Minidumper: Get the nexe's build ID from PT_NOTE when available

src/untrusted/minidump_generator/build_id.c

+/*
+ * Copyright (c) 2013 The Native Client Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include <assert.h>
+#include <string.h>
+
+#include "native_client/src/include/elf32.h"

[bradn, 2013/05/10 18:51:38]

link.h ?

[Mark Seaborn, 2013/05/10 19:29:41]

That's a glibc header related to the dynamic linker.  We want this to work with
newlib and with static linking.

+
+
+extern const Elf32_Ehdr __ehdr_start __attribute__((weak));

[bradn, 2013/05/10 18:51:38]

Why does this need the weak attribute?

[Mark Seaborn, 2013/05/10 19:29:41]

Comment added (copied from tls.c).

+
+static uintptr_t note_align(uintptr_t value) {
+  return (value + 3) & ~3;
+}
+
+int nacl_get_build_id(const char **data, size_t *size) {
+  if (&__ehdr_start != NULL &&
+      __ehdr_start.e_ident[EI_CLASS] == ELFCLASS32 &&

[bradn, 2013/05/10 18:51:38]

The deep nesting is kind of undesirable as this is an error case? Return in the
inverted case?

[Mark Seaborn, 2013/05/10 19:29:41]

It's not really an error, just "not found when searching".

We might change the x86 nacl-gcc toolchains to provide the build ID by defining
a __build_id symbol via the linker script.  In that case, that will be a second
build ID source to check in nacl_get_build_id() after checking __ehdr_start.

+      __ehdr_start.e_phentsize == sizeof(Elf32_Phdr)) {
+    int phnum = __ehdr_start.e_phnum;
+    uintptr_t start_addr = (uintptr_t) &__ehdr_start;
+    Elf32_Phdr *phdrs = (Elf32_Phdr *) (start_addr + __ehdr_start.e_phoff);

[bradn, 2013/05/10 18:51:38]

Wouldn't using using dl_iterate_phdr be preferable? I.e. using a public
interface?

[Mark Seaborn, 2013/05/10 19:29:41]

dl_iterate_phdr() is a glibc interface.  We want this to work with newlib and
static linking.

+    for (int i = 0; i < phnum; ++i) {
+      Elf32_Phdr *phdr = &phdrs[i];
+      if (phdr->p_type == PT_NOTE) {
+        Elf32_Nhdr *note = (Elf32_Nhdr *) (start_addr + phdr->p_offset);
+        uintptr_t note_end = (uintptr_t) note + phdr->p_memsz;
+        while ((uintptr_t) note < note_end) {
+          uintptr_t name_ptr = (uintptr_t) &note[1];
+          assert(name_ptr <= note_end);
+          uintptr_t desc_ptr = note_align(name_ptr + note->n_namesz);
+          uintptr_t next_ptr = note_align(desc_ptr + note->n_descsz);
+          assert(next_ptr <= note_end);
+          if (note->n_type == NT_GNU_BUILD_ID &&
+              note->n_namesz == sizeof(ELF_NOTE_GNU) &&
+              memcmp((const char *) name_ptr, ELF_NOTE_GNU,
+                     sizeof(ELF_NOTE_GNU)) == 0) {
+            *data = (const char *) desc_ptr;
+            *size = note->n_descsz;
+            return 1;
+          }
+          note = (Elf32_Nhdr *) next_ptr;
+        }
+      }
+    }
+  }
+  return 0;
+}