[ELF] - Linkerscript: ignore DATA_SEGMENT_RELRO_END expression

[grimar]

Summary

DATA_SEGMENT_RELRO_END expression is used to protect first 3 entries of
.got.plt with relro technologie.
Since we decided not to support this in lld, I also see no reasons to try implement this in LS,
therefore expression is just ignored.

[grimar]

I just realized that is not so simple. Even if DATA_SEGMENT_RELRO_END functionality itself mostly can be ignored, it still should do alignment of location counter to page boundaries if -z relro is used I think.

[grimar]

I think we can support this in a next way:

1. DATA_SEGMENT_ALIGN is always just ignored.
2. When -z relro is not present then DATA_SEGMENT_RELRO_END is ignored.
3. When -z relro is present, DATA_SEGMENT_RELRO_END just aligns the location counter position to the page boundary.

That way we will have the same behavior we have now without the script. Any opinions ?

[ruiu]

Can you give me a pointer to a real-world use of DATA_SEGMENT_RELRO_END?

[davide]

In D22676#493843, @ruiu wrote:

Can you give me a pointer to a real-world use of DATA_SEGMENT_RELRO_END?

https://svnweb.freebsd.org/base/head/sys/conf/ldscript.amd64?revision=284870&view=markup#l146

ELF/LinkerScript.cpp

uint64_t ExprParser::parsePrimary() {
   // Since we do not support the optimization from comment above,
   // we can just ignore that command.
   if (Tok == "DATA_SEGMENT_END") {
     expect("(");
     expect(".");
     expect(")");
     return Dot;
   }
+  if (Tok == "DATA_SEGMENT_RELRO_END") {
+    expect("(");
+    next();
+    expect(",");
+    next();
+    expect(")");
+    return Dot;
+  }
   uint64_t V = 0;
   if (Tok.getAsInteger(0, V))
     setError("malformed number: " + Tok);
   return V;
 }
 
 uint64_t ExprParser::parseTernary(uint64_t Cond) {
   next();

test/ELF/linkerscript-locationcounter.s

 # RUN:  .neq : { *(.neq) } \
 # RUN:  . = CONSTANT (MAXPAGESIZE) * 0x24; \
 # RUN:  .maxpagesize : { *(.maxpagesize) } \
 # RUN:  . = CONSTANT (COMMONPAGESIZE) * 0x25; \
 # RUN:  .commonpagesize : { *(.commonpagesize) } \
 # RUN:  . = DATA_SEGMENT_ALIGN (CONSTANT (MAXPAGESIZE), CONSTANT (COMMONPAGESIZE)); \
 # RUN:  .datasegmentalign : { *(.datasegmentalign) } \
 # RUN:  . = DATA_SEGMENT_END (.); \
+# RUN:  . = 0x27000; \
+# RUN:  . = DATA_SEGMENT_RELRO_END (24, .); \
+# RUN:  .datasegmentrelroend : { *(.datasegmentrelroend) } \
 # RUN: }" > %t.script
 # RUN: ld.lld %t --script %t.script -o %t2
 # RUN: llvm-readobj -s %t2 | FileCheck %s
 
 # CHECK: Section {
 # CHECK:   Index: 1
 # CHECK:   Name: .plus
 # CHECK-NEXT:   Type: SHT_PROGBITS

 # CHECK-NEXT:   Address: 0x26008
 # CHECK-NEXT:   Offset:
 # CHECK-NEXT:   Size:
 # CHECK-NEXT:   Link:
 # CHECK-NEXT:   Info:
 # CHECK-NEXT:   AddressAlignment:
 # CHECK-NEXT:   EntrySize:
 # CHECK-NEXT: }
+# CHECK-NEXT: Section {
+# CHECK-NEXT:   Index:
+# CHECK-NEXT:   Name: .datasegmentrelroend
+# CHECK-NEXT:   Type: SHT_PROGBITS
+# CHECK-NEXT:   Flags [
+# CHECK-NEXT:     SHF_ALLOC
+# CHECK-NEXT:   ]
+# CHECK-NEXT:   Address: 0x27000
+# CHECK-NEXT:   Offset: 0x11000
+# CHECK-NEXT:   Size:
+# CHECK-NEXT:   Link:
+# CHECK-NEXT:   Info:
+# CHECK-NEXT:   AddressAlignment:
+# CHECK-NEXT:   EntrySize:
+# CHECK-NEXT: }
 
 ## Mailformed number error.
 # RUN: echo "SECTIONS { \
 # RUN:  . = 0x12Q41; \
 # RUN: }" > %t.script
 # RUN: not ld.lld %t --script %t.script -o %t2 2>&1 | \
 # RUN:  FileCheck --check-prefix=NUMERR %s
 # NUMERR: malformed number: 0x12Q41

 .section .maxpagesize, "a"
 .quad 0
 
 .section .commonpagesize, "a"
 .quad 0
 
 .section .datasegmentalign, "a"
 .quad 0
+
+.section .datasegmentrelroend, "a"
+.quad 0