[PATCH v2] Search for DWZ files in debug-file-directories as well
Sergio Durigan Junior
sergiodj@sergiodj.net
Sat Nov 28 20:51:37 GMT 2020
On Wednesday, November 25 2020, Luis Machado wrote:
> Hi,
>
> I like the idea of GDB trying harder to find a particular
> file. Sometimes it is a bit frustrating to deal with GDB and hardcoded
> paths.
>
> With that said, the code is a bit confusing as-is, maybe because there
> is a lot of fiddling with the paths etc.
>
> I think putting the new code into a function with the appropriate name
> would help here. Your choice.
Thanks for the review.
I will see about putting this code into a separate function.
> In any case, it looks OK for me as-is. Just a nit below...
>
> On 11/18/20 11:27 PM, Sergio Durigan Junior via Gdb-patches wrote:
>> Changes from v1:
>> - Addressed Simon's comments (new comment explaining how we try to
>> match for the current debug-file-directory; properly use
>> .erase/.insert methods -- keeping in mind that they modify the
>> string in-place).
>>
>> When Debian (and Ubuntu) builds its binaries, it (still) doesn't use
>> dwz's "--relative" option. This causes their debuginfo files to
>> carry a .gnu_debugaltlink section containing a full pathname to the
>> DWZ alt debug file, like this:
>> $ readelf -wk /usr/bin/cat
>> Contents of the .gnu_debugaltlink section:
>> Separate debug info file:
>> /usr/lib/debug/.dwz/x86_64-linux-gnu/coreutils.debug
>> Build-ID (0x14 bytes):
>> ee 76 5d 71 97 37 ce 46 99 44 32 bb e8 a9 1a ef 99 96 88 db
>> Contents of the .gnu_debuglink section:
>> Separate debug info file:
>> 06d3bee37b8c7e67b31cb2689cb351102ae73b.debug
>> CRC value: 0x53267655
>> This usually works OK, because most of the debuginfo files installed
>> via apt will be present in /usr/lib/debug anyway. However, imagine
>> the following scenario:
>> - You are using /usr/bin/cat, it crashes on you and generates a
>> corefile.
>> - You don't want/need to "apt install" the debuginfo file for
>> coreutils from the repositories. Instead, you already have the
>> debuginfo files in a separate directory (e.g., $HOME/dbgsym).
>> - You start GDB and "set debug-file-directory $HOME/dbgsym".
>> You then get the following message:
>> $ gdb -ex 'set debug-file-directory ./dbgsym/usr/lib/debug' -ex
>> 'file /bin/cat' -ex 'core-file ./cat.core'
>> GNU gdb (Ubuntu 10.1-0ubuntu1) 10.1
>> ...
>> Reading symbols from /bin/cat...
>> Reading symbols from /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id/bc/06d3bee37b8c7e67b31cb2689cb351102ae73b.debug...
>> could not find '.gnu_debugaltlink' file for /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id/bc/06d3bee37b8c7e67b31cb2689cb351102ae73b.debug
>> This error happens because GDB is trying to locate the build-id
>> link (inside /home/sergio/gdb/dbgsym/usr/lib/debug/.build-id) for the
>> DWZ alt debug file, which doesn't exist. Arguably, this is a problem
>> with how dh_dwz works in Debian, and it's something I'm also planning
>> to tackle. But, back at the problem at hand.
>> Besides not being able to find the build-id link in the directory
>> mentioned above, GDB also tried to open the DWZ alt file using its
>> filename. The problem here is that, since we don't have the distro's
>> debuginfo installed, it can't find anything under /usr/lib/debug that
>> satisfies it.
>> It occurred to me that a good way to workaround this problem is to
>> actually try to locate the DWZ alt debug file inside the
>> debug-file-directories (that were likely provided by the user). So
>> this is what the proposed patch does.
>> The idea here is simple: get the filename extracted from the
>> .gnu_debugaltlink section, and manipulate it in order to replace the
>> initial part of the path (everything before "/.dwz/") by whatever
>> debug-file-directories the user might have provided.
>> I talked with Mark Wielaard and he agrees this is a sensible
>> approach.
>> In fact, apparently this is something that eu-readelf also does.
>> I regtested this code, and no regressions were found.
>> 2020-11-14 Sergio Durigan Junior <sergiodj@sergiodj.net>
>> * dwarf2/read.c (dwarf2_get_dwz_file): Convert 'filename' to a
>> std::string. Implement ability to search for DWZ files in the
>> debug-file-directories provided by the user as well.
>> ---
>> gdb/dwarf2/read.c | 73 +++++++++++++++++++++++++++++++++++++++++++----
>> 1 file changed, 67 insertions(+), 6 deletions(-)
>> diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
>> index 3c59826291..c462b9bb2c 100644
>> --- a/gdb/dwarf2/read.c
>> +++ b/gdb/dwarf2/read.c
>> @@ -2190,7 +2190,7 @@ locate_dwz_sections (bfd *abfd, asection *sectp, dwz_file *dwz_file)
>> struct dwz_file *
>> dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd)
>> {
>> - const char *filename;
>> + std::string filename;
>> bfd_size_type buildid_len_arg;
>> size_t buildid_len;
>> bfd_byte *buildid;
>> @@ -2216,19 +2216,17 @@ dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd)
>> filename = data.get ();
>> - std::string abs_storage;
>> - if (!IS_ABSOLUTE_PATH (filename))
>> + if (!IS_ABSOLUTE_PATH (filename.c_str ()))
>> {
>> gdb::unique_xmalloc_ptr<char> abs
>> = gdb_realpath (bfd_get_filename (per_bfd->obfd));
>> - abs_storage = ldirname (abs.get ()) + SLASH_STRING +
>> filename;
>> - filename = abs_storage.c_str ();
>> + filename = ldirname (abs.get ()) + SLASH_STRING + filename;
>> }
>> /* First try the file name given in the section. If that
>> doesn't
>> work, try to use the build-id instead. */
>> - gdb_bfd_ref_ptr dwz_bfd (gdb_bfd_open (filename, gnutarget));
>> + gdb_bfd_ref_ptr dwz_bfd (gdb_bfd_open (filename.c_str (), gnutarget));
>> if (dwz_bfd != NULL)
>> {
>> if (!build_id_verify (dwz_bfd.get (), buildid_len, buildid))
>> @@ -2238,6 +2236,69 @@ dwarf2_get_dwz_file (dwarf2_per_bfd *per_bfd)
>> if (dwz_bfd == NULL)
>> dwz_bfd = build_id_to_debug_bfd (buildid_len, buildid);
>> + if (dwz_bfd == nullptr)
>> + {
>> + /* If the user has provided us with different
>> + debug-file-directories, we can try them in order. */
>> + size_t dwz_pos = filename.find ("/.dwz/");
>> +
>> + if (dwz_pos != std::string::npos)
>> + {
>> + std::vector<gdb::unique_xmalloc_ptr<char>> debugdir_vec
>> + = dirnames_to_char_ptr_vec (debug_file_directory);
>> +
>> + for (const gdb::unique_xmalloc_ptr<char> &debugdir : debugdir_vec)
>> + {
>> + /* The idea is to iterate over the
>> + debug-file-directories provided by the user and
>> + replace the hard-coded path in the "filename" by each
>> + debug-file-directory.
>> +
>> + For example, suppose that filename is:
>> +
>> + /usr/lib/debug/.dwz/foo.dwz
>> +
>> + And suppose that we have "$HOME/bar" as the
>> + debug-file-directory. We would then adjust filename
>> + to look like:
>> +
>> + $HOME/bar/.dwz/foo.dwz
>> +
>> + which would hopefully allow us to find the alt debug
>> + file. */
>> + std::string ddir = debugdir.get ();
>> +
>> + /* Check whether the beginning of FILENAME is DDIR. If
>> + it is, then we are dealing with a file which we
>> + already attempted to open before, so we just skip it
>> + and continue processing the reamining
>
> remaining?
Fixed.
>> + debug-file-directories. */
>
> debug-file-directories doesn't exist in the code. I suppose this
> should be "debug file directories" or "debug-file-directory's"?
OK, I see your rationale. I don't mind having debug-file-directories,
but I replaced it with "debug file directories".
Thanks for the review.
--
Sergio
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