| Message ID | 1449768978-3872-1-git-send-email-dkg@fifthhorseman.net |
|---|---|
| State | New |
| Headers | show |
On Thu 2015-12-10 12:36:18 -0500, Daniel Kahn Gillmor wrote: > Work on the reproducible-builds project [0] has identified that build > paths are one cause of output variation between builds. This > changeset allows users to avoid this variation when building C objects > with debug symbols, while leaving the default behavior unchanged. I've now opened a bugzilla issue about this as well, with patch attached: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68848 --dkg
On Thu, 10 Dec 2015, Daniel Kahn Gillmor wrote: > Specifically, if the first character of the "old" argument is a > literal $, then gcc will treat it as an environment variable name, and > use the value of the env var for prefix mapping. I don't think a literal $ in option arguments is a good idea; it's far too hard to pass through a sequence of shells and makefiles that you typically get in recursive make. You end up with things like '-Wl,-rpath,'\''\\\$$\$$\\\$$\$$ORIGIN'\''/../' (part of a process for using $ORIGIN when linking GDB) if you try.
On Thu 2015-12-10 18:59:33 -0500, Joseph Myers wrote: > On Thu, 10 Dec 2015, Daniel Kahn Gillmor wrote: > >> Specifically, if the first character of the "old" argument is a >> literal $, then gcc will treat it as an environment variable name, and >> use the value of the env var for prefix mapping. > > I don't think a literal $ in option arguments is a good idea; it's far too > hard to pass through a sequence of shells and makefiles that you typically > get in recursive make. You end up with things like > '-Wl,-rpath,'\''\\\$$\$$\\\$$\$$ORIGIN'\''/../' (part of a process for > using $ORIGIN when linking GDB) if you try. yow, that's truly monstrous! Is there a different symbol or string you'd be OK using instead for the same approach? What about looking for an "ENV:" prefix? so something like: -fdebug-prefix-map=ENV:SOURCE_BUILD_DIR=/usr/src wdyt? I could rework the patch pretty easily if that seems acceptable. --dkg
I just found a related patch again that we have in NetBSD's gcc that allows fixing __FILE__ references. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=47047 No progress since 2010 in getting this included though :( Thomas On Thu, Dec 10, 2015 at 12:36:18PM -0500, Daniel Kahn Gillmor wrote: > Work on the reproducible-builds project [0] has identified that build > paths are one cause of output variation between builds. This > changeset allows users to avoid this variation when building C objects > with debug symbols, while leaving the default behavior unchanged. > > Background > ---------- > > gcc includes the build path in any generated DWARF debugging symbols, > specifically in DW_AT_comp_dir, but allows the embedded path to be > changed via -fdebug-prefix-map. > > When -fdebug-prefix-map is used with the current build path, it > removes the build path from DW_AT_comp_dir but places it instead in > DW_AT_producer, so the reproducibility problem isn't resolved. > > When building software for binary redistribution, the actual build > path on the build machine is irrelevant, and doesn't need to be > exposed in the debug symbols. > > Resolution > ---------- > > This patch extends the first argument to -fdebug-prefix-map ("old") to > be able to read from the environment, which allows a packager to avoid > embedded build paths in the debugging symbols with something like: > > export SOURCE_BUILD_DIR="$(pwd)" > gcc -fdebug-prefix-map=\$SOURCE_BUILD_DIR=/usr/src > > Details > ------- > > Specifically, if the first character of the "old" argument is a > literal $, then gcc will treat it as an environment variable name, and > use the value of the env var for prefix mapping. > > As a result, DW_AT_producer contains the literal envvar name, > DW_AT_comp_dir contains the transformed build path, and the actual > build path is not at all present in the generated object file. > > This has been tested successfully on amd64 machines, and i see no > reason why it would be platform-specific. > > More discussion of alternate approaches considered and discarded in > the development of this change can be found at [1] for those > interested. > > Feedback welcome! > > [0] https://reproducible-builds.org > [1] https://lists.alioth.debian.org/pipermail/reproducible-builds/Week-of-Mon-20151130/004051.html > --- > gcc/doc/invoke.texi | 4 +++- > gcc/final.c | 27 +++++++++++++++++++++++++-- > 2 files changed, 28 insertions(+), 3 deletions(-) > > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi > index 5256031..234432f 100644 > --- a/gcc/doc/invoke.texi > +++ b/gcc/doc/invoke.texi > @@ -6440,7 +6440,9 @@ link processing time. Merging is enabled by default. > @item -fdebug-prefix-map=@var{old}=@var{new} > @opindex fdebug-prefix-map > When compiling files in directory @file{@var{old}}, record debugging > -information describing them as in @file{@var{new}} instead. > +information describing them as in @file{@var{new}} instead. If > +@file{@var{old}} starts with a @samp{$}, the corresponding environment > +variable will be dereferenced, and its value will be used instead. > > @item -fno-dwarf2-cfi-asm > @opindex fdwarf2-cfi-asm > diff --git a/gcc/final.c b/gcc/final.c > index 8cb5533..bc43b61 100644 > --- a/gcc/final.c > +++ b/gcc/final.c > @@ -1525,6 +1525,9 @@ add_debug_prefix_map (const char *arg) > { > debug_prefix_map *map; > const char *p; > + char *env; > + const char *old; > + size_t oldlen; > > p = strchr (arg, '='); > if (!p) > @@ -1532,9 +1535,29 @@ add_debug_prefix_map (const char *arg) > error ("invalid argument %qs to -fdebug-prefix-map", arg); > return; > } > + if (*arg == '$') > + { > + env = xstrndup (arg+1, p - (arg+1)); > + old = getenv(env); > + if (!old) > + { > + warning (0, "environment variable %qs not set in argument to " > + "-fdebug-prefix-map", env); > + free(env); > + return; > + } > + oldlen = strlen(old); > + free(env); > + } > + else > + { > + old = xstrndup (arg, p - arg); > + oldlen = p - arg; > + } > + > map = XNEW (debug_prefix_map); > - map->old_prefix = xstrndup (arg, p - arg); > - map->old_len = p - arg; > + map->old_prefix = old; > + map->old_len = oldlen; > p++; > map->new_prefix = xstrdup (p); > map->new_len = strlen (p); > -- > 2.6.2 > > _______________________________________________ > rb-general@lists.reproducible-builds.org mailing list > > To unsubscribe or change your options, please visit: > https://lists.reproducible-builds.org/listinfo/rb-general >
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi index 5256031..234432f 100644 --- a/gcc/doc/invoke.texi +++ b/gcc/doc/invoke.texi @@ -6440,7 +6440,9 @@ link processing time. Merging is enabled by default. @item -fdebug-prefix-map=@var{old}=@var{new} @opindex fdebug-prefix-map When compiling files in directory @file{@var{old}}, record debugging -information describing them as in @file{@var{new}} instead. +information describing them as in @file{@var{new}} instead. If +@file{@var{old}} starts with a @samp{$}, the corresponding environment +variable will be dereferenced, and its value will be used instead. @item -fno-dwarf2-cfi-asm @opindex fdwarf2-cfi-asm diff --git a/gcc/final.c b/gcc/final.c index 8cb5533..bc43b61 100644 --- a/gcc/final.c +++ b/gcc/final.c @@ -1525,6 +1525,9 @@ add_debug_prefix_map (const char *arg) { debug_prefix_map *map; const char *p; + char *env; + const char *old; + size_t oldlen; p = strchr (arg, '='); if (!p) @@ -1532,9 +1535,29 @@ add_debug_prefix_map (const char *arg) error ("invalid argument %qs to -fdebug-prefix-map", arg); return; } + if (*arg == '$') + { + env = xstrndup (arg+1, p - (arg+1)); + old = getenv(env); + if (!old) + { + warning (0, "environment variable %qs not set in argument to " + "-fdebug-prefix-map", env); + free(env); + return; + } + oldlen = strlen(old); + free(env); + } + else + { + old = xstrndup (arg, p - arg); + oldlen = p - arg; + } + map = XNEW (debug_prefix_map); - map->old_prefix = xstrndup (arg, p - arg); - map->old_len = p - arg; + map->old_prefix = old; + map->old_len = oldlen; p++; map->new_prefix = xstrdup (p); map->new_len = strlen (p);
Work on the reproducible-builds project [0] has identified that build paths are one cause of output variation between builds. This changeset allows users to avoid this variation when building C objects with debug symbols, while leaving the default behavior unchanged. Background ---------- gcc includes the build path in any generated DWARF debugging symbols, specifically in DW_AT_comp_dir, but allows the embedded path to be changed via -fdebug-prefix-map. When -fdebug-prefix-map is used with the current build path, it removes the build path from DW_AT_comp_dir but places it instead in DW_AT_producer, so the reproducibility problem isn't resolved. When building software for binary redistribution, the actual build path on the build machine is irrelevant, and doesn't need to be exposed in the debug symbols. Resolution ---------- This patch extends the first argument to -fdebug-prefix-map ("old") to be able to read from the environment, which allows a packager to avoid embedded build paths in the debugging symbols with something like: export SOURCE_BUILD_DIR="$(pwd)" gcc -fdebug-prefix-map=\$SOURCE_BUILD_DIR=/usr/src Details ------- Specifically, if the first character of the "old" argument is a literal $, then gcc will treat it as an environment variable name, and use the value of the env var for prefix mapping. As a result, DW_AT_producer contains the literal envvar name, DW_AT_comp_dir contains the transformed build path, and the actual build path is not at all present in the generated object file. This has been tested successfully on amd64 machines, and i see no reason why it would be platform-specific. More discussion of alternate approaches considered and discarded in the development of this change can be found at [1] for those interested. Feedback welcome! [0] https://reproducible-builds.org [1] https://lists.alioth.debian.org/pipermail/reproducible-builds/Week-of-Mon-20151130/004051.html --- gcc/doc/invoke.texi | 4 +++- gcc/final.c | 27 +++++++++++++++++++++++++-- 2 files changed, 28 insertions(+), 3 deletions(-)