Re: [PATCH 1/2] gdb: Add soname to build-id mapping for core files

[Aaron Merey <amerey@redhat.com>]

On Fri, Mar 11, 2022 at 10:23 AM Tom Tromey <tom@tromey.com> wrote:
> > +         if (soname.get () != nullptr)
>
> I don't think you need the .get here.

Fixed.

> > +  std::unordered_map<char *, const bfd_build_id *> filename_map;
>
> I suspect this code is intended to use the contents of the string as the 
> key -- but this will actually use the pointer value as the key.
>
> If the pointer value is intended, some comment somewhere to that effect
> -- explaining why it's an ok approach -- would be good to have.

After taking another look at this I don't think filename_map is needed
at all.  The idea behind it was to use build-ids to reaquire downloaded
files in core_target::build_file_mappings from the debuginfod cache.
However this is unnecessary since bfd_map in build_file_mappings already
makes previously opened files (including those downloaded) available
using just a filename.

A build-id is only needed when trying to aquire the file for the first
time.  This is facilitated by vma_map in linux_read_core_file_mappings.

> > +      if (sec->flags & SEC_LOAD
> > +       && get_elf_backend_data (cbfd)->elf_backend_core_find_build_id
> > +            (cbfd, (bfd_vma) sec->filepos))
>
> The get_elf_backend_data call has to have parens around it according to
> GNU / gdb style.

Fixed.

> > +  /* Read the soname from the string table.  */  
> > +  gdb::byte_vector dynstr_buf;
> > +  if (!gdb_bfd_get_full_section_contents (abfd.get (), dynstr, &dynstr_buf))
> > +    return {}; 
> > + 
> > +  return make_unique_xstrdup ((char *) dynstr_buf.data () + idx);
>
> What if the data isn't \0-terminated?  Maybe some size checking (not
> sure if needed) and make_unique_xstrndup would be safer.

Added a check to make sure \0 exists between the start of the soname
and the end of the section.

I also replaced basename with lbasename in get_cbfd_soname_build_id.

Aaron

---
 gdb/corelow.c    | 11 ++++++
 gdb/linux-tdep.c | 24 ++++++++++++-
 gdb/solib.c      | 87 ++++++++++++++++++++++++++++++++++++++++++++++++
 gdb/solib.h      | 20 +++++++++++
 4 files changed, 141 insertions(+), 1 deletion(-)

diff --git a/gdb/corelow.c b/gdb/corelow.c
index 1579e6bc2b8..9876f4d920e 100644
--- a/gdb/corelow.c
+++ b/gdb/corelow.c
@@ -282,6 +282,17 @@ core_target::build_file_mappings ()
 
 	/* Set target_section fields.  */
 	m_core_file_mappings.emplace_back (start, end, sec);
+
+	/* If this is a bfd of a shared library, record its soname
+	   and build id.  */
+	if (build_id != nullptr)
+	  {
+	    gdb::unique_xmalloc_ptr<char> soname
+	      = gdb_bfd_read_elf_soname (bfd->filename);
+	    if (soname != nullptr)
+	      set_cbfd_soname_build_id (current_program_space->cbfd,
+					soname.get (), build_id);
+	  }
       });
 
   normalize_mem_ranges (&m_core_unavailable_mappings);
diff --git a/gdb/linux-tdep.c b/gdb/linux-tdep.c
index d4868902ac3..52cdaae034b 100644
--- a/gdb/linux-tdep.c
+++ b/gdb/linux-tdep.c
@@ -44,6 +44,7 @@
 #include "solib-svr4.h"
 
 #include <ctype.h>
+#include <unordered_map>
 
 /* This enum represents the values that the user can choose when
    informing the Linux kernel about which memory mappings will be
@@ -1185,6 +1186,22 @@ linux_read_core_file_mappings
   if (f != descend)
     warning (_("malformed note - filename area is too big"));
 
+  const bfd_build_id *orig_build_id = cbfd->build_id;
+  std::unordered_map<ULONGEST, const bfd_build_id *> vma_map;
+
+  /* Search for solib build-ids in the core file.  Each time one is found,
+     map the start vma of the corresponding elf header to the build-id.  */
+  for (bfd_section *sec = cbfd->sections; sec != nullptr; sec = sec->next)
+    {
+      cbfd->build_id = nullptr;
+
+      if (sec->flags & SEC_LOAD
+	  && (get_elf_backend_data (cbfd)->elf_backend_core_find_build_id
+	       (cbfd, (bfd_vma) sec->filepos)))
+	vma_map[sec->vma] = cbfd->build_id;
+    }
+
+  cbfd->build_id = orig_build_id;
   pre_loop_cb (count);
 
   for (int i = 0; i < count; i++)
@@ -1198,8 +1215,13 @@ linux_read_core_file_mappings
       descdata += addr_size;
       char * filename = filenames;
       filenames += strlen ((char *) filenames) + 1;
+      const bfd_build_id *build_id = nullptr;
+      auto vma_map_it = vma_map.find (start);
+
+      if (vma_map_it != vma_map.end ())
+	build_id = vma_map_it->second;
 
-      loop_cb (i, start, end, file_ofs, filename, nullptr);
+      loop_cb (i, start, end, file_ofs, filename, build_id);
     }
 }
 
diff --git a/gdb/solib.c b/gdb/solib.c
index 8bcbfa22df1..8ecea0f89e7 100644
--- a/gdb/solib.c
+++ b/gdb/solib.c
@@ -23,6 +23,7 @@
 #include <fcntl.h>
 #include "symtab.h"
 #include "bfd.h"
+#include "build-id.h"
 #include "symfile.h"
 #include "objfiles.h"
 #include "gdbcore.h"
@@ -519,6 +520,55 @@ solib_bfd_open (const char *pathname)
   return abfd;
 }
 
+/* Mapping of a core file's shared library sonames to their respective
+   build-ids.  Added to the registries of core file bfds.  */
+
+typedef std::unordered_map<std::string, std::string> soname_build_id_map;
+
+/* Key used to associate a soname_build_id_map to a core file bfd.  */
+
+static const struct bfd_key<soname_build_id_map> cbfd_soname_build_id_data_key;
+
+/* See solib.h.  */
+
+void
+set_cbfd_soname_build_id (gdb_bfd_ref_ptr abfd,
+			  const char *soname,
+			  const bfd_build_id *build_id)
+{
+  gdb_assert (abfd.get () != nullptr);
+  gdb_assert (soname != nullptr);
+  gdb_assert (build_id != nullptr);
+
+  soname_build_id_map *mapptr = cbfd_soname_build_id_data_key.get (abfd.get ());
+
+  if (mapptr == nullptr)
+    mapptr = cbfd_soname_build_id_data_key.emplace (abfd.get ());
+
+  (*mapptr)[soname] = build_id_to_string (build_id);
+}
+
+/* See solib.h.  */
+
+gdb::unique_xmalloc_ptr<char>
+get_cbfd_soname_build_id (gdb_bfd_ref_ptr abfd, const char *soname)
+{
+  if (abfd.get () == nullptr || soname == nullptr)
+    return {};
+
+  soname_build_id_map *mapptr
+    = cbfd_soname_build_id_data_key.get (abfd.get ());
+
+  if (mapptr == nullptr)
+    return {};
+
+  auto it = mapptr->find (lbasename (soname));
+  if (it == mapptr->end ())
+    return {};
+
+  return make_unique_xstrdup (it->second.c_str ());
+}
+
 /* Given a pointer to one of the shared objects in our list of mapped
    objects, use the recorded name to open a bfd descriptor for the
    object, build a section table, relocate all the section addresses
@@ -1586,6 +1636,43 @@ gdb_bfd_scan_elf_dyntag (const int desired_dyntag, bfd *abfd, CORE_ADDR *ptr,
   return 0;
 }
 
+/* See solib.h.  */
+
+gdb::unique_xmalloc_ptr<char>
+gdb_bfd_read_elf_soname (const char *filename)
+{
+  gdb_bfd_ref_ptr abfd = gdb_bfd_open (filename, gnutarget);
+
+  if (abfd == nullptr)
+    return {};
+
+  /* Check that ABFD is an ET_DYN ELF file.  */
+  if (!bfd_check_format (abfd.get (), bfd_object)
+      || !(bfd_get_file_flags (abfd.get ()) & DYNAMIC))
+    return {};
+
+  CORE_ADDR idx;
+  if (!gdb_bfd_scan_elf_dyntag (DT_SONAME, abfd.get (), &idx, nullptr))
+    return {};
+
+  struct bfd_section *dynstr = bfd_get_section_by_name (abfd.get (), ".dynstr");
+  int sect_size = bfd_section_size (dynstr);
+  if (dynstr == nullptr || sect_size <= idx)
+    return {};
+
+  /* Read soname from the string table.  */
+  gdb::byte_vector dynstr_buf;
+  if (!gdb_bfd_get_full_section_contents (abfd.get (), dynstr, &dynstr_buf))
+    return {};
+
+  /* Ensure soname is null-terminated before returning a copy.  */
+  char *soname = (char *) dynstr_buf.data () + idx;
+  if (strnlen (soname, sect_size - idx) == sect_size - idx)
+    return {};
+
+  return make_unique_xstrdup (soname);
+}
+
 /* Lookup the value for a specific symbol from symbol table.  Look up symbol
    from ABFD.  MATCH_SYM is a callback function to determine whether to pick
    up a symbol.  DATA is the input of this callback function.  Return NULL
diff --git a/gdb/solib.h b/gdb/solib.h
index 2258b0ba6a0..cd6c8a81104 100644
--- a/gdb/solib.h
+++ b/gdb/solib.h
@@ -118,6 +118,12 @@ extern CORE_ADDR gdb_bfd_lookup_symbol_from_symtab (bfd *abfd,
 extern int gdb_bfd_scan_elf_dyntag (const int desired_dyntag, bfd *abfd,
 				    CORE_ADDR *ptr, CORE_ADDR *ptr_addr);
 
+/* If FILENAME refers to an ELF shared object then attempt to return the
+   string referred to by its DT_SONAME tag.   */
+
+extern gdb::unique_xmalloc_ptr<char> gdb_bfd_read_elf_soname
+  (const char *filename);
+
 /* Enable or disable optional solib event breakpoints as appropriate.  */
 
 extern void update_solib_breakpoints (void);
@@ -126,4 +132,18 @@ extern void update_solib_breakpoints (void);
 
 extern void handle_solib_event (void);
 
+/* Associate SONAME with BUILD_ID in ABFD's registry so that it can be
+   retrieved with get_cbfd_soname_build_id.  */
+
+extern void set_cbfd_soname_build_id (gdb_bfd_ref_ptr abfd,
+				      const char *soname,
+				      const bfd_build_id *build_id);
+
+/* If SONAME had a build-id associated with it in ABFD's registry by a
+   previous call to set_cbfd_soname_build_id then return the build-id
+   as a NULL-terminated hex string.  */
+
+extern gdb::unique_xmalloc_ptr<char> get_cbfd_soname_build_id
+  (gdb_bfd_ref_ptr abfd, const char *soname);
+
 #endif /* SOLIB_H */
-- 
2.35.1