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.

diff --git a/ChangeLog b/ChangeLog
index 456b9ba8bbd6f055ca49e6a4ede014be7ddcc95d..7cc8f6e8054d9145ef2d7666ccb6fb5b2e819bb9 100644 (file)
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2019-06-13  Szabolcs Nagy  <szabolcs.nagy@arm.com>
+
+       * 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.
+
 2019-06-13  Szabolcs Nagy  <szabolcs.nagy@arm.com>
 
        * elf/elf.h (STO_AARCH64_VARIANT_PCS): Define.

diff --git a/sysdeps/aarch64/dl-dtprocnum.h b/sysdeps/aarch64/dl-dtprocnum.h
new file mode 100644 (file)
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 823eefba460eed9b4933c7c38179cb5e9180ead8..4f27637b2012a74fb8cfc2e73b09ed29eb7e2a50 100644 (file)
--- 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 ba74fe10e12e2a525aa62c1d3dea5ecb3790c6b2..7f801b14db2fa0cd2df7cafc1e7a1f9d629455dc 100644 (file)
--- 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.  */
 };