[binutils-gdb] RISC-V: Ouput __global_pointer$ as dynamic symbol when generating dynamic PDE.

[Nelson Chu]

https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=18b9872261b950a8d10a4ae6ccb8f2acdaebc3e6

commit 18b9872261b950a8d10a4ae6ccb8f2acdaebc3e6
Author: Nelson Chu <nelson.chu@sifive.com>
Date:   Tue Dec 15 19:03:34 2020 -0800

    RISC-V: Ouput __global_pointer$ as dynamic symbol when generating dynamic PDE.
    
    When the ifunc resolver is in the executable, we may relax the variables
    to gp-relative access instruction in the ifunc resolver, or in other functions
    that called by the ifunc resolver.  But this will cause the uninitialized
    gp problem since the ifunc need to be resolved at the early runtime, that
    is at the pre-load stage, but we set the gp until the startup code.
    
    At first, we try to add a new dynamic tag, DT_RISCV_GP, to stroe the gp value
    and let ld.so can init the gp register early, before the pre-load stage.  But
    we need to extend the ABI if we want to add a new dynamic tag.  Therefore,
    in the psabi discussion, we try another solution, which was suggested by the
    lld and FreeBSD linker experts, to let ld.so set the gp earlier - make sure
    __global_pointer$ is output as a dynamic symbol when we are generating pde,
    since we only do the relaxation for it.  Afterwards, ld.so can search the
    DT_SYMTAB to get the gp value, and set the gp register before resolving ifunc.
    
    bfd/
        * elfnn-riscv.c (allocate_dynrelocs): When we are generating pde, make
          sure gp symbol is output as a dynamic symbol.

Diff:
---
 bfd/ChangeLog     | 5 +++++
 bfd/elfnn-riscv.c | 9 +++++++++
 2 files changed, 14 insertions(+)

diff --git a/bfd/ChangeLog b/bfd/ChangeLog
index f1ae342b4c3..20ccea5920e 100644
--- a/bfd/ChangeLog
+++ b/bfd/ChangeLog
@@ -1,3 +1,8 @@
+2021-01-05  Nelson Chu  <nelson.chu@sifive.com>
+
+	* elfnn-riscv.c (allocate_dynrelocs): When we are generating pde,
+	make sure gp symbol is output as a dynamic symbol.
+
 2021-01-04  H.J. Lu  <hongjiu.lu@intel.com>
 
 	PR ld/26256
diff --git a/bfd/elfnn-riscv.c b/bfd/elfnn-riscv.c
index 047f31b4eac..9da415e604e 100644
--- a/bfd/elfnn-riscv.c
+++ b/bfd/elfnn-riscv.c
@@ -1083,6 +1083,15 @@ allocate_dynrelocs (struct elf_link_hash_entry *h, void *inf)
   htab = riscv_elf_hash_table (info);
   BFD_ASSERT (htab != NULL);
 
+  /* When we are generating pde, make sure gp symbol is output as a
+     dynamic symbol.  Then ld.so can set the gp register earlier, before
+     resolving the ifunc.  */
+  if (!bfd_link_pic (info)
+      && htab->elf.dynamic_sections_created
+      && strcmp (h->root.root.string, RISCV_GP_SYMBOL) == 0
+      && !bfd_elf_link_record_dynamic_symbol (info, h))
+    return FALSE;
+
   /* Since STT_GNU_IFUNC symbols must go through PLT, we handle them
      in the allocate_ifunc_dynrelocs and allocate_local_ifunc_dynrelocs,
      if they are defined and referenced in a non-shared object.  */