[PATCH] D77007: [ELF] Allow SHF_LINK_ORDER and non-SHF_LINK_ORDER to be mixed

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: grimar, psmith, ruiu.
Herald added subscribers: llvm-commits, arichardson, emaste.
Herald added a reviewer: espindola.
Herald added a project: LLVM.

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 over-constrained. This patch allows mixed flags with the
requirement that SHF_LINK_ORDER sections must be contiguous.
This requirement is caused by an interaction of Linux kernel

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

and clang's -fpatchable-function-entry=N[,M] implementation which sets
the SHF_LINK_ORDER flag (D72215 <https://reviews.llvm.org/D72215>) to fix a number of garbage collection
issues.

This patch also imposes one restriction: SHF_LINK_ORDER sections cannot
be separated by a symbol assignment or a BYTE command.

Link: https://github.com/ClangBuiltLinux/linux/issues/953


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D77007

Files:
  lld/ELF/OutputSections.cpp
  lld/ELF/Writer.cpp
  lld/test/ELF/linkerscript/linkorder.s


Index: lld/test/ELF/linkerscript/linkorder.s
===================================================================
--- lld/test/ELF/linkerscript/linkorder.s
+++ lld/test/ELF/linkerscript/linkorder.s
@@ -35,7 +35,7 @@
 
 ## 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 +43,9 @@
 
 ## 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:
Index: lld/ELF/Writer.cpp
===================================================================
--- lld/ELF/Writer.cpp
+++ lld/ELF/Writer.cpp
@@ -1562,17 +1562,30 @@
     // but sort must consider them all at once.
     std::vector<InputSection **> scriptSections;
     std::vector<InputSection *> sections;
+    bool started = false, stopped = false;
     for (BaseCommand *base : sec->sectionCommands) {
       if (auto *isd = dyn_cast<InputSectionDescription>(base)) {
         for (InputSection *&isec : isd->sections) {
-          scriptSections.push_back(&isec);
-          sections.push_back(isec);
+          if (!(isec->flags & SHF_LINK_ORDER)) {
+            if (started)
+              stopped = true;
+          } else if (stopped) {
+            error(toString(isec) + ": SHF_LINK_ORDER sections in " + sec->name +
+                  " are not contiguous");
+          } else {
+            started = true;
 
-          InputSection *link = isec->getLinkOrderDep();
-          if (!link->getParent())
-            error(toString(isec) + ": sh_link points to discarded section " +
-                  toString(link));
+            scriptSections.push_back(&isec);
+            sections.push_back(isec);
+
+            InputSection *link = isec->getLinkOrderDep();
+            if (!link->getParent())
+              error(toString(isec) + ": sh_link points to discarded section " +
+                    toString(link));
+          }
         }
+      } else if (started) {
+        stopped = true;
       }
     }
 
Index: lld/ELF/OutputSections.cpp
===================================================================
--- lld/ELF/OutputSections.cpp
+++ lld/ELF/OutputSections.cpp
@@ -114,8 +114,7 @@
     flags = isec->flags;
   } else {
     // Otherwise, check if new type or flags are compatible with existing ones.
-    unsigned mask = SHF_TLS | SHF_LINK_ORDER;
-    if ((flags & mask) != (isec->flags & mask))
+    if ((flags ^ isec->flags) & SHF_TLS)
       error("incompatible section flags for " + name + "\n>>> " + toString(isec) +
             ": 0x" + utohexstr(isec->flags) + "\n>>> output section " + name +
             ": 0x" + utohexstr(flags));
@@ -366,8 +365,9 @@
     // all InputSections in the OutputSection have the same dependency.
     if (auto *ex = dyn_cast<ARMExidxSyntheticSection>(first))
       link = ex->getLinkOrderDep()->getParent()->sectionIndex;
-    else if (auto *d = first->getLinkOrderDep())
-      link = d->getParent()->sectionIndex;
+    else if (first->flags & SHF_LINK_ORDER)
+      if (auto *d = first->getLinkOrderDep())
+        link = d->getParent()->sectionIndex;
   }
 
   if (type == SHT_GROUP) {


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