[Bug ld/23560] New: ld generates bad aarch64 veneers/long branch stubs

[rafaelauler at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=23560

            Bug ID: 23560
           Summary: ld generates bad aarch64 veneers/long branch stubs
           Product: binutils
           Version: 2.31
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: ld
          Assignee: unassigned at sourceware dot org
          Reporter: rafaelauler at gmail dot com
  Target Milestone: ---

Hi,

I noticed ld is failing to generate long branch stubs for aarch64 under certain
conditions. After debugging the issue to try to understand what was happening,
I observed that the function elfNN_aarch64_size_stubs() has a loop to compute
the veneers, cache them into a special hash table and then redo layout (calling
the layout_sections_again callback), doing this until no new stubs are needed.
It then writes the stubs later by traversing the stub hash table with
aarch64_build_one_stub() calls in elfNN_aarch64_build_stubs().

The problem is that layout_sections_again may change a symbol value (by
changing its containing section's output_offset) but will not update the hash
entry containing the stub destination, which was pre-computed with a now
outdated value, thus generating a veneer with a bad target.

I could reproduce this on ld 2.31, but I wrote the patch for ld 2.36.

diff --git a/binutils-2.26.1/bfd/elfnn-aarch64.c
b/binutils-2.26.1/bfd/elfnn-aarch64.c
--- a/binutils-2.26.1/bfd/elfnn-aarch64.c
+++ b/binutils-2.26.1/bfd/elfnn-aarch64.c
@@ -4163,6 +4163,9 @@ elfNN_aarch64_size_stubs (bfd *output_bfd,
                    {
                      /* The proper stub has already been created.  */
                      free (stub_name);
+                     /* Always update this stub target since it may have
+                        changed after layout */
+                     stub_entry->target_value = sym_value + irela->r_addend;
                      continue;
                    }

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