
> > about a typo in the commit message.
> >
> > > My concern is the limit in the open file descriptors in case the data
> > > touched a lot of different libraries. The DSO code has some logic to
> > > deal with it but I'm not sure if we can share that since libdw seems to
> > > want to own the FD.
> >
> > The code opens the FD:
> >
> > + fd = open(dso_name, O_RDONLY);
> > + if (fd < 0)
> > + return 0;
> > +
> > + dwfl = dwfl_begin(&offline_callbacks);
> > + if (!dwfl) {
> > + close(fd);
> > + return 0;
> > + }
> >
> > It then uses the FD and closes it (the close is hidden in libdw itself):
> >
> > + /*
> > + * If the report is successful, the file descriptor fd
> > is consumed
> > + * and closed by the Dwfl. If not, it is not closed.
> > + */
> > + mod = dwfl_report_offline(dwfl, dso_name, dso_name, fd);
> >
> > So it is possible we exhaust all the file descriptors if there are
> > multiple concurrent calls to libdw__addr2line and every dso has
> > missing libdw dwfl data... but because the open/close are close
> > together and that 1 FD is say small to the FDs needed for the
> > cmd__addr2line, I don't think it is a problem we need to specifically
> > handle. Were the FD kept open until the dso was deleted, I'd agree
> > with you.
>
> Oh, I thought libdwfl keeps the FD open until you call dwfl_end().
> Are you sure the dwfl function would close the FD? I've quickly checked
> the code, but it doesn't seem to close if you pass an FD manually.
So in the documentation there is:
https://sourceware.org/git/?p=elfutils.git;a=blob;f=libdwfl/libdwfl.h;h=90523283c9359b09a16709a8422917a44257932f;hb=HEAD#l155
```
/* ...On success, FD is consumed by the
library, ... */
extern Dwfl_Module *dwfl_report_elf (Dwfl *dwfl, const char *name,
const char *file_name, int fd,
GElf_Addr base, bool add_p_vaddr);
/* Similar, but report the module for offline use. ... */
extern Dwfl_Module *dwfl_report_offline (Dwfl *dwfl, const char *name,
const char *file_name, int fd);
```
Running with gdb I see:
```
$ perf record perf test -w noploop
Lowering default frequency rate from 4000 to 2500.
Please consider tweaking /proc/sys/kernel/perf_event_max_sample_rate.
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.611 MB perf.data (2534 samples) ]
$ gdb --args perf --no-pager annotate -l --stdio
...
Reading symbols from /tmp/perf/perf...
(gdb) b libdw__addr2line
Breakpoint 1 at 0x392b98: file util/libdw.c, line 25.
(gdb) r
Starting program: /tmp/perf/perf --no-pager annotate -l --stdio
...
Breakpoint 1, libdw__addr2line (dso_name=0x555557b41270
"/root/.debug/.build-id/4c/72a04e484e8636631219f4454cc64a674a5f57/elf",
addr=1903613,
file=0x7fffffffa830, line_nr=0x7fffffffa82c, dso=0x5555569eba20,
unwind_inlines=false, node=0x0, sym=0x0) at util/libdw.c:25
25 {
(gdb) n
31 Dwfl *dwfl = dso__a2l_libdw(dso);
(gdb)
38 if (!dwfl) {
(gdb)
45 fd = open(dso_name, O_RDONLY);
(gdb)
46 if (fd < 0)
(gdb) p fd
$1 = 5
(gdb) n
49 dwfl = dwfl_begin(&offline_callbacks);
(gdb) b close
Breakpoint 2 at 0x7fffedf09760: close. (15 locations)
(gdb) c
Continuing.
Breakpoint 2.1, __GI___close (fd=5) at ../sysdeps/unix/sysv/linux/close.c:27
warning: 27 ../sysdeps/unix/sysv/linux/close.c: No such file or directory
(gdb) bt
#0 __GI___close (fd=5) at ../sysdeps/unix/sysv/linux/close.c:27
#1 0x00007ffff7e0e122 in ?? () from /lib/x86_64-linux-gnu/libdw.so.1
#2 0x00007ffff7e0e6f7 in ?? () from /lib/x86_64-linux-gnu/libdw.so.1
#3 0x00005555558e6c3c in libdw__addr2line (dso_name=0x555557b41270
"/root/.debug/.build-id/4c/72a04e484e8636631219f4454cc64a674a5f57/elf",
addr=1903613, file=0x7fffffffa830, line_nr=0x7fffffffa82c,
dso=0x5555569eba20, unwind_inlines=false, node=0x0, sym=0x0) at
util/libdw.c:59
#4 0x000055555584c861 in addr2line (dso_name=0x555557b41270
"/root/.debug/.build-id/4c/72a04e484e8636631219f4454cc64a674a5f57/elf",
addr=1903613, file=0x7fffffffa830, line_nr=0x7fffffffa82c,
dso=0x5555569eba20, unwind_inlines=false, node=0x0, sym=0x0)
at util/srcline.c:124
#5 0x000055555584ca9c in __get_srcline (dso=0x5555569eba20,
addr=1903613, sym=0x0, show_sym=false, show_addr=true,
unwind_inlines=false,
ip=1903613) at util/srcline.c:181
#6 0x000055555584ce21 in get_srcline (dso=0x5555569eba20,
addr=1903613, sym=0x0, show_sym=false, show_addr=true, ip=1903613)
at util/srcline.c:259
#7 0x0000555555754a63 in annotation__calc_lines
(notes=0x5555586d02d0, ms=0x555558665600, root=0x7fffffffa9a0) at
util/annotate.c:1651
#8 0x0000555555754b20 in symbol__calc_lines (ms=0x555558665600,
root=0x7fffffffa9a0) at util/annotate.c:1663
#9 0x0000555555754f22 in hist_entry__tty_annotate (he=0x555558665560,
evsel=0x5555561dc6e0) at util/annotate.c:1728
#10 0x00005555555a829a in hist_entry__stdio_annotate
(he=0x555558665560, evsel=0x5555561dc6e0, ann=0x7fffffffcc90) at
builtin-annotate.c:332
#11 0x00005555555a8ed4 in hists__find_annotations
(hists=0x5555561dc998, evsel=0x5555561dc6e0, ann=0x7fffffffcc90) at
builtin-annotate.c:545
#12 0x00005555555a9404 in __cmd_annotate (ann=0x7fffffffcc90) at
builtin-annotate.c:655
#13 0x00005555555aa5c6 in cmd_annotate (argc=0, argv=0x7fffffffe318)
at builtin-annotate.c:949
#14 0x0000555555655e11 in run_builtin (p=0x555556128650
<commands+432>, argc=3, argv=0x7fffffffe318) at perf.c:349
#15 0x00005555556560a9 in handle_internal_command (argc=3,
argv=0x7fffffffe318) at perf.c:401
#16 0x0000555555656202 in run_argv (argcp=0x7fffffffe10c,
argv=0x7fffffffe100) at perf.c:445
#17 0x000055555565654b in main (argc=3, argv=0x7fffffffe318) at perf.c:553
(gdb) frame 3
#3 0x00005555558e6c3c in libdw__addr2line (dso_name=0x555557b41270
"/root/.debug/.build-id/4c/72a04e484e8636631219f4454cc64a674a5f57/elf",
addr=1903613, file=0x7fffffffa830, line_nr=0x7fffffffa82c,
dso=0x5555569eba20, unwind_inlines=false, node=0x0, sym=0x0) at
util/libdw.c:59
59 mod = dwfl_report_offline(dwfl, dso_name, dso_name, fd);
```
So I think everything is working as explained. The open happens
returning fd 5, dwfl_report_offline is called and on success it closes
fd 5. Were the dwfl_report_offline to fail then the error path in
libdw__addr2line closes fd.
Looking at older versions of the code (10 years old) the header file
documentation is the same, so I'm not sure why you are seeing a
different behavior.
Thanks,
Ian
> Thanks,
> Namhyung
>
> >
> > > Also, have you checked if this generates the exactly same output with
> > > other implementations?
> >
> > So the code passes `perf test` and I was checking functionality with
> > perf annotate, top, etc. What I saw looked good, but it may not have
> > been exhaustive. I didn't specifically create a test that compares the
> > output of the different addr2line implementations. Such a test would
> > be possible, it's not something we've done elsewhere.
> >
> > Thanks,
> > Ian
> >
> > > Thanks,
> > > Namhyung
> > >
> > > >
> > > > Signed-off-by: Ian Rogers <irogers@...gle.com>
> > > > ---
> > > > tools/perf/util/Build | 1 +
> > > > tools/perf/util/dso.c | 2 +
> > > > tools/perf/util/dso.h | 11 +++
> > > > tools/perf/util/libdw.c | 136 ++++++++++++++++++++++++++++++++++++++
> > > > tools/perf/util/libdw.h | 60 +++++++++++++++++
> > > > tools/perf/util/srcline.c | 5 ++
> > > > 6 files changed, 215 insertions(+)
> > > > create mode 100644 tools/perf/util/libdw.c
> > > > create mode 100644 tools/perf/util/libdw.h
> > > >
> > > > diff --git a/tools/perf/util/Build b/tools/perf/util/Build
> > > > index 1c2a43e1dc68..2bed6274e248 100644
> > > > --- a/tools/perf/util/Build
> > > > +++ b/tools/perf/util/Build
> > > > @@ -224,6 +224,7 @@ perf-util-$(CONFIG_LIBDW) += dwarf-regs-powerpc.o
> > > > perf-util-$(CONFIG_LIBDW) += dwarf-regs-x86.o
> > > > perf-util-$(CONFIG_LIBDW) += debuginfo.o
> > > > perf-util-$(CONFIG_LIBDW) += annotate-data.o
> > > > +perf-util-$(CONFIG_LIBDW) += libdw.o
> > > >
> > > > perf-util-$(CONFIG_LIBDW_DWARF_UNWIND) += unwind-libdw.o
> > > > perf-util-$(CONFIG_LOCAL_LIBUNWIND) += unwind-libunwind-local.o
> > > > diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
> > > > index 344e689567ee..06980844c014 100644
> > > > --- a/tools/perf/util/dso.c
> > > > +++ b/tools/perf/util/dso.c
> > > > @@ -32,6 +32,7 @@
> > > > #include "string2.h"
> > > > #include "vdso.h"
> > > > #include "annotate-data.h"
> > > > +#include "libdw.h"
> > > >
> > > > static const char * const debuglink_paths[] = {
> > > > "%.0s%s",
> > > > @@ -1605,6 +1606,7 @@ void dso__delete(struct dso *dso)
> > > > auxtrace_cache__free(RC_CHK_ACCESS(dso)->auxtrace_cache);
> > > > dso_cache__free(dso);
> > > > dso__free_a2l(dso);
> > > > + dso__free_a2l_libdw(dso);
> > > > dso__free_symsrc_filename(dso);
> > > > nsinfo__zput(RC_CHK_ACCESS(dso)->nsinfo);
> > > > mutex_destroy(dso__lock(dso));
> > > > diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
> > > > index f8ccb9816b89..4aee23775054 100644
> > > > --- a/tools/perf/util/dso.h
> > > > +++ b/tools/perf/util/dso.h
> > > > @@ -268,6 +268,7 @@ DECLARE_RC_STRUCT(dso) {
> > > > const char *short_name;
> > > > const char *long_name;
> > > > void *a2l;
> > > > + void *a2l_libdw;
> > > > char *symsrc_filename;
> > > > #if defined(__powerpc__)
> > > > void *dwfl; /* DWARF debug info */
> > > > @@ -334,6 +335,16 @@ static inline void dso__set_a2l(struct dso *dso, void *val)
> > > > RC_CHK_ACCESS(dso)->a2l = val;
> > > > }
> > > >
> > > > +static inline void *dso__a2l_libdw(const struct dso *dso)
> > > > +{
> > > > + return RC_CHK_ACCESS(dso)->a2l_libdw;
> > > > +}
> > > > +
> > > > +static inline void dso__set_a2l_libdw(struct dso *dso, void *val)
> > > > +{
> > > > + RC_CHK_ACCESS(dso)->a2l_libdw = val;
> > > > +}
> > > > +
> > > > static inline unsigned int dso__a2l_fails(const struct dso *dso)
> > > > {
> > > > return RC_CHK_ACCESS(dso)->a2l_fails;
> > > > diff --git a/tools/perf/util/libdw.c b/tools/perf/util/libdw.c
> > > > new file mode 100644
> > > > index 000000000000..c4331fa8e1a3
> > > > --- /dev/null
> > > > +++ b/tools/perf/util/libdw.c
> > > > @@ -0,0 +1,136 @@
> > > > +// SPDX-License-Identifier: GPL-2.0
> > > > +#include "dso.h"
> > > > +#include "libdw.h"
> > > > +#include "srcline.h"
> > > > +#include "symbol.h"
> > > > +#include "dwarf-aux.h"
> > > > +#include <fcntl.h>
> > > > +#include <unistd.h>
> > > > +#include <elfutils/libdwfl.h>
> > > > +
> > > > +void dso__free_a2l_libdw(struct dso *dso)
> > > > +{
> > > > + Dwfl *dwfl = dso__a2l_libdw(dso);
> > > > +
> > > > + if (dwfl) {
> > > > + dwfl_end(dwfl);
> > > > + dso__set_a2l_libdw(dso, NULL);
> > > > + }
> > > > +}
> > > > +
> > > > +int libdw__addr2line(const char *dso_name, u64 addr,
> > > > + char **file, unsigned int *line_nr,
> > > > + struct dso *dso, bool unwind_inlines,
> > > > + struct inline_node *node, struct symbol *sym)
> > > > +{
> > > > + static const Dwfl_Callbacks offline_callbacks = {
> > > > + .find_debuginfo = dwfl_standard_find_debuginfo,
> > > > + .section_address = dwfl_offline_section_address,
> > > > + .find_elf = dwfl_build_id_find_elf,
> > > > + };
> > > > + Dwfl *dwfl = dso__a2l_libdw(dso);
> > > > + Dwfl_Module *mod;
> > > > + Dwfl_Line *dwline;
> > > > + Dwarf_Addr bias;
> > > > + const char *src;
> > > > + int lineno;
> > > > +
> > > > + if (!dwfl) {
> > > > + /*
> > > > + * Initialize Dwfl session.
> > > > + * We need to open the DSO file to report it to libdw.
> > > > + */
> > > > + int fd;
> > > > +
> > > > + fd = open(dso_name, O_RDONLY);
> > > > + if (fd < 0)
> > > > + return 0;
> > > > +
> > > > + dwfl = dwfl_begin(&offline_callbacks);
> > > > + if (!dwfl) {
> > > > + close(fd);
> > > > + return 0;
> > > > + }
> > > > +
> > > > + /*
> > > > + * If the report is successful, the file descriptor fd is consumed
> > > > + * and closed by the Dwfl. If not, it is not closed.
> > > > + */
> > > > + mod = dwfl_report_offline(dwfl, dso_name, dso_name, fd);
> > > > + if (!mod) {
> > > > + dwfl_end(dwfl);
> > > > + close(fd);
> > > > + return 0;
> > > > + }
> > > > +
> > > > + dwfl_report_end(dwfl, /*removed=*/NULL, /*arg=*/NULL);
> > > > + dso__set_a2l_libdw(dso, dwfl);
> > > > + } else {
> > > > + /* Dwfl session already initialized, get module for address. */
> > > > + mod = dwfl_addrmodule(dwfl, addr);
> > > > + }
> > > > +
> > > > + if (!mod)
> > > > + return 0;
> > > > +
> > > > + /* Find source line information for the address. */
> > > > + dwline = dwfl_module_getsrc(mod, addr);
> > > > + if (!dwline)
> > > > + return 0;
> > > > +
> > > > + /* Get line information. */
> > > > + src = dwfl_lineinfo(dwline, &addr, &lineno, /*col=*/NULL, /*mtime=*/NULL, /*length=*/NULL);
> > > > +
> > > > + if (file)
> > > > + *file = src ? strdup(src) : NULL;
> > > > + if (line_nr)
> > > > + *line_nr = lineno;
> > > > +
> > > > + /* Optionally unwind inline function call chain. */
> > > > + if (unwind_inlines && node && src) {
> > > > + Dwarf_Die *cudie = dwfl_module_addrdie(mod, addr, &bias);
> > > > + Dwarf_Die *scopes = NULL;
> > > > + int nscopes;
> > > > +
> > > > + if (!cudie)
> > > > + return 1;
> > > > +
> > > > + nscopes = die_get_scopes(cudie, addr, &scopes);
> > > > + if (nscopes > 0) {
> > > > + int i;
> > > > + const char *call_file = src;
> > > > + unsigned int call_line = lineno;
> > > > +
> > > > + for (i = 0; i < nscopes; i++) {
> > > > + Dwarf_Die *die = &scopes[i];
> > > > + struct symbol *inline_sym;
> > > > + char *srcline = NULL;
> > > > + int tag = dwarf_tag(die);
> > > > +
> > > > + /* We are interested in inlined subroutines. */
> > > > + if (tag != DW_TAG_inlined_subroutine &&
> > > > + tag != DW_TAG_subprogram)
> > > > + continue;
> > > > +
> > > > + inline_sym = new_inline_sym(dso, sym, dwarf_diename(die));
> > > > +
> > > > + if (call_file)
> > > > + srcline = srcline_from_fileline(call_file, call_line);
> > > > +
> > > > + inline_list__append(inline_sym, srcline, node);
> > > > +
> > > > + /* Update call site for next level. */
> > > > + if (tag == DW_TAG_inlined_subroutine) {
> > > > + call_file = die_get_call_file(die);
> > > > + call_line = die_get_call_lineno(die);
> > > > + } else {
> > > > + /* Reached the root subprogram. */
> > > > + break;
> > > > + }
> > > > + }
> > > > + free(scopes);
> > > > + }
> > > > + }
> > > > +
> > > > + return 1;
> > > > +}
> > > > diff --git a/tools/perf/util/libdw.h b/tools/perf/util/libdw.h
> > > > new file mode 100644
> > > > index 000000000000..0f8d7b4a11a5
> > > > --- /dev/null
> > > > +++ b/tools/perf/util/libdw.h
> > > > @@ -0,0 +1,60 @@
> > > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > > +#ifndef PERF_LIBDW_H
> > > > +#define PERF_LIBDW_H
> > > > +
> > > > +#include <linux/types.h>
> > > > +
> > > > +struct dso;
> > > > +struct inline_node;
> > > > +struct symbol;
> > > > +
> > > > +#ifdef HAVE_LIBDW_SUPPORT
> > > > +/*
> > > > + * libdw__addr2line - Convert address to source location using libdw
> > > > + * @dso_name: Name of the DSO
> > > > + * @addr: Address to resolve
> > > > + * @file: Pointer to return filename (caller must free)
> > > > + * @line_nr: Pointer to return line number
> > > > + * @dso: The dso struct
> > > > + * @unwind_inlines: Whether to unwind inline function calls
> > > > + * @node: Inline node list to append to
> > > > + * @sym: The symbol associated with the address
> > > > + *
> > > > + * This function initializes a Dwfl context for the DSO if not already present,
> > > > + * finds the source line information for the given address, and optionally
> > > > + * resolves inline function call chains.
> > > > + *
> > > > + * Returns 1 on success (found), 0 on failure (not found).
> > > > + */
> > > > +int libdw__addr2line(const char *dso_name, u64 addr, char **file,
> > > > + unsigned int *line_nr, struct dso *dso,
> > > > + bool unwind_inlines, struct inline_node *node,
> > > > + struct symbol *sym);
> > > > +
> > > > +/*
> > > > + * dso__free_a2l_libdw - Free libdw resources associated with the DSO
> > > > + * @dso: The dso to free resources for
> > > > + *
> > > > + * This function cleans up the Dwfl context used for addr2line lookups.
> > > > + */
> > > > +void dso__free_a2l_libdw(struct dso *dso);
> > > > +
> > > > +#else /* HAVE_LIBDW_SUPPORT */
> > > > +
> > > > +static inline int libdw__addr2line(const char *dso_name __maybe_unused,
> > > > + u64 addr __maybe_unused, char **file __maybe_unused,
> > > > + unsigned int *line_nr __maybe_unused,
> > > > + struct dso *dso __maybe_unused,
> > > > + bool unwind_inlines __maybe_unused,
> > > > + struct inline_node *node __maybe_unused,
> > > > + struct symbol *sym __maybe_unused)
> > > > +{
> > > > + return 0;
> > > > +}
> > > > +
> > > > +static inline void dso__free_a2l_libdw(struct dso *dso __maybe_unused)
> > > > +{
> > > > +}
> > > > +#endif /* HAVE_LIBDW_SUPPORT */
> > > > +
> > > > +#endif /* PERF_LIBDW_H */
> > > > diff --git a/tools/perf/util/srcline.c b/tools/perf/util/srcline.c
> > > > index 27c0966611ab..4b456c4d4138 100644
> > > > --- a/tools/perf/util/srcline.c
> > > > +++ b/tools/perf/util/srcline.c
> > > > @@ -6,6 +6,7 @@
> > > > #include "libbfd.h"
> > > > #include "llvm.h"
> > > > #include "symbol.h"
> > > > +#include "libdw.h"
> > > >
> > > > #include <inttypes.h>
> > > > #include <string.h>
> > > > @@ -120,6 +121,10 @@ static int addr2line(const char *dso_name, u64 addr, char **file, unsigned int *
> > > > {
> > > > int ret;
> > > >
> > > > + ret = libdw__addr2line(dso_name, addr, file, line_nr, dso, unwind_inlines, node, sym);
> > > > + if (ret > 0)
> > > > + return ret;
> > > > +
> > > > ret = llvm__addr2line(dso_name, addr, file, line_nr, dso, unwind_inlines, node, sym);
> > > > if (ret > 0)
> > > > return ret;
> > > > --
> > > > 2.52.0.rc2.455.g230fcf2819-goog
> > > >
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