ELF: Add a -z glibc-228-compat flag for working around an old glibc bug.

lld/ELF/Config.h

@@ -366,6 +366,7 @@ struct Config {
   bool zCopyreloc;
   bool zForceBti;
   bool zForceIbt;
+  bool zGlibc228Compat;
   bool zGlobal;
   bool zHazardplt;
   bool zIfuncNoplt;

lld/ELF/Driver.cpp

@@ -1559,6 +1559,7 @@ static void readConfigs(Ctx &ctx, opt::InputArgList &args) {
   ctx.arg.zForceBti = hasZOption(args, "force-bti");
   ctx.arg.zForceIbt = hasZOption(args, "force-ibt");
   ctx.arg.zGcs = getZGcs(ctx, args);
+  ctx.arg.zGlibc228Compat = hasZOption(args, "glibc-228-compat");
   ctx.arg.zGlobal = hasZOption(args, "global");
   ctx.arg.zGnustack = getZGnuStack(args);
   ctx.arg.zHazardplt = hasZOption(args, "hazardplt");

lld/ELF/Writer.cpp

@@ -2277,6 +2277,19 @@ static uint64_t computeFlags(Ctx &ctx, uint64_t flags) {
     return PF_R | PF_W | PF_X;
   if (ctx.arg.executeOnly && (flags & PF_X))
     return flags & ~PF_R;
+
+  // The -z glibc-228-compat flag is used for binaries utilizing IFUNCs which
+  // need to be compatible with glibc versions containing a bug that was fixed
+  // in commit b5c45e83753b27dc538dff2d55d4410c385cf3a4 which was released in
+  // version 2.29. The bug causes glibc to mprotect the .text section as RW
+  // while calling ifunc resolvers in binaries linked with -z notext, leading to
+  // a SIGSEGV at startup time. By setting the W flag on the executable section
+  // we work around the bug by avoiding the code path that does the mprotect. It
+  // is recommended that binaries linked with this flag contain startup code
+  // (e.g. in .init_array) that remaps the executable section as non-writable.
+  if (ctx.arg.zGlibc228Compat && !ctx.arg.zText && (flags & PF_X))
+    return flags | PF_W;
+
   return flags;
 }