> rm -rf /usr/lib/debug/.build-id

May I suggest you don't do that :)

Could you give an actual example (where the user doesn't blow away the system .build-id directory)? I do think this is a bug. But I am wondering how common this is. In general dwz is a system/packager tool, installing .debug and dwz multi files without corresponding build-ids would normally be a packaging bug.

.gnu_debugaltlink being relative to the .debug file makes sense. But that is somewhat hard to get at in general. We don't know where the given Elf for a Dwarf comes from (it might not even be a file).

The original design had new libdw methods dwarf_getalt () and dwarf_addalt () that let you set it on a new Dwarf. Those didn't make it in the final patch. We might have to introduce those then, so eu-readelf can use them to explicitly set when they cannot be found by build-id.

See https://lists.fedorahosted.org/pipermail/elfutils-devel/2012-June/002352.html for the original design including those methods.

(In reply to Mark Wielaard from comment #1)
> Could you give an actual example (where the user doesn't blow away the
> system .build-id directory)?

rpmbuildlocal -bb prelink.spec
exx prelink-debuginfo-0.5.0-1.fc19.x86_64.rpm
eu-readelf --debug-dump=info prelink-debuginfo-0.5.0-1.fc19.x86_64/usr/lib/debug/usr/sbin/prelink.debug

The first two lines are custom scripts with obvious functionality.


So Dwarf maybe should contain its filename.

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This is still an issue as discussed in a recent thread on elfutils-devel:
https://lists.fedorahosted.org/pipermail/elfutils-devel/2013-December/thread.html#3585

This was fixed in elfutils 0.159.

libdw: Support for DWZ multifile forms DW_FORM_GNU_ref_alt and
       DW_FORM_GNU_strp_alt is now enabled by default and no longer
       experimental. Added new functions dwarf_getalt and dwarf_setalt
       to get or set the alternative debug file used for the alt FORMs.

libdwfl: The dwfl_build_id_find_debuginfo and dwfl_standard_find_debuginfo
         functions will now try to resolve and set the alternative debug file.

That last change makes sure that if the build-id based approach doesn't work the fallback using a relative path name is used.

eu-readelf uses libdwfl to open the .debug file and so gets the dwz alt-file resolved automagically.

With elfutils-0.159-1.fc21.x86_64, util-linux-debuginfo-2.25-0.2.fc21.x86_64 and sudo mv /usr/lib/debug/.build-id /usr/lib/debug/no.build-id

$ strace -e trace=open eu-readelf --debug-dump=info /usr/lib/debug/usr/bin/cal.debug

open("/usr/lib/debug/.build-id/46/828366f0fe68634cfc16f27440598388d7b3df.debug", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/debug/usr/bin/../../.dwz/util-linux-2.25-0.2.fc21.x86_64", O_RDONLY) = 4