[Object] Fix handling of Elf_Nhdr with sh_addralign=8

The generic ABI says:

> Padding is present, if necessary, to ensure 8 or 4-byte alignment for the next note entry (depending on whether the file is a 64-bit or 32-bit object). Such padding is not included in descsz.

Our parsing code currently aligns n_namesz. Fix the bug by aligning the start
offset of the descriptor instead. This issue has been benign because the primary
uses of sh_addralign=8 notes are `.note.gnu.property`, where
`sizeof(Elf_Nhdr) + sizeof("GNU") = 16` (already aligned by 8).

In practice, many 64-bit systems incorrectly use sh_addralign=4 notes.
We can use sh_addralign (= p_align) to decide the descriptor padding.
Treat an alignment of 0 and 1 as 4. This approach matches modern GNU readelf
(since 2018).

We have a few tests incorrectly using sh_addralign=0. We may make our behavior
stricter after fixing these tests.

Linux kernel dumped core files use `p_align=0` notes, so we need to support the
case for compatibility.

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D150022

1 files changed, 5 insertions, 4 deletions

diff --git a/lld/ELF/InputFiles.cpp b/lld/ELF/InputFiles.cpp
index b8291fb4307b..311e2c9a9e54 100644
--- a/lld/ELF/InputFiles.cpp
+++ b/lld/ELF/InputFiles.cpp
@@ -885,12 +885,13 @@ template <class ELFT> static uint32_t readAndFeatures(const InputSection &sec) {
   while (!data.empty()) {
     // Read one NOTE record.
     auto *nhdr = reinterpret_cast<const Elf_Nhdr *>(data.data());
-    if (data.size() < sizeof(Elf_Nhdr) || data.size() < nhdr->getSize())
+    if (data.size() < sizeof(Elf_Nhdr) ||
+        data.size() < nhdr->getSize(sec.addralign))
       reportFatal(data.data(), "data is too short");
 
     Elf_Note note(*nhdr);
     if (nhdr->n_type != NT_GNU_PROPERTY_TYPE_0 || note.getName() != "GNU") {
-      data = data.slice(nhdr->getSize());
+      data = data.slice(nhdr->getSize(sec.addralign));
       continue;
     }
 
@@ -899,7 +900,7 @@ template <class ELFT> static uint32_t readAndFeatures(const InputSection &sec) {
                                   : GNU_PROPERTY_X86_FEATURE_1_AND;
 
     // Read a body of a NOTE record, which consists of type-length-value fields.
-    ArrayRef<uint8_t> desc = note.getDesc();
+    ArrayRef<uint8_t> desc = note.getDesc(sec.addralign);
     while (!desc.empty()) {
       const uint8_t *place = desc.data();
       if (desc.size() < 8)
@@ -924,7 +925,7 @@ template <class ELFT> static uint32_t readAndFeatures(const InputSection &sec) {
     }
 
     // Go to next NOTE record to look for more FEATURE_1_AND descriptions.
-    data = data.slice(nhdr->getSize());
+    data = data.slice(nhdr->getSize(sec.addralign));
   }
 
   return featuresSet;