[ELF] Place .note in the first page to ensure they are available in core files

[MaskRay]

Summary

Other large sections (e.g. .rela.dyn .dynstr) may push .note.* off the
first page. They won't be available in core files if RLIMIT_CORE is
limited.

This patch gives priority to alloctable SHT_NOTE sections so that they
are assuredly in the first page and will be available in core files.
They are small and contain important information (e.g. .note.gnu.build-id
identifies the origin of the core, .note.tag stores NT_FREEBSD_ABI_TAG).

Note: gold Output_section_order has a similar rule:

// Loadable read-only note sections come next so that the PT_NOTE
// segment is on the first page of the executable.
ORDER_RO_NOTE,

[MaskRay]

gold

[MaskRay]

Forget to upload test file...

[MaskRay]

Add comment about FreeBSD .note.tag

[ruiu]

LGTM

ELF/Writer.cpp

773–777

This exiting comment is a bit pointless. Let's update:

Put .note sections at the beginning of an output file so that they are likely to be included in a core file even if core file size is limited by ulimit. In particular, we want a .note.gnu.build-id and a .note.tag to be included in a core to match core files with executable files.

[MaskRay]

Update comments

ELF/Writer.cpp

 // section should be placed in the file.  Instead of using simple
 // numbers (0,1,2...), we use a series of flags. One for each decision
 // point when placing the section.
 // Using flags has two key properties:
 // * It is easy to check if a give branch was taken.
 // * It is easy two see how similar two ranks are (see getRankProximity).
 enum RankFlags {
   RF_NOT_ADDR_SET = 1 << 18,
-  RF_NOT_INTERP = 1 << 17,
-  RF_NOT_ALLOC = 1 << 16,
-  RF_WRITE = 1 << 15,
-  RF_EXEC_WRITE = 1 << 14,
-  RF_EXEC = 1 << 13,
-  RF_RODATA = 1 << 12,
-  RF_NON_TLS_BSS = 1 << 11,
-  RF_NON_TLS_BSS_RO = 1 << 10,
-  RF_NOT_TLS = 1 << 9,
-  RF_BSS = 1 << 8,
-  RF_NOTE = 1 << 7,
+  RF_NOT_ALLOC = 1 << 17,
+  RF_NOT_INTERP = 1 << 16,
+  RF_NOT_NOTE = 1 << 15,
+  RF_WRITE = 1 << 14,
+  RF_EXEC_WRITE = 1 << 13,
+  RF_EXEC = 1 << 12,
+  RF_RODATA = 1 << 11,
+  RF_NON_TLS_BSS = 1 << 10,
+  RF_NON_TLS_BSS_RO = 1 << 9,
+  RF_NOT_TLS = 1 << 8,
+  RF_BSS = 1 << 7,
   RF_PPC_NOT_TOCBSS = 1 << 6,
   RF_PPC_TOCL = 1 << 5,
   RF_PPC_TOC = 1 << 4,
   RF_PPC_GOT = 1 << 3,
   RF_PPC_BRANCH_LT = 1 << 2,
   RF_MIPS_GPREL = 1 << 1,
   RF_MIPS_NOT_GOT = 1 << 0
 };
 
 static unsigned getSectionRank(const OutputSection *Sec) {
   unsigned Rank = 0;
 
   // We want to put section specified by -T option first, so we
   // can start assigning VA starting from them later.
   if (Config->SectionStartMap.count(Sec->Name))
     return Rank;
   Rank |= RF_NOT_ADDR_SET;
 
+  // Allocatable sections go first to reduce the total PT_LOAD size and
+  // so debug info doesn't change addresses in actual code.
+  if (!(Sec->Flags & SHF_ALLOC))
+    return Rank | RF_NOT_ALLOC;
+
   // Put .interp first because some loaders want to see that section
   // on the first page of the executable file when loaded into memory.
   if (Sec->Name == ".interp")
     return Rank;
   Rank |= RF_NOT_INTERP;
 
-  // Allocatable sections go first to reduce the total PT_LOAD size and
-  // so debug info doesn't change addresses in actual code.
-  if (!(Sec->Flags & SHF_ALLOC))
-    return Rank | RF_NOT_ALLOC;
+  // Put .note sections (which make up one PT_NOTE) at the beginning so that
+  // they are likely to be included in a core file even if core file size is
+  // limited. In particular, we want a .note.gnu.build-id and a .note.tag to be
+  // included in a core to match core files with executables.
+  if (Sec->Type == SHT_NOTE)
+    return Rank;
+  Rank |= RF_NOT_NOTE;
 
   // Sort sections based on their access permission in the following
   // order: R, RX, RWX, RW.  This order is based on the following
   // considerations:
   // * Read-only sections come first such that they go in the
   //   PT_LOAD covering the program headers at the start of the file.
   // * Read-only, executable sections come next.
   // * Writable, executable sections follow such that .plt on
static unsigned getSectionRank(const OutputSection *Sec) {
   if (!IsTls)
     Rank |= RF_NOT_TLS;
 
   // Within the TLS initialization block, the non-nobits sections need to appear
   // first.
   if (IsNoBits)
     Rank |= RF_BSS;
 
-  // We create a NOTE segment for contiguous .note sections, so make
-  // them contigous if there are more than one .note section with the
-  // same attributes.
-  if (Sec->Type == SHT_NOTE)
-    Rank |= RF_NOTE;
-
   // Some architectures have additional ordering restrictions for sections
   // within the same PT_LOAD.
   if (Config->EMachine == EM_PPC64) {
     // PPC64 has a number of special SHT_PROGBITS+SHF_ALLOC+SHF_WRITE sections
     // that we would like to make sure appear is a specific order to maximize
     // their coverage by a single signed 16-bit offset from the TOC base
     // pointer. Conversely, the special .tocbss section should be first among
     // all SHT_NOBITS sections. This will put it next to the loaded special

test/ELF/note-first-page.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: ld.lld %t.o --build-id=md5 --shared -o %t.so
+# RUN: llvm-readelf -S %t.so | FileCheck %s
+
+# Check .note.gnu.build-id is placed before other potentially large sections
+# (.dynsym .dynstr (and .rela.dyn in PIE)). This ensures the note information
+# available in core files because various core dumpers ensure the first page is
+# available.
+
+# CHECK: [ 1] .note.gnu.build-id
+# CHECK: [ 2] .dynsym