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[RFC] Apply compilation dir to source_path lookup
- From: Mike Gulick <mgulick at mathworks dot com>
- To: "gdb-patches at sourceware dot org" <gdb-patches at sourceware dot org>
- Cc: Mike Gulick <mgulick at mathworks dot com>
- Date: Thu, 5 Sep 2019 22:40:48 +0000
- Subject: [RFC] Apply compilation dir to source_path lookup
Hi,
I'm having trouble getting gdb to find the corresponding source for a
binary whose compilation directory was rewritten using
-fdebug-prefix-map. I am doing this in order to make builds
reproducible. I am using this gcc switch to remove a portion of the
compile directory, e.g.
$ cd $HOME/test/src/
$ gcc -g -o test -fdebug-prefix-map=$HOME= test.c
$ dwarfdump test
...
DW_AT_name test.c
DW_AT_comp_dir /test/src
...
When attempting to debug the resulting binary, gdb is unable to find the
source file even if I add the removed path $HOME to the gdb source
path:
$ cd $HOME
$ gdb --nh ~/test/src/test
GNU gdb (GDB) 8.3
...
(gdb) directory /home/mgulick
Source directories searched: /home/mgulick:$cdir:$cwd
(gdb) info sources
Source files for which symbols have been read in:
/test/src/test.c
Source files for which symbols will be read in on demand:
(gdb) b test.c:3
Breakpoint 1 at 0x664: file test.c, line 3.
(gdb) r
Starting program: /mathworks/home/mgulick/test/src/test
Breakpoint 1, main () at test.c:3
3 test.c: No such file or directory.
It turns out that only the source file, i.e. DW_AT_name, is used when
searching the source path. This is surprising given the example with
'/usr/src/foo-1.0/lib/foo.c' and '/mnt/cross' in the gdb documentation
here: https://sourceware.org/gdb/onlinedocs/gdb/Source-Path.html. It
seems uncommon for DW_AT_name to be the full path to the source file in
most programs I have come across. In that example from the
documentation, it is likely that DW_AT_name would be 'lib/foo.c' and
DW_AT_comp_dir would be '/usr/src/foo-1.0'.
I have implemented two different approaches to handle this, both of
which work, and I wanted feedback on a) whether you think this is a
legitimate bug/feature, and b) which approach you prefer.
Approach 1 is to include another pass in find_and_open_source where the
compilation directory is prepended to the filename before searching for
the file with openp. This allows the example I provided above to work
as-is.
Approach 2 is to allow '$cdir' to appear as part of an entry in the
source_path. Currently it is only allowed to exist as a standalone
entry. This would allow me to say 'directory /home/mgulick/$cdir', and
find_and_open_source would expand the $cdir part of this string to the
compilation directory.
I prefer approach 1 for a couple of reasons:
1) It is simpler for users to understand. It doesn't really require
understanding the difference between DW_AT_comp_dir and DW_AT_name. It
will match the source file in /home/mgulick/test.c, /test/src/test.c, or
/home/mgulick/test/src/test.c. Of course, since '$cdir' is still in
source_path, it will also look in /test/src/test/src/test.c, which is a
little confusing, but shouldn't be a problem (and we could explicitly
remove '$cdir' from the path when doing this search).
2) It seems to match the existing gdb documentation.
I dislike approach 2 because it makes the directory strings more
complicated, and it also means I'm unable to have a directory named
'/home/mgulick/$cdir' (bizarre, I know, but such things are possible).
Here is a preliminary patch for the first approach. I will go ahead and
add a test-case and re-send a formal patch review request if this seems
like an acceptable solution.
Thanks,
Mike
diff --git a/gdb/source.c b/gdb/source.c
index b27f210802..b7b5741028 100644
--- a/gdb/source.c
+++ b/gdb/source.c
@@ -1033,7 +1033,35 @@ find_and_open_source (const char *filename,
openp_flags flags = OPF_SEARCH_IN_PATH;
if (basenames_may_differ)
flags |= OPF_RETURN_REALPATH;
+
+ /* Try to locate file using filename */
result = openp (path, flags, filename, OPEN_MODE, fullname);
+ if (result < 0 && dirname != NULL)
+ {
+ /* Try to locate file using compilation dir + filename. This is helpful
+ if part of the compilation directory was removed, e.g. using gcc's
+ -fdebug-prefix-map, and we have added the missing prefix to
+ source_path. */
+ /* Allocate space for dirname, possibly an extra slash, the filename,
+ and terminating null */
+ char * cdir_filename = (char *)
+ alloca (strlen (dirname) + strlen(SLASH_STRING) + strlen (filename) + 1);
+
+ cdir_filename = strcpy (cdir_filename, dirname);
+ int len = strlen (cdir_filename); /* last char in cdir_filename */
+
+ /* Add directory separator if not provided by dirname or filename */
+ if (!(IS_DIR_SEPARATOR (cdir_filename[len]) ||
+ IS_DIR_SEPARATOR (filename[0])))
+ strcat (cdir_filename, SLASH_STRING);
+ else if (IS_DIR_SEPARATOR (cdir_filename[len]) &&
+ IS_DIR_SEPARATOR (filename[0]))
+ /* Both provide a slash, use only one */
+ cdir_filename[len] = '\0';
+ strcat(cdir_filename,filename);
+
+ result = openp(path, flags, cdir_filename, OPEN_MODE, fullname);
+ }
if (result < 0)
{
/* Didn't work. Try using just the basename. */