From d45df094353b8b0b5f1f76d9f5f828c34595fe11 Mon Sep 17 00:00:00 2001 From: Fangrui Song Date: Fri, 3 May 2019 00:35:49 +0000 Subject: [PATCH] [ELF] Place SHT_NOTE sections with the same alignment into one PT_NOTE Summary: While the generic ABI requires notes to be 8-byte aligned in ELF64, many vendor-specific notes (from Linux, NetBSD, Solaris, etc) use 4-byte alignment. In a PT_NOTE segment, if 4-byte aligned notes are followed by an 8-byte aligned note, the possible 4-byte padding may make consumers fail to parse the 8-byte aligned note. See PR41000 for a recent report about .note.gnu.property (NT_GNU_PROPERTY_TYPE_0). (Note, for NT_GNU_PROPERTY_TYPE_0, the consumers should probably migrate to PT_GNU_PROPERTY, but the alignment issue affects other notes as well.) To fix the issue, don't mix notes with different alignments in one PT_NOTE. If compilers emit 4-byte aligned notes before 8-byte aligned notes, we'll create at most 2 segments. sh_size%sh_addralign=0 is actually implied by the rule for linking unrecognized sections (in generic ABI), so we don't have to check that. Notes that match in name, type and attribute flags are concatenated into a single output section. The compilers have to ensure sh_size%sh_addralign=0 to make concatenated notes parsable. An alternative approach is to create a PT_NOTE for each SHT_NOTE, but we'll have to incur the sizeof(Elf64_Phdr)=56 overhead every time a new note section is introduced. Reviewers: ruiu, jakehehrlich, phosek, jhenderson, pcc, espindola Subscribers: emaste, arichardson, krytarowski, fedor.sergeev, llvm-commits Tags: #llvm Differential Revision: https://reviews.llvm.org/D61296 llvm-svn: 359853 --- lld/ELF/Writer.cpp | 5 +++-- lld/test/ELF/build-id.s | 6 +++--- lld/test/ELF/note-alignment.s | 36 ++++++++++++++++++++++++++++++++++++ 3 files changed, 42 insertions(+), 5 deletions(-) create mode 100644 lld/test/ELF/note-alignment.s diff --git a/lld/ELF/Writer.cpp b/lld/ELF/Writer.cpp index c20b5c4..b6a70a4 100644 --- a/lld/ELF/Writer.cpp +++ b/lld/ELF/Writer.cpp @@ -1999,11 +1999,12 @@ template std::vector Writer::createPhdrs() { if (Config->ZWxneeded) AddHdr(PT_OPENBSD_WXNEEDED, PF_X); - // Create one PT_NOTE per a group of contiguous .note sections. + // Create one PT_NOTE per a group of contiguous SHT_NOTE sections with the + // same alignment. PhdrEntry *Note = nullptr; for (OutputSection *Sec : OutputSections) { if (Sec->Type == SHT_NOTE && (Sec->Flags & SHF_ALLOC)) { - if (!Note || Sec->LMAExpr) + if (!Note || Sec->LMAExpr || Note->LastSec->Alignment != Sec->Alignment) Note = AddHdr(PT_NOTE, PF_R); Note->add(Sec); } else { diff --git a/lld/test/ELF/build-id.s b/lld/test/ELF/build-id.s index f03a271..ebfcb08 100644 --- a/lld/test/ELF/build-id.s +++ b/lld/test/ELF/build-id.s @@ -65,15 +65,15 @@ _start: # DEFAULT: Contents of section .note.test: # DEFAULT: Contents of section .note.gnu.build-id: # DEFAULT-NEXT: 04000000 08000000 03000000 474e5500 ............GNU. -# DEFAULT-NEXT: 894c04e8 fbf5556b +# DEFAULT-NEXT: 95849665 2621c734 # MD5: Contents of section .note.gnu.build-id: # MD5-NEXT: 04000000 10000000 03000000 474e5500 ............GNU. -# MD5-NEXT: 6a51bbd7 9e8ee3f9 2e02d213 711cfec9 +# MD5-NEXT: 1882c01f 71698eed 229b3994 eb554c80 # SHA1: Contents of section .note.gnu.build-id: # SHA1-NEXT: 04000000 14000000 03000000 474e5500 ............GNU. -# SHA1-NEXT: 9a8618b1 d6fd0e5c eda73dd8 76de5596 +# SHA1-NEXT: 96820adf d90d5470 0a0c32ff a88c4017 # UUID: Contents of section .note.gnu.build-id: # UUID-NEXT: 04000000 10000000 03000000 474e5500 ............GNU. diff --git a/lld/test/ELF/note-alignment.s b/lld/test/ELF/note-alignment.s new file mode 100644 index 0000000..417721f --- /dev/null +++ b/lld/test/ELF/note-alignment.s @@ -0,0 +1,36 @@ +# REQUIRES: x86 +# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o %t.o +# RUN: ld.lld %t.o -o %t +# RUN: llvm-readelf -l %t | FileCheck %s + +# Check that we don't mix 4-byte and 8-byte aligned notes in one PT_LOAD. +# The possible 4-byte padding before the 8-byte align note may make consumers +# fail to parse it. + +# CHECK: NOTE {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} 0x000004 0x000004 R 0x4 +# CHECK: NOTE {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} 0x000010 0x000010 R 0x8 +# CHECK: NOTE {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} {{0x[0-9a-f]+}} 0x000008 0x000008 R 0x4 + +# CHECK: 03 .note.a +# CHECK-NEXT: 04 .note.b .note.c +# CHECK-NEXT: 05 .note.d .note.e + +.section .note.a, "a", @note +.align 4 +.long 0 + +.section .note.b, "a", @note +.align 8 +.quad 0 + +.section .note.c, "a", @note +.align 8 +.quad 0 + +.section .note.d, "a", @note +.align 4 +.long 0 + +.section .note.e, "a", @note +.align 4 +.long 0 -- 2.7.4