[libc-commits] [libc] [libc][linux] add support to parse PT_GNU_PROPERTY (PR #174772)

[Jakob Koschel via libc-commits]

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@@ -0,0 +1,137 @@
+//===-- Implementation file of gnu_property_section -----------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+#include "startup/linux/gnu_property_section.h"
+
+#include "hdr/elf_proxy.h"
+#include "hdr/link_macros.h"
+#include "src/__support/CPP/string_view.h"
+#include "src/__support/macros/config.h"
+#include "src/string/memory_utils/utils.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+// The program property note is basically a note (Elf64_Nhdr) prepended with:
+// * n_name[4]: should always be "GNU\0"
+// * n_desc: an array of n_descsz bytes with program property entries
+// Since we are casting a memory address into this struct, the layout needs to
+// *exactly* match.
+struct Elf64_ProgramPropertyNote {
+  Elf64_Nhdr nhdr;
+  unsigned char n_name[4];
+  unsigned char n_desc[0]; // the size of 'n_desc' depends on n_descsz and is
+                           // not known statically.
+};
+
+// 32-bit variant for ProgramPropertyNote.
+struct Elf32_ProgramPropertyNote {
+  Elf32_Nhdr nhdr;
+  unsigned char n_name[4];
+  unsigned char n_desc[0]; // the size of 'n_desc' depends on n_descsz and is
+                           // not known statically.
+};
+
+// A program property consists of a type, the data size, followed by the actual
+// data and potential padding (aligning it to 64bit).
+// Since we are casting a memory address into this struct, the layout needs to
+// *exactly* match (The padding is ommited since it doesn't have actual
+// content).
+// pr_data needs to be ElfW_Word aligned, therefore the whole struct needs to be
+// aligned.
+struct Elf64_ProgramProperty {
+  Elf64_Word pr_type;
+  Elf64_Word pr_datasz;
+  unsigned char pr_data[0];
+};
+
+// 32-bit variant for ProgramProperty.
+struct Elf32_ProgramProperty {
+  Elf32_Word pr_type;
+  Elf32_Word pr_datasz;
+  unsigned char pr_data[0];
+};
+
+bool GnuPropertySection::parse(const ElfW(Phdr) * gnu_property_phdr,
+                               const ElfW(Addr) base) {
+  if (!gnu_property_phdr)
+    return false;
+
+  const auto note_nhdr_size = gnu_property_phdr->p_memsz;
+  // Sanity check we are using the correct phdr and the memory size is large
+  // enough to fit the program property note.
+  if (gnu_property_phdr->p_type != PT_GNU_PROPERTY ||
+      note_nhdr_size < sizeof(ElfW(ProgramPropertyNote)))
+    return false;
+
+  const ElfW(ProgramPropertyNote) *note_nhdr =
+      reinterpret_cast<ElfW(ProgramPropertyNote) *>(base +
+                                                    gnu_property_phdr->p_vaddr);
+  if (!note_nhdr)
+    return false;
+
+  const ElfW(Word) nhdr_desc_size = note_nhdr->nhdr.n_descsz;
+
+  // sizeof(*note_nhdr) does not include the size of n_desc,
+  // since it is not known at compile time.
+  // The size of it combined with n_descsz cannot exceed the total size of the
+  // program property note.
+  if ((sizeof(*note_nhdr) + nhdr_desc_size) > note_nhdr_size)
+    return false;
+
+  if (note_nhdr->nhdr.n_namesz != 4 ||
+      note_nhdr->nhdr.n_type != NT_GNU_PROPERTY_TYPE_0 ||
+      cpp::string_view(reinterpret_cast<const char *>(note_nhdr->n_name), 4) !=
+          cpp::string_view("GNU", 4))
+    return false;
+
+  // program property note is valid, we can parse the program property array.
+  ElfW(Word) offset = 0;
+  while (offset < nhdr_desc_size) {
+    if ((nhdr_desc_size - offset) < sizeof(ElfW(ProgramProperty)))
+      // We can no longer even fit the statically known size of
+      // ProgramProperty. Doing the cast and reading pr_type/pr_datasz is no
+      // longer in bounds.
+      break;
----------------
jakos-sec wrote:

Done, actually it allowed optimizing the condition a lot.

https://github.com/llvm/llvm-project/pull/174772