Add debugger rendezvous DT_DEBUG .dynamic entry

[emaste]

Summary

The runtime linker may store a pointer to a data structure used by debuggers. This allows gdb and lldb to debug lld-linked binaries.

I can combine it as an else case in the existing if (Config->EMachine == EM_MIPS) block if that's preferred.

[emaste]

• add test
• add DT_DEBUG only to executables (use the presence of an entrypoint as a proxy for this)

[• rafael]

Why is mips special?

[emaste]

In D15775#316823, @rafael wrote:

Why is mips special?

MIPS's .dyanamic section is read-only. Instead of using DT_DEBUG for the pointer to dyld information it uses a separate tag DT_MIPS_RLD_MAP which points to storage in the read-write .rld_map section, which in turn points to the dyld information.

I added the support for DT_MIPS_RLD_MAP in D1890 which has some more background.

[• rafael]

That is pretty horrible. Any chance we could start producing a rw dynamic
section on MIPS too? With that we could at least hope for one day dropping
the MIPS specific field.

[emaste]

In D15775#316831, @rafael wrote:

That is pretty horrible. Any chance we could start producing a rw dynamic
section on MIPS too? With that we could at least hope for one day dropping
the MIPS specific field.

DT_MIPS_RLD_MAP is specified in the ABI and a migration would be a long process, but it shouldn't hurt anything to emit DT_DEBUG in MIPS binaries too. GNU ld did so until relatively recently. I'm happy to go ahead with or without the Config->EMachine != EM_MIPS.

[• rafael]

I would much prefer without. Thanks.

[emaste]

Drop MIPS special-case; DT_DEBUG is not currently used on MIPS but it doesn't hurt anything to leave the entry there either.

[davide]

test/ELF/dt_tags.s

1 ↗

(On Diff #43628)

Is this really required now?

[davide]

Ignore my previous comment, I missed the triple bit. This looks fine to me.

[emaste]

test/ELF/dt_tags.s

1 ↗

(On Diff #43628)

I think it is, based on the -triple below, but I'm not an expert in the way the tests are set up. Most other tests have it, or e.g. REQUIRES: x86,ppc for tests that use both x86 & ppc triples.

[emaste]

@ruiu or @rafael, ok with this version?

[emaste]

Other tests need updating for this change - I'll upload a new version next week.

[emaste]

Update existing tests for growth in .dynamic section

[emaste]

test/ELF/dt_tags.s

10 ↗

(On Diff #43741)

Note that this is also implicitly tested in dynamic-relocs.s

14 ↗

(On Diff #43741)

However I believe this is not tested elsewhere

[emaste]

Ping

[ruiu]

LGTM

ELF/OutputSections.cpp

if (Config->ZOrigin) {
     DtFlags1 |= DF_1_ORIGIN;
   }
 
   if (DtFlags)
     ++NumEntries; // DT_FLAGS
   if (DtFlags1)
     ++NumEntries; // DT_FLAGS_1
 
+  if (Config->EMachine != EM_MIPS)
+    ++NumEntries; // DT_DEBUG
+
   if (Config->EMachine == EM_MIPS) {
     ++NumEntries; // DT_MIPS_RLD_VERSION
     ++NumEntries; // DT_MIPS_FLAGS
     ++NumEntries; // DT_MIPS_BASE_ADDRESS
     ++NumEntries; // DT_MIPS_SYMTABNO
     ++NumEntries; // DT_MIPS_LOCAL_GOTNO
     ++NumEntries; // DT_MIPS_GOTSYM;
     ++NumEntries; // DT_PLTGOT
template <class ELFT> void DynamicSection<ELFT>::writeTo(uint8_t *Buf) {
   if (InitSym)
     WritePtr(DT_INIT, getSymVA<ELFT>(*InitSym));
   if (FiniSym)
     WritePtr(DT_FINI, getSymVA<ELFT>(*FiniSym));
   if (DtFlags)
     WriteVal(DT_FLAGS, DtFlags);
   if (DtFlags1)
     WriteVal(DT_FLAGS_1, DtFlags1);
+  if (Config->EMachine != EM_MIPS)
+    WriteVal(DT_DEBUG, 0);
 
   // See "Dynamic Section" in Chapter 5 in the following document
   // for detailed description:
   // ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf
   if (Config->EMachine == EM_MIPS) {
     WriteVal(DT_MIPS_RLD_VERSION, 1);
     WriteVal(DT_MIPS_FLAGS, RHF_NOTPOT);
     WritePtr(DT_MIPS_BASE_ADDRESS, Target->getVAStart());