[ELF] Merge .openbsd.randomdata.* sections into a single .openbsd.randomdata section when linking

[brad]

Summary

Merge .openbsd.randomdata.* sections into a single .openbsd.randomdata section when linking.

[MaskRay]

Can you describe the purpose of the section and why it needs to be suffixed? If .openbsd.randomdata. is only used in very few places, writing a linker script is a more appropriate solution.

This patch needs a test case in section-name.s if it is justified.

[jrtc27]

Isn't it useless without emitting a PT_OPENBSD_RANDOMIZE to cover it? (reading the somewhat terse https://github.com/openbsd/src/blob/master/libexec/ld.so/SPECS.randomdata)

[brad]

I spoke to the author of the diff and asked for some feedback. He said...

Picking a random userland .o file from the build: readelf -a shows that it
contain multiple randomdata sections:

.openbsd.randomdata.retguard.130
.openbsd.randomdata.retguard.236
.openbsd.randomdata.retguard.2936
.openbsd.randomdata.retguard.443
.openbsd.randomdata.retguard.2600

The suffix here is a hash generated by the compiler. This is something
done by default by the compiler on openbsd, so if we were to instead use a
linker script for this then (almost) *EVERY* program built on openbsd
would require a linker script.

As for "isn't this useless without putting them in PT_OPENBSD_RANDOMIZE
segment", well, sure, so add the four chunks under gnu/llvm which define &
mention that symbol...but I think some may argue that doing that should
only be done if the binary's 'OS' field in the header is the openbsd
value, so that may create a different kind of pushback.

[grimar]

Well, if the default compiler on openbsd does it, I think the change is reasonable. @MaskRay?
This patch, as was mentioned, needs a test case.

Also, I believe LLD already creates the PT_OPENBSD_RANDOMIZE segment and adds the .openbsd.randomdata
section to it already in Writer.cpp:

// PT_OPENBSD_RANDOMIZE is an OpenBSD-specific feature. That makes
// the dynamic linker fill the segment with random data.
if (OutputSection *cmd = findSection(".openbsd.randomdata", partNo))
  addHdr(PT_OPENBSD_RANDOMIZE, cmd->getPhdrFlags())->add(cmd);

[brad]

I'm not really good with coming up with tests, but especially with lld. Any suggestions? Any further comments?

[grimar]

In D87219#2325905, @brad wrote:

I'm not really good with coming up with tests, but especially with lld. Any suggestions? Any further comments?

I think you need to update lld\test\ELF\section-name.s.

[brad]

Add test.

[MaskRay]

This seems like work of a default linker script. Nowadays you may use OVERWRITE_SECTIONS to create a new output section description: https://maskray.me/blog/2021-07-04-sections-and-overwrite-sections

lld/ELF/LinkerScript.cpp

if (config->zKeepTextSectionPrefix)
         if (isSectionPrefix(v.substr(5), s->name.substr(5)))
           return v;
     return ".text";
   }
 
   for (StringRef v :
        {".data.rel.ro", ".data", ".rodata", ".bss.rel.ro", ".bss",
         ".gcc_except_table", ".init_array", ".fini_array", ".tbss", ".tdata",
-        ".ARM.exidx", ".ARM.extab", ".ctors", ".dtors"})
+        ".openbsd.randomdata", ".ARM.exidx", ".ARM.extab", ".ctors", ".dtors"})
     if (isSectionPrefix(v, s->name))
       return v;
 
   return s->name;
 }
 
 uint64_t ExprValue::getValue() const {
   if (sec)

lld/test/ELF/section-name.s

 .section .data.rel.ro.local.a,"aw",%progbits
 .byte 0
 .section .tbss.foo,"aGwT",@nobits,foo,comdat
 .byte 0
 .section .gcc_except_table.foo,"aG",@progbits,foo,comdat
 .byte 0
 .section .tdata.foo,"aGwT",@progbits,foo,comdat
 .byte 0
+.section .openbsd.randomdata.retguard.1723,"aw"
+.byte 0
 
 // CHECK:  1 .rodata  00000002
 // CHECK:  2 .gcc_except_table 00000001
 // CHECK:  3 .text         00000002
 // CHECK:  4 .tdata        00000001
 // CHECK:  5 .tbss         00000001
 // CHECK:  6 .data.rel.ro  00000004
-// CHECK:  7 .data         00000002
-// CHECK:  8 .foo.a        00000001
-// CHECK:  9 .foo          00000001
-// CHECK: 10 .bss          00000002
-// CHECK: 11 .comment      00000008
-// CHECK: 12 .symtab       00000030
-// CHECK: 13 .shstrtab     00000075
-// CHECK: 14 .strtab       00000008
+// CHECK:  7 .openbsd.randomdata 00000001
+// CHECK:  8 .data         00000002
+// CHECK:  9 .foo.a        00000001
+// CHECK: 10 .foo          00000001
+// CHECK: 11 .bss          00000002
+// CHECK: 12 .comment      00000008
+// CHECK: 13 .symtab       00000030
+// CHECK: 14 .shstrtab     00000089
+// CHECK: 15 .strtab       00000008