[PATCH] D55800: [ELF] Place .note in the first page to ensure they are available in core files

[Fangrui Song via Phabricator via llvm-commits]

MaskRay updated this revision to Diff 178595.
MaskRay edited the summary of this revision.
MaskRay added a comment.
Herald added a subscriber: krytarowski.

Add comment about FreeBSD .note.tag


Repository:
  rLLD LLVM Linker

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D55800/new/

https://reviews.llvm.org/D55800

Files:
  ELF/Writer.cpp
  test/ELF/note-first-page.s


Index: test/ELF/note-first-page.s
===================================================================
--- /dev/null
+++ test/ELF/note-first-page.s
@@ -0,0 +1,12 @@
+# 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
Index: ELF/Writer.cpp
===================================================================
--- ELF/Writer.cpp
+++ ELF/Writer.cpp
@@ -730,17 +730,17 @@
 // * 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,
@@ -759,17 +759,26 @@
     return Rank;
   Rank |= RF_NOT_ADDR_SET;
 
-  // 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 .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;
+
+  // 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. They contain information important in core files (e.g.
+  // .note.gnu.build-id identifies the origin of the core, .note.tag stores
+  // NT_FREEBSD_ABI_TAG). Ensure they are in the first page.
+  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:
@@ -832,12 +841,6 @@
   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) {


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