Fix PR gdb/17917 Lookup build-id in remote binaries

GDB doesn't support loading debug files using build-id from remote
target filesystems.

This is the case when gdbserver attached to a process and a gdb target
remote occurs over tcp.

With this change we make build-id lookups possible:

    (gdb) show debug-file-directory
    The directory where separate debug symbols are searched for is "/usr/local/lib/debug".
    (gdb) set debug-file-directory /usr/lib/debug
    (gdb) show sysroot
    The current system root is "target:".
    (gdb) target extended-remote :46615
    Remote debugging using :46615
    warning: Can not parse XML target description; XML support was disabled at compile time
    Reading /usr/sbin/mariadbd from remote target...
    warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
    Reading /usr/sbin/mariadbd from remote target...
    Reading symbols from target:/usr/sbin/mariadbd...
    Reading /usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading /usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading symbols from target:/usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug...
    Reading /lib/x86_64-linux-gnu/libpcre2-8.so.0 from remote target...
    ...

Before this change, the lookups would have been (GNU gdb (GDB) Fedora 10.2-3.fc34):

    (gdb) target extended-remote :46615
    Remote debugging using :46615
    Reading /usr/sbin/mariadbd from remote target...
    warning: File transfers from remote targets can be slow. Use "set sysroot" to access files locally instead.
    Reading /usr/sbin/mariadbd from remote target...
    Reading symbols from target:/usr/sbin/mariadbd...
    Reading /usr/sbin/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading /usr/sbin/.debug/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading /usr/lib/debug//usr/sbin/0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading /usr/lib/debug/usr/sbin//0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Reading target:/usr/lib/debug/usr/sbin//0a874dca5a7ff831396ddc0785d939a192efe3.debug from remote target...
    Missing separate debuginfo for target:/usr/sbin/mariadbd
    Try: dnf --enablerepo='*debug*' install /usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug
    (No debugging symbols found in target:/usr/sbin/mariadbd)

Observe it didn't look for
/usr/lib/debug/.build-id/6e/0a874dca5a7ff831396ddc0785d939a192efe3.debug
on the remote target (where it is) and expected them to be installed
locally.

As a minor optimization, this also changes the build-id lookup such that
if sysroot is empty, no second lookup of the same location is performed.

Change-Id: I5181696d271c325a25a0805a8defb8ab7f9b3f55
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17917

1 files changed, 12 insertions, 9 deletions

diff --git a/gdb/build-id.c b/gdb/build-id.c
index 553d6ce..d68b7a7 100644
--- a/gdb/build-id.c
+++ b/gdb/build-id.c
@@ -81,9 +81,15 @@ build_id_to_debug_bfd_1 (const std::string &link, size_t build_id_len,
     }
 
   /* lrealpath() is expensive even for the usually non-existent files.  */
-  gdb::unique_xmalloc_ptr<char> filename;
-  if (access (link.c_str (), F_OK) == 0)
-    filename.reset (lrealpath (link.c_str ()));
+  gdb::unique_xmalloc_ptr<char> filename_holder;
+  const char *filename = nullptr;
+  if (startswith (link, TARGET_SYSROOT_PREFIX))
+    filename = link.c_str ();
+  else if (access (link.c_str (), F_OK) == 0)
+    {
+      filename_holder.reset (lrealpath (link.c_str ()));
+      filename = filename_holder.get ();
+    }
 
   if (filename == NULL)
     {
@@ -94,7 +100,7 @@ build_id_to_debug_bfd_1 (const std::string &link, size_t build_id_len,
     }
 
   /* We expect to be silent on the non-existing files.  */
-  gdb_bfd_ref_ptr debug_bfd = gdb_bfd_open (filename.get (), gnutarget);
+  gdb_bfd_ref_ptr debug_bfd = gdb_bfd_open (filename, gnutarget);
 
   if (debug_bfd == NULL)
     {
@@ -162,12 +168,9 @@ build_id_to_bfd_suffix (size_t build_id_len, const bfd_byte *build_id,
 
       /* Try to look under the sysroot as well.  If the sysroot is
 	 "/the/sysroot", it will give
-	 "/the/sysroot/usr/lib/debug/.build-id/ab/cdef.debug".
+	 "/the/sysroot/usr/lib/debug/.build-id/ab/cdef.debug".  */
 
-	 Don't do it if the sysroot is the target system ("target:").  It
-	 could work in theory, but the lrealpath in build_id_to_debug_bfd_1
-	 only works with local paths.  */
-      if (gdb_sysroot != TARGET_SYSROOT_PREFIX)
+      if (!gdb_sysroot.empty ())
 	{
 	  link = gdb_sysroot + link;
 	  debug_bfd = build_id_to_debug_bfd_1 (link, build_id_len, build_id);