[glibc/release/2.28/master] aarch64: Fix DT_AARCH64_VARIANT_PCS handling [BZ #26798]

[Szabolcs Nagy]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=d81114e0aaedc30d82956d3a21ae6777b3fbba3c

commit d81114e0aaedc30d82956d3a21ae6777b3fbba3c
Author: Szabolcs Nagy <szabolcs.nagy@arm.com>
Date:   Thu Oct 22 17:55:01 2020 +0100

    aarch64: Fix DT_AARCH64_VARIANT_PCS handling [BZ #26798]
    
    The variant PCS support was ineffective because in the common case
    linkmap->l_mach.plt == 0 but then the symbol table flags were ignored
    and normal lazy binding was used instead of resolving the relocs early.
    (This was a misunderstanding about how GOT[1] is setup by the linker.)
    
    In practice this mainly affects SVE calls when the vector length is
    more than 128 bits, then the top bits of the argument registers get
    clobbered during lazy binding.
    
    Fixes bug 26798.
    
    (cherry picked from commit 558251bd8785760ad40fcbfeaaee5d27fa5b0fe4)

Diff:
---
 sysdeps/aarch64/dl-machine.h | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/sysdeps/aarch64/dl-machine.h b/sysdeps/aarch64/dl-machine.h
index 9617cb754f..c7ae423417 100644
--- a/sysdeps/aarch64/dl-machine.h
+++ b/sysdeps/aarch64/dl-machine.h
@@ -388,13 +388,6 @@ 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 (map->l_mach.plt == 0)
-	{
-	  /* Prelinking.  */
-	  *reloc_addr += l_addr;
-	  return;
-	}
-
       if (1) /* DT_AARCH64_VARIANT_PCS is not available, so always check.  */
 	{
 	  /* Check the symbol table for variant PCS symbols.  */
@@ -418,7 +411,10 @@ elf_machine_lazy_rel (struct link_map *map,
 	    }
 	}
 
-      *reloc_addr = map->l_mach.plt;
+      if (map->l_mach.plt == 0)
+	*reloc_addr += l_addr;
+      else
+	*reloc_addr = map->l_mach.plt;
     }
   else if (__builtin_expect (r_type == AARCH64_R(TLSDESC), 1))
     {