[Bug target/77408] New: Copy relocation turns read-only data into writable

["hjl.tools at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77408

            Bug ID: 77408
           Summary: Copy relocation turns read-only data into writable
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
                CC: ubizjak at gmail dot com
  Target Milestone: ---
            Target: x86

On x86, copy relocation is used in executable to access external data
defined in shared object as if it is defined locally. At run-time,
dynamic linker copies symbol data from shared object to executable and
its references from shared objects are resolved by GLOB_DAT relocation.
Since the copy of symbol data in executable is writable even if the
original symbol in shared object is read-only, this is a potential
security risk.

We can avoid copy relocation by always using PIC model to access
external data symbol.  If the external symbol is defined locally in
executable, linker can optimize instructions on memory operand with
GOTPCRELX/GOT32X relocation against external symbol into a different
form on immediate operand.