[ELF] Allow SHF_LINK_ORDER and non-SHF_LINK_ORDER to be mixed

Currently, `error: incompatible section flags for .rodata` is reported
when we mix SHF_LINK_ORDER and non-SHF_LINK_ORDER sections in an output section.

This is overconstrained. This patch allows mixed flags with the
requirement that SHF_LINK_ORDER sections must be contiguous. Mixing
flags is used by Linux aarch64 (https://github.com/ClangBuiltLinux/linux/issues/953)

  .init.data : { ... KEEP(*(__patchable_function_entries)) ... }

When the integrated assembler is enabled, clang's -fpatchable-function-entry=N[,M]
implementation sets the SHF_LINK_ORDER flag (D72215) to fix a number of
garbage collection issues.

Strictly speaking, the ELF specification does not require contiguous
SHF_LINK_ORDER sections but for many current uses of SHF_LINK_ORDER like
.ARM.exidx/__patchable_function_entries there has been a requirement for
the sections to be contiguous on top of the requirements of the ELF
specification.

This patch also imposes one restriction: SHF_LINK_ORDER sections cannot
be separated by a symbol assignment or a BYTE command. Not allowing BYTE
is a natural extension that a non-SHF_LINK_ORDER cannot be a separator.
Symbol assignments can delimiter the contents of SHF_LINK_ORDER
sections.  Allowing SHF_LINK_ORDER sections across symbol assignments
(especially __start_/__stop_) can make things hard to explain. The
restriction should not be a problem for practical use cases.

Reviewed By: psmith

Differential Revision: https://reviews.llvm.org/D77007

(cherry picked from commit 673e81eee4fa3ffa38736f1063e6c4fa2d9278b0)

2 files changed, 7 insertions, 20 deletions

diff --git a/lld/test/ELF/linkerscript/linkorder.s b/lld/test/ELF/linkerscript/linkorder.s
index 118c2e6b073..eab4819af8c 100644
--- a/lld/test/ELF/linkerscript/linkorder.s
+++ b/lld/test/ELF/linkerscript/linkorder.s
@@ -7,6 +7,8 @@
 # RUN: ld.lld -T %t.lds %t.o -o %t
 # RUN: llvm-readelf -S -x .rodata -x .text %t | FileCheck %s
 
+# CHECK:      [ 1] .rodata   {{.*}} AL 3
+# CHECK:      [ 3] .text     {{.*}} AX 0
 # CHECK:      Hex dump of section '.rodata':
 # CHECK-NEXT: 00020103
 # CHECK:      Hex dump of section '.text':
@@ -17,6 +19,8 @@
 # RUN: ld.lld -T %t1.lds %t.o -o %t1
 # RUN: llvm-readelf -S -x .rodata -x .text %t1 | FileCheck --check-prefix=CHECK1 %s
 
+# CHECK1:      [ 1] .rodata   {{.*}} AL 3
+# CHECK1:      [ 3] .text     {{.*}} AX 0
 # CHECK1:      Hex dump of section '.rodata':
 # CHECK1-NEXT: 00010203
 # CHECK1:      Hex dump of section '.text':
@@ -35,7 +39,7 @@
 
 ## Non-contiguous SHF_LINK_ORDER sections, separated by a BYTE.
 # RUN: echo 'SECTIONS { .rodata : {*(.rodata.foo) BYTE(0) *(.rodata.bar)} }' > %terr1.lds
-# RUN: ld.lld -T %terr1.lds %t.o -o /dev/null
+# RUN: not ld.lld -T %terr1.lds %t.o -o /dev/null 2>&1 | FileCheck --check-prefix=ERR %s
 
 ## Non-contiguous SHF_LINK_ORDER sections, separated by a non-SHF_LINK_ORDER section.
 # RUN: echo 'SECTIONS { .rodata : {*(.rodata.foo) *(.text) *(.rodata.bar)} }' > %terr2.lds
@@ -43,9 +47,9 @@
 
 ## Non-contiguous SHF_LINK_ORDER sections, separated by a symbol assignment.
 # RUN: echo 'SECTIONS { .rodata : {*(.rodata.foo) a = .; *(.rodata.bar)} }' > %terr3.lds
-# RUN: ld.lld -T %terr3.lds %t.o -o /dev/null
+# RUN: not ld.lld -T %terr3.lds %t.o -o /dev/null 2>&1 | FileCheck --check-prefix=ERR %s
 
-# ERR: error: incompatible section flags for .rodata
+# ERR: error: {{.*}}.o:(.rodata.bar): SHF_LINK_ORDER sections in .rodata are not contiguous
 
 .global _start
 _start:
diff --git a/lld/test/ELF/linkorder-err3.s b/lld/test/ELF/linkorder-err3.s
deleted file mode 100644
index 2e36ab45767..00000000000
--- a/lld/test/ELF/linkorder-err3.s
+++ /dev/null
@@ -1,17 +0,0 @@
-# REQUIRES: x86
-
-# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
-# RUN: not ld.lld %t.o -o /dev/null 2>&1 | FileCheck %s
-
-# CHECK:      error: incompatible section flags for .bar
-# CHECK-NEXT: >>> {{.*}}.o:(.bar): 0x2
-# CHECK-NEXT: >>> output section .bar: 0x82
-
-.section .foo,"a",@progbits
-.quad 0
-
-.section .bar,"ao",@progbits,.foo,unique,1
-.quad 0
-
-.section .bar,"a",@progbits,unique,2
-.quad 1