aarch64: handle STO_AARCH64_VARIANT_PCS

Avoid lazy binding of symbols that may follow a variant PCS with different
register usage convention from the base PCS.

Currently the lazy binding entry code does not preserve all the registers
required for AdvSIMD and SVE vector calls.  Saving and restoring all
registers unconditionally may break existing binaries, even if they never
use vector calls, because of the larger stack requirement for lazy
resolution, which can be significant on an SVE system.

The solution is to mark all symbols in the symbol table that may follow
a variant PCS so the dynamic linker can handle them specially.  In this
patch such symbols are always resolved at load time, not lazily.

So currently LD_AUDIT for variant PCS symbols are not supported, for that
the _dl_runtime_profile entry needs to be changed e.g. to unconditionally
save/restore all registers (but pass down arg and retval registers to
pltentry/exit callbacks according to the base PCS).

This patch also removes a __builtin_expect from the modified code because
the branch prediction hint did not seem useful.

	* sysdeps/aarch64/dl-dtprocnum.h: New file.
	* sysdeps/aarch64/dl-machine.h (DT_AARCH64): Define.
	(elf_machine_runtime_setup): Handle DT_AARCH64_VARIANT_PCS.
	(elf_machine_lazy_rel): Check STO_AARCH64_VARIANT_PCS and bind such
	symbols at load time.
	* sysdeps/aarch64/linkmap.h (struct link_map_machine): Add variant_pcs.

3 files changed, 60 insertions, 4 deletions

diff --git a/sysdeps/aarch64/dl-dtprocnum.h b/sysdeps/aarch64/dl-dtprocnum.h
new file mode 100644
index 0000000..4ac2adf
--- /dev/null
+++ b/sysdeps/aarch64/dl-dtprocnum.h
@@ -0,0 +1,21 @@
+/* Configuration of lookup functions.  AArch64 version.
+   Copyright (C) 2019 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library.  If not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* Number of extra dynamic section entries for this architecture.  By
+   default there are none.  */
+#define DT_THISPROCNUM	DT_AARCH64_NUM
diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index 823eefb..4f27637 100644
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -27,6 +27,9 @@
 #include <dl-irel.h>
 #include <cpu-features.c>
 
+/* Translate a processor specific dynamic tag to the index in l_info array.  */
+#define DT_AARCH64(x) (DT_AARCH64_##x - DT_LOPROC + DT_NUM)
+
 /* Return nonzero iff ELF header is compatible with the running host.  */
 static inline int __attribute__ ((unused))
 elf_machine_matches_host (const ElfW(Ehdr) *ehdr)
@@ -102,6 +105,10 @@ elf_machine_runtime_setup (struct link_map *l, int lazy, int profile)
 	}
     }
 
+  /* Check if STO_AARCH64_VARIANT_PCS needs to be handled.  */
+  if (l->l_info[DT_AARCH64 (VARIANT_PCS)])
+    l->l_mach.variant_pcs = 1;
+
   return lazy;
 }
 
@@ -388,10 +395,37 @@ elf_machine_lazy_rel (struct link_map *map,
   /* Check for unexpected PLT reloc type.  */
   if (__builtin_expect (r_type == AARCH64_R(JUMP_SLOT), 1))
     {
-      if (__builtin_expect (map->l_mach.plt, 0) == 0)
-	*reloc_addr += l_addr;
-      else
-	*reloc_addr = map->l_mach.plt;
+      if (map->l_mach.plt == 0)
+	{
+	  /* Prelinking.  */
+	  *reloc_addr += l_addr;
+	  return;
+	}
+
+      if (__glibc_unlikely (map->l_mach.variant_pcs))
+	{
+	  /* Check the symbol table for variant PCS symbols.  */
+	  const Elf_Symndx symndx = ELFW (R_SYM) (reloc->r_info);
+	  const ElfW (Sym) *symtab =
+	    (const void *)D_PTR (map, l_info[DT_SYMTAB]);
+	  const ElfW (Sym) *sym = &symtab[symndx];
+	  if (__glibc_unlikely (sym->st_other & STO_AARCH64_VARIANT_PCS))
+	    {
+	      /* Avoid lazy resolution of variant PCS symbols.  */
+	      const struct r_found_version *version = NULL;
+	      if (map->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
+		{
+		  const ElfW (Half) *vernum =
+		    (const void *)D_PTR (map, l_info[VERSYMIDX (DT_VERSYM)]);
+		  version = &map->l_versions[vernum[symndx] & 0x7fff];
+		}
+	      elf_machine_rela (map, reloc, sym, version, reloc_addr,
+				skip_ifunc);
+	      return;
+	    }
+	}
+
+      *reloc_addr = map->l_mach.plt;
     }
   else if (__builtin_expect (r_type == AARCH64_R(TLSDESC), 1))
     {
diff --git a/sysdeps/aarch64/linkmap.h b/sysdeps/aarch64/linkmap.h
index ba74fe1..7f801b1 100644
--- a/sysdeps/aarch64/linkmap.h
+++ b/sysdeps/aarch64/linkmap.h
@@ -20,4 +20,5 @@ struct link_map_machine
 {
   ElfW(Addr) plt;	  /* Address of .plt */
   void *tlsdesc_table;	  /* Address of TLS descriptor hash table.  */
+  int variant_pcs;	  /* If set, PLT calls may follow a variant PCS.  */
 };