libdw: Add support for DWZ multifile forms DW_FORM_GNU_ref_alt/strp_alt.

DWZ multifile forms http://www.dwarfstd.org/ShowIssue.php?issue=120604.1
DW_FORM_GNU_ref_alt and DW_FORM_GNU_strp_alt reference an alternative
debuginfo file.  dwarf_begin and dwarf_begin_elf will try to use this
automatically.  There are no user visible changes to the libdw interface.

dwarf_formref_die, dwarf_formstring and dwarf_formudata can now return
a Dwarf_Die which comes from a CU in the alternative Dwarf descriptor.

__libdw_read_offset was adjusted to take an alternative Dwarf descriptor
into account.

Signed-off-by: Mark Wielaard <mjw@redhat.com>

1 files changed, 123 insertions, 0 deletions

diff --git a/libdw/dwarf_begin_elf.c b/libdw/dwarf_begin_elf.c
index 3e01800a..fd957701 100644
--- a/libdw/dwarf_begin_elf.c
+++ b/libdw/dwarf_begin_elf.c
@@ -31,12 +31,17 @@
 # include <config.h>
 #endif
 
+#include <assert.h>
+#include <inttypes.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include <string.h>
 #include <unistd.h>
+#include <sys/types.h>
 #include <sys/stat.h>
+#include <fcntl.h>
 
 #include "libdwP.h"
 
@@ -66,6 +71,110 @@ static const char dwarf_scnnames[IDX_last][17] =
 };
 #define ndwarf_scnnames (sizeof (dwarf_scnnames) / sizeof (dwarf_scnnames[0]))
 
+internal_function int
+__check_build_id (Dwarf *dw, const uint8_t *build_id, const size_t id_len)
+{
+  if (dw == NULL)
+    return -1;
+
+  Elf *elf = dw->elf;
+  Elf_Scn *scn = elf_nextscn (elf, NULL);
+  if (scn == NULL)
+    return -1;
+
+  do
+    {
+      GElf_Shdr shdr_mem;
+      GElf_Shdr *shdr = gelf_getshdr (scn, &shdr_mem);
+      if (likely (shdr != NULL) && shdr->sh_type == SHT_NOTE)
+	{
+	  size_t pos = 0;
+	  GElf_Nhdr nhdr;
+	  size_t name_pos;
+	  size_t desc_pos;
+	  Elf_Data *data = elf_getdata (scn, NULL);
+	  while ((pos = gelf_getnote (data, pos, &nhdr, &name_pos,
+				      &desc_pos)) > 0)
+	    if (nhdr.n_type == NT_GNU_BUILD_ID
+	        && nhdr.n_namesz == sizeof "GNU"
+		&& ! memcmp (data->d_buf + name_pos, "GNU", sizeof "GNU"))
+	      return (nhdr.n_descsz == id_len
+		      && ! memcmp (data->d_buf + desc_pos,
+				   build_id, id_len)) ? 0 : 1;
+        }
+      }
+    while ((scn = elf_nextscn (elf, scn)) != NULL);
+
+  return -1;
+}
+
+/* Try to open an debug alt link by name, checking build_id.
+   Marks free_alt on success, return NULL on failure.  */
+static Dwarf *
+try_debugaltlink (Dwarf *result, const char *try_name,
+		   const uint8_t *build_id, const size_t id_len)
+{
+  int fd = open (try_name, O_RDONLY);
+  if (fd > 0)
+    {
+      result->alt_dwarf = INTUSE (dwarf_begin) (fd, DWARF_C_READ);
+      if (result->alt_dwarf != NULL)
+	{
+	  Elf *elf = result->alt_dwarf->elf;
+	  if (__check_build_id (result->alt_dwarf, build_id, id_len) == 0
+	      && elf_cntl (elf, ELF_C_FDREAD) == 0)
+	    {
+	      close (fd);
+	      result->free_alt = 1;
+	      return result;
+	    }
+	  INTUSE (dwarf_end) (result->alt_dwarf);
+	}
+      close (fd);
+    }
+  return NULL;
+}
+
+/* For dwz multifile support, ignore if it looks wrong.  */
+static Dwarf *
+open_debugaltlink (Dwarf *result, const char *alt_name,
+		   const uint8_t *build_id, const size_t id_len)
+{
+  /* First try the name itself, it is either an absolute path or
+     a relative one.  Sadly we don't know relative from where at
+     this point.  */
+  if (try_debugaltlink (result, alt_name, build_id, id_len) != NULL)
+    return result;
+
+  /* Lets try based on the build-id.  This is somewhat distro specific,
+     we are following the Fedora implementation described at
+  https://fedoraproject.org/wiki/Releases/FeatureBuildId#Find_files_by_build_ID
+   */
+#define DEBUG_PREFIX "/usr/lib/debug/.build-id/"
+#define PREFIX_LEN sizeof (DEBUG_PREFIX)
+  char id_name[PREFIX_LEN + 1 + id_len * 2 + sizeof ".debug" - 1];
+  strcpy (id_name, DEBUG_PREFIX);
+  int n = snprintf (&id_name[PREFIX_LEN  - 1],
+		    4, "%02" PRIx8 "/", (uint8_t) build_id[0]);
+  assert (n == 3);
+  for (size_t i = 1; i < id_len; ++i)
+    {
+      n = snprintf (&id_name[PREFIX_LEN - 1 + 3 + (i - 1) * 2],
+		    3, "%02" PRIx8, (uint8_t) build_id[i]);
+      assert (n == 2);
+    }
+  strcpy (&id_name[PREFIX_LEN - 1 + 3 + (id_len - 1) * 2],
+	  ".debug");
+
+  if (try_debugaltlink (result, id_name, build_id, id_len))
+    return result;
+
+  /* Everything failed, mark this Dwarf as not having an alternate,
+     but don't fail the load.  The user may want to set it by hand
+     before usage.  */
+  result->alt_dwarf = NULL;
+  return result;
+}
 
 static Dwarf *
 check_section (Dwarf *result, GElf_Ehdr *ehdr, Elf_Scn *scn, bool inscngrp)
@@ -110,6 +219,20 @@ check_section (Dwarf *result, GElf_Ehdr *ehdr, Elf_Scn *scn, bool inscngrp)
       return NULL;
     }
 
+  /* For dwz multifile support, ignore if it looks wrong.  */
+  if (strcmp (scnname, ".gnu_debugaltlink") == 0)
+    {
+      Elf_Data *data = elf_getdata (scn, NULL);
+      if (data != NULL && data->d_size != 0)
+	{
+	  const char *alt_name = data->d_buf;
+	  const void *build_id = memchr (data->d_buf, '\0', data->d_size);
+	  const int id_len = data->d_size - (build_id - data->d_buf + 1);
+	  if (alt_name && build_id && id_len > 0)
+	    return open_debugaltlink (result, alt_name, build_id + 1, id_len);
+	}
+    }
+
 
   /* Recognize the various sections.  Most names start with .debug_.  */
   size_t cnt;