[PATCH] Don't use realpath on objfile names

[Tom Tromey]

Currently gdb uses realpath when finding an objfile.  However, this
yields surprising results when the given name is a symlink -- if the
symlink is retargeted, re-running will not cause a change.

This patch fixes the problem by changing gdb to compute the absolute
path rather than the real path.

This area has some history.  PR gdb/15415 changed this area to try to
preserve the basename of the executable, so that programs that check
argv[0] will continue to work correctly.

There's also PR gdb/15934, which proposes preserving the original name
for use in argv[0] and in "info inferior".  I haven't done this.  The
"info inferior" part might not be too difficult (unsure) but I wasn't
sure how argv[0] could really be preserved given relative file names,
the use of shell to startup, and the user's ability to change gdb's
working directory.

I think this patch does fix PR symtab/24143, which concerns finding
separate debug files when symlinks are used.  The included test does
not test exactly this scenario, though.

Regression tested on x86-64 Fedora 34.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=24143
---
 gdb/exec.c                             | 14 ++-----
 gdb/symfile.c                          |  2 +-
 gdb/testsuite/gdb.base/symlink-exe.exp | 57 ++++++++++++++++++++++++++
 3 files changed, 62 insertions(+), 11 deletions(-)
 create mode 100644 gdb/testsuite/gdb.base/symlink-exe.exp

diff --git a/gdb/exec.c b/gdb/exec.c
index 84c36473abb..077db790ef6 100644
--- a/gdb/exec.c
+++ b/gdb/exec.c
@@ -386,7 +386,7 @@ exec_file_attach (const char *filename, int from_tty)
   else
     {
       int load_via_target = 0;
-      const char *scratch_pathname, *canonical_pathname;
+      const char *scratch_pathname;
       int scratch_chan;
       char **matching;
 
@@ -398,7 +398,7 @@ exec_file_attach (const char *filename, int from_tty)
 	    load_via_target = 1;
 	}
 
-      gdb::unique_xmalloc_ptr<char> canonical_storage, scratch_storage;
+      gdb::unique_xmalloc_ptr<char> scratch_storage;
       if (load_via_target)
 	{
 	  /* gdb_bfd_fopen does not support "target:" filenames.  */
@@ -409,7 +409,6 @@ exec_file_attach (const char *filename, int from_tty)
 
 	  scratch_pathname = filename;
 	  scratch_chan = -1;
-	  canonical_pathname = scratch_pathname;
 	}
       else
 	{
@@ -437,19 +436,14 @@ exec_file_attach (const char *filename, int from_tty)
 	    perror_with_name (filename);
 
 	  scratch_pathname = scratch_storage.get ();
-
-	  /* gdb_bfd_open (and its variants) prefers canonicalized
-	     pathname for better BFD caching.  */
-	  canonical_storage = gdb_realpath (scratch_pathname);
-	  canonical_pathname = canonical_storage.get ();
 	}
 
       gdb_bfd_ref_ptr temp;
       if (write_files && !load_via_target)
-	temp = gdb_bfd_fopen (canonical_pathname, gnutarget,
+	temp = gdb_bfd_fopen (scratch_pathname, gnutarget,
 			      FOPEN_RUB, scratch_chan);
       else
-	temp = gdb_bfd_open (canonical_pathname, gnutarget, scratch_chan);
+	temp = gdb_bfd_open (scratch_pathname, gnutarget, scratch_chan);
       current_program_space->set_exec_bfd (std::move (temp));
 
       if (!current_program_space->exec_bfd ())
diff --git a/gdb/symfile.c b/gdb/symfile.c
index dcd217c0a51..239513415ba 100644
--- a/gdb/symfile.c
+++ b/gdb/symfile.c
@@ -1707,7 +1707,7 @@ symfile_bfd_open (const char *name)
 
       /* Look down path for it, allocate 2nd new malloc'd copy.  */
       desc = openp (getenv ("PATH"),
-		    OPF_TRY_CWD_FIRST | OPF_RETURN_REALPATH,
+		    OPF_TRY_CWD_FIRST,
 		    expanded_name.get (), O_RDONLY | O_BINARY, &absolute_name);
 #if defined(__GO32__) || defined(_WIN32) || defined (__CYGWIN__)
       if (desc < 0)
diff --git a/gdb/testsuite/gdb.base/symlink-exe.exp b/gdb/testsuite/gdb.base/symlink-exe.exp
new file mode 100644
index 00000000000..4a45f06b775
--- /dev/null
+++ b/gdb/testsuite/gdb.base/symlink-exe.exp
@@ -0,0 +1,57 @@
+# Copyright 2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+if {![isnative]} {
+    unsupported "symlink-exe.exp not supported on non-native target"
+    return -1
+}
+
+if {[is_remote host]} {
+    unsupported "symlink-exe.exp not supported on remote host"
+    return -1
+}
+
+# We don't really care about the contents, but we're going to build
+# two executables here and they need different source file names.
+standard_testfile main.c argv0-symlink.c
+
+if {[build_executable ${testfile}.exp ${testfile}1 ${srcfile}] == -1} {
+    return -1
+}
+
+set status [remote_exec host \
+		"ln -sf ${testfile}1 [standard_output_file ${testfile}]"]
+if {[lindex $status 0] != 0} {
+    unsupported "symlink-exe.exp - host does not support symbolic links"
+    return -1
+}
+
+clean_restart $testfile
+gdb_test "run" ".*"
+
+# Ensure the new file has a newer mtime.
+sleep 1
+if {[build_executable ${testfile}.exp ${testfile}2 ${srcfile2}] == -1} {
+    return -1
+}
+set status [remote_exec host \
+		"ln -sf ${testfile}2 [standard_output_file ${testfile}]"]
+if {[lindex $status 0] != 0} {
+    unsupported "symlink-exe.exp - host does not support symbolic links"
+    return -1
+}
+
+gdb_test "run" ".*${testfile}.* has changed; re-reading symbols.*" \
+    "run and re-read symbols"
-- 
2.34.1