[llvm-dev] [lld] avoid emitting PLT entries for ifuncs
Rui Ueyama via llvm-dev
llvm-dev at lists.llvm.org
Wed Aug 22 01:27:55 PDT 2018
Hi Mark,
Although I do understand your motivation to add this feature, because the
proposed change works only with a specific loader, I'd explorer other
options before adding a new feature to the linker.
So the problem for the kernel loader is to know all locations from where
the control jumps to ifunc PLT entries. Usually, once lld is done with
linking, all traces of such relocations are discarded because they are no
longer needed.
However, if you pass the -emit-relocs option to the linker, lld keeps all
relocations that have already been resolved in an output executable. By
analyzing a relocation table in a resulting executable, you could find all
locations where the ifunc PLT is called. Then, you can construct a new
table for your linker, embed it to the executable using objcopy or
something like that, and then let the kernel loader interpret it.
Have you considered that?
On Tue, Aug 21, 2018 at 10:48 PM Mark Johnston via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello,
>
> We've recently started using ifuncs in the x86(_64) FreeBSD kernel.
> Currently lld will emit a PLT entry for each ifunc, so ifunc calls are
> more expensive that those of regular functions. In our kernel, this
> overhead isn't really necessary: if lld instead emits PC-relative
> relocations for each ifunc call site, where each relocation references
> a symbol of type GNU_IFUNC, then during boot we can resolve each
> call site and apply the relocation before mapping the kernel text
> read-only. Then, ifunc calls have the same overhead as regular function
> calls.
>
> To implement this optimization, I wrote an lld patch to add
> "-z ifunc-noplt". When this option is specified, lld does not create
> PLT entries for ifuncs and instead passes the existing PC-relative
> relocation through to the output file. The patch is below; I tested it
> with lld 7.0 and the patch applied without modifications to the sources
> in trunk.
>
> I'm wondering if such an option would be acceptable in upstream lld, and
> whether anyone had comments on my implementation. The patch is lacking
> tests, and I had some questions:
> - How should "-z ifunc-noplt" interact with "-z text"? Should the
> invoker be required to additionally specify "-z notext"?
> - Could "-z ifunc-noplt" be subsumed by a more general mechanism which
> tells lld not to apply constant relocations and instead pass them
> through to the output file? I could imagine using such mechanism
> to make it possible to dynamically enable retpoline at boot time.
> It could also be useful for implementing static DTrace trace points.
>
> Thanks,
> -Mark
>
> diff --git a/ELF/Config.h b/ELF/Config.h
> index 5dc7f5321..b5a3d3266 100644
> --- a/ELF/Config.h
> +++ b/ELF/Config.h
> @@ -182,6 +182,7 @@ struct Configuration {
> bool ZCopyreloc;
> bool ZExecstack;
> bool ZHazardplt;
> + bool ZIfuncnoplt;
> bool ZInitfirst;
> bool ZKeepTextSectionPrefix;
> bool ZNodelete;
> diff --git a/ELF/Driver.cpp b/ELF/Driver.cpp
> index aced1edca..e7896cedf 100644
> --- a/ELF/Driver.cpp
> +++ b/ELF/Driver.cpp
> @@ -340,7 +340,8 @@ static bool getZFlag(opt::InputArgList &Args,
> StringRef K1, StringRef K2,
>
> static bool isKnown(StringRef S) {
> return S == "combreloc" || S == "copyreloc" || S == "defs" ||
> - S == "execstack" || S == "hazardplt" || S == "initfirst" ||
> + S == "execstack" || S == "hazardplt" || S == "ifunc-noplt" ||
> + S == "initfirst" ||
> S == "keep-text-section-prefix" || S == "lazy" || S == "muldefs"
> ||
> S == "nocombreloc" || S == "nocopyreloc" || S == "nodelete" ||
> S == "nodlopen" || S == "noexecstack" ||
> @@ -834,6 +835,7 @@ void LinkerDriver::readConfigs(opt::InputArgList
> &Args) {
> Config->ZCopyreloc = getZFlag(Args, "copyreloc", "nocopyreloc", true);
> Config->ZExecstack = getZFlag(Args, "execstack", "noexecstack", false);
> Config->ZHazardplt = hasZOption(Args, "hazardplt");
> + Config->ZIfuncnoplt = hasZOption(Args, "ifunc-noplt");
> Config->ZInitfirst = hasZOption(Args, "initfirst");
> Config->ZKeepTextSectionPrefix = getZFlag(
> Args, "keep-text-section-prefix", "nokeep-text-section-prefix",
> false);
> diff --git a/ELF/Relocations.cpp b/ELF/Relocations.cpp
> index 8f60aa3d2..a54d87e43 100644
> --- a/ELF/Relocations.cpp
> +++ b/ELF/Relocations.cpp
> @@ -361,6 +361,10 @@ static bool isStaticLinkTimeConstant(RelExpr E,
> RelType Type, const Symbol &Sym,
> R_TLSLD_HINT>(E))
> return true;
>
> + // The computation involves output from the ifunc resolver.
> + if (Sym.isGnuIFunc() && Config->ZIfuncnoplt)
> + return false;
> +
> // These never do, except if the entire file is position dependent or if
> // only the low bits are used.
> if (E == R_GOT || E == R_PLT || E == R_TLSDESC)
> @@ -808,6 +812,10 @@ static void processRelocAux(InputSectionBase &Sec,
> RelExpr Expr, RelType Type,
> Sec.Relocations.push_back({Expr, Type, Offset, Addend, &Sym});
> return;
> }
> + if (Sym.isGnuIFunc() && Config->ZIfuncnoplt) {
> + InX::RelaDyn->addReloc(Type, &Sec, Offset, &Sym, Addend, R_ADDEND,
> Type);
> + return;
> + }
> bool CanWrite = (Sec.Flags & SHF_WRITE) || !Config->ZText;
> if (CanWrite) {
> // R_GOT refers to a position in the got, even if the symbol is
> preemptible.
> @@ -977,7 +985,7 @@ static void scanReloc(InputSectionBase &Sec,
> OffsetGetter &GetOffset, RelTy *&I,
> // all dynamic symbols that can be resolved within the executable will
> // actually be resolved that way at runtime, because the main exectuable
> // is always at the beginning of a search list. We can leverage that
> fact.
> - if (Sym.isGnuIFunc())
> + if (Sym.isGnuIFunc() && !Config->ZIfuncnoplt)
> Expr = toPlt(Expr);
> else if (!Sym.IsPreemptible && Expr == R_GOT_PC &&
> !isAbsoluteValue(Sym))
> Expr = Target->adjustRelaxExpr(Type, RelocatedAddr, Expr);
> diff --git a/ELF/Writer.cpp b/ELF/Writer.cpp
> index 90462ecc7..418133ebd 100644
> --- a/ELF/Writer.cpp
> +++ b/ELF/Writer.cpp
> @@ -1570,8 +1570,11 @@ template <class ELFT> void
> Writer<ELFT>::finalizeSections() {
> applySynthetic({InX::EhFrame},
> [](SyntheticSection *SS) { SS->finalizeContents(); });
>
> - for (Symbol *S : Symtab->getSymbols())
> + for (Symbol *S : Symtab->getSymbols()) {
> S->IsPreemptible |= computeIsPreemptible(*S);
> + if (S->isGnuIFunc() && Config->ZIfuncnoplt)
> + S->ExportDynamic = true;
> + }
>
> // Scan relocations. This must be done after every symbol is declared
> so that
> // we can correctly decide if a dynamic relocation is needed.
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