Patchwork [RFC] Apply compilation dir to source_path lookup

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Submitter Andrew Burgess
Date Sept. 7, 2019, 11:50 p.m.
Message ID <20190907235059.GN6076@embecosm.com>
Download mbox | patch
Permalink /patch/34446/
State New
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Comments

Andrew Burgess - Sept. 7, 2019, 11:50 p.m.
* Mike Gulick <mgulick@mathworks.com> [2019-09-05 22:40:48 +0000]:

> 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'.

That depends on how you compile your source:

   gcc -o my.exe /usr/src/foo-1.0/lib/foo.c

would result in DW_AT_name being '/usr/src/foo-1.0/lib/foo.c' I
believe.

So I think the example is fine, though I don't think it really goes
into enough details.

> 
> 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).

I agree with your approach, I think it's nicer if things can "just
work" for the user, and this seems like a good approach.

> 
> 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.

Do you have a copyright assignment in place?  Any patch over a couple
of trivial lines requires an assignment.  Others might correct me, but
this patch would seem to be big enough to me.

> 
> 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 */

Comments need to end with '.', and there should always be two spaces
after a '.' in comments.

>    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 */

You need two spaces at the end of your comments, and can you fold
these two comments into one.

> +      char * cdir_filename = (char *)
> +	alloca (strlen (dirname) + strlen(SLASH_STRING) + strlen (filename) + 1);

Whitespace between function name and opening parenthesis.

> +
> +      cdir_filename = strcpy (cdir_filename, dirname);
> +      int len = strlen (cdir_filename);    /* last char in cdir_filename */

Comments should be formatted as sentences, so capital 'L' here, and
'.  ' and the end.

> +
> +      /* Add directory separator if not provided by dirname or filename */
> +      if (!(IS_DIR_SEPARATOR (cdir_filename[len]) ||
> +	    IS_DIR_SEPARATOR (filename[0])))

Break the line before the '||'.

> +        strcat (cdir_filename, SLASH_STRING);
> +      else if (IS_DIR_SEPARATOR (cdir_filename[len]) &&
> +	       IS_DIR_SEPARATOR (filename[0]))

Same, line break before '&&'.

> +	/* Both provide a slash, use only one */

Comment formatting.

> +	cdir_filename[len] = '\0';
> +      strcat(cdir_filename,filename);

Whitespace before opening parenthesis.

> +
> +      result = openp(path, flags, cdir_filename, OPEN_MODE, fullname);

And again.

> +    }
>    if (result < 0)
>      {
>        /* Didn't work.  Try using just the basename.  */

You'll also need to write a short ChangeLog entry for the patch.  I
also wonder if the function could be simplified by using std::string
to build the new string rather than all of the strlen/strcpy/strcat
calls?  Though I don't think that would be a requirement for
acceptance.

You might also consider extending the documentation relating to source
path searching to better describe the process - as you discuss above,
the existing documentation is not that clear on the subject.

Finally, you'd probably want to write a test.... however, I was
wondering how a test for this might be written, and ended up writing
one myself.  Feel free to use this as a starting point, or just
include this as is if it helps.

Thanks,
Andrew

---

commit c10a25ca9a1eb8f7e2cbab9b8fd652fe9feb16de
Author: Andrew Burgess <andrew.burgess@embecosm.com>
Date:   Sun Sep 8 00:19:48 2019 +0100

    gdb/testsuite: New test for mapped compilation directory
    
    Ensure GDB can find source files when compiling using the
    -fdebug-prefix-map GCC option.  The problem is that the compilation
    directory is not tried against other directories in the directory
    search path.
    
    gdb/testsuite/ChangeLog:
    
            * gdb.base/source-dir.c: New file.
            * gdb.base/source-dir.exp: Add extra test for mapped compilation
            directory.

Patch

diff --git a/gdb/testsuite/gdb.base/source-dir.c b/gdb/testsuite/gdb.base/source-dir.c
new file mode 100644
index 00000000000..a9bce4a1445
--- /dev/null
+++ b/gdb/testsuite/gdb.base/source-dir.c
@@ -0,0 +1,5 @@ 
+int
+main ()
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/source-dir.exp b/gdb/testsuite/gdb.base/source-dir.exp
index 048c0e95161..ecfb74ef27d 100644
--- a/gdb/testsuite/gdb.base/source-dir.exp
+++ b/gdb/testsuite/gdb.base/source-dir.exp
@@ -15,9 +15,127 @@ 
 
 standard_testfile
 
-gdb_start
+proc search_dir_list { dirs } {
+    set output "\r\nSource directories searched: "
+    append output [join $dirs "\[:;\]"]
 
-set foo "/nOtExStInG"
+    return ${output}
+}
 
-gdb_test "directory $foo/a $foo/b $foo/c" "\r\nSource directories searched: $foo/a\[:;\]$foo/b\[:;\]$foo/c\[:;\]\\\$cdir\[:;\]\\\$cwd"
-gdb_test "directory $foo/b $foo/d $foo/c" "\r\nSource directories searched: $foo/b\[:;\]$foo/d\[:;\]$foo/c\[:;\]$foo/a\[:;\]\\\$cdir\[:;\]\\\$cwd"
+
+# Check that adding directories to the search path changes the order
+# in which directories are searched.
+proc test_changing_search_directory {} {
+    gdb_start
+
+    set foo "/nOtExStInG"
+
+    gdb_test "directory $foo/a $foo/b $foo/c" \
+	[search_dir_list [list \
+			      "$foo/a" \
+			      "$foo/b" \
+			      "$foo/c" \
+			      "\\\$cdir" \
+			      "\\\$cwd"]]
+    gdb_test "directory $foo/b $foo/d $foo/c" \
+	[search_dir_list [list \
+			      "$foo/b" \
+			      "$foo/d" \
+			      "$foo/c" \
+			      "$foo/a" \
+			      "\\\$cdir" \
+			      "\\\$cwd"]]
+    gdb_exit
+}
+
+# Test that the compilation directory can also be extended with a
+# prefix from the directory search path in order to find source files.
+proc test_truncated_comp_dir {} {
+    global srcfile srcdir subdir binfile
+
+    # When we run this test the current directory will be something
+    # like this:
+    #     /some/path/to/gdb/build/testsuite/
+    # We are going to copy the source file out of the source tree into
+    # a location like this:
+    #     /some/path/to/gdb/build/testsuite/output/gdb.base/soure-dir/
+    #
+    # We will then switch to this directory and compile the source
+    # file, however, we will ask GCC to remove this prefix from the
+    # compilation directory in the debug info:
+    #     /some/path/to/gdb/build/testsuite/output/
+    #
+    # As a result the debug information will look like this:
+    #
+    #     DW_AT_name        : source-dir.c
+    #     DW_AT_comp_dir    : /gdb.base/source-dir
+    #
+    # Finally we switch back to this directory:
+    #     /some/path/to/gdb/build/testsuite/
+    #
+    # and start GDB.  There was a time when GDB would be unable to
+    # find the source file no matter what we added to the directory
+    # search path, this should now be fixed.
+
+    set original_dir [pwd]
+    set working_dir [standard_output_file ""]
+    cd ${working_dir}
+
+    set strip_dir [file normalize "${working_dir}/../.."]
+
+    set new_srcfile [standard_output_file ${srcfile}]
+    remote_exec host "cp ${srcdir}/${subdir}/${srcfile} ${new_srcfile}"
+
+    set options \
+	"debug additional_flags=-fdebug-prefix-map=${strip_dir}="
+    if  { [gdb_compile "${srcfile}" "${binfile}" \
+	       executable ${options}] != "" } {
+	untested "failed to compile"
+	return -1
+    }
+
+    cd ${original_dir}
+
+    clean_restart ${binfile}
+
+    gdb_test_no_output "set directories \$cdir:\$cwd"
+    gdb_test "show directories" \
+	"\r\nSource directories searched: \\\$cdir\[:;\]\\\$cwd"
+
+    if ![runto_main] then {
+	fail "can't run to main"
+	return 0
+    }
+
+    gdb_test "info source" \
+	[multi_line \
+	     "Current source file is ${srcfile}" \
+	     "Compilation directory is \[^\n\r\]+" \
+	     "${srcfile}: No such file or directory."]
+
+    gdb_test "dir $strip_dir" \
+	[search_dir_list [list \
+			      "$strip_dir" \
+			      "\\\$cdir" \
+			      "\\\$cwd"]]
+    gdb_test "list" [multi_line \
+			 "1\[ \t\]+int" \
+			 "2\[ \t\]+main \\(\\)" \
+			 "3\[ \t\]+\\{" \
+			 "4\[ \t\]+return 0;" \
+			 "5\[ \t\]+\\}" ]
+
+    gdb_test "info source" \
+	[multi_line \
+	     "Current source file is ${srcfile}" \
+	     "Compilation directory is \[^\n\r\]+" \
+	     "Located in ${new_srcfile}" \
+	     "Contains 5 lines." \
+	     "Source language is c." \
+	     "Producer is \[^\n\r\]+" \
+	     "\[^\n\r\]+" \
+	     "\[^\n\r\]+" ]
+}
+
+test_changing_search_directory
+test_truncated_comp_dir