PowerPC64 .branch_lt size change leads to "stubs don't match calculated size"

Alan Modra amodra@gmail.com
Fri Jan 26 05:30:00 GMT 2018


I haven't analyzed this myself, I'm relying on Nick's excellent
analysis.  What I believe is happening is that after some number of
stub sizing iterations, a long-branch stub needs to be converted to a
plt-branch, but either due to stub alignment or other stubs shrinking
in size, the stub group section size doesn't change.

That means we exit from ppc64_elf_size_stubs after sizing with an
incorrect layout, in fact the additional .branch_lt entry overlays
.got!  Since .TOC. is normally set to .got + 0x8000 the stub sizing
code decides that entry is within +/-32k of the TOC pointer and so a
three insn stub is sufficient.  When we come to build the stubs using
a correct non-overlaying layout, a four insn plt-branch stub is
generated and the stub group size doesn't match that calculated

	* elf64-ppc.c (ppc64_elf_size_stubs): Iterate sizing when
	.branch_lt changes size.

diff --git a/bfd/elf64-ppc.c b/bfd/elf64-ppc.c
index b2d288b..5cbb035 100644
--- a/bfd/elf64-ppc.c
+++ b/bfd/elf64-ppc.c
@@ -12702,6 +12702,9 @@ ppc64_elf_size_stubs (struct bfd_link_info *info)
 	    stub_sec->flags &= ~SEC_RELOC;
+      if (htab->stub_iteration <= STUB_SHRINK_ITER
+	  || htab->brlt->rawsize < htab->brlt->size)
+	htab->brlt->rawsize = htab->brlt->size;
       htab->brlt->size = 0;
       htab->brlt->reloc_count = 0;
       htab->brlt->flags &= ~SEC_RELOC;
@@ -12757,6 +12760,9 @@ ppc64_elf_size_stubs (struct bfd_link_info *info)
       if (group == NULL
+	  && (htab->brlt->rawsize == htab->brlt->size
+	      || (htab->stub_iteration > STUB_SHRINK_ITER
+		  && htab->brlt->rawsize > htab->brlt->size))
 	  && (htab->glink_eh_frame == NULL
 	      || htab->glink_eh_frame->rawsize == htab->glink_eh_frame->size))

Alan Modra
Australia Development Lab, IBM

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