On Thu, Oct 14, 2021 at 3:23 AM Peter Zijlstra [off-list ref] wrote:

On Wed, Oct 13, 2021 at 11:16:44AM -0700, Sami Tolvanen wrote:

quoted

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 1f2ae708b223..5fe31523e51f 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c

@@ -63,6 +63,23 @@ bool arch_callee_saved_reg(unsigned char reg)
      }
 }

+unsigned long arch_cfi_section_reloc_offset(struct reloc *reloc)
+{
+     if (!reloc->addend)
+             return 0;
+
+     if (reloc->type == R_X86_64_PC32 || reloc->type == R_X86_64_PLT32)
+             return reloc->addend + 4;
+
+     return reloc->addend;
+}
+
+unsigned long arch_cfi_jump_reloc_offset(unsigned long offset)
+{
+     /* offset to the relocation in a jmp instruction */
+     return offset + 1;
+}
+
 unsigned long arch_dest_reloc_offset(int addend)
 {
      return addend + 4;

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index b18f0055b50b..cd09c93c34fb 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c

quoted

@@ -575,6 +580,49 @@ static int read_rela_reloc(struct section *sec, int i, struct reloc *reloc, unsi
      return 0;
 }

+/*
+ * CONFIG_CFI_CLANG replaces function relocations to refer to an intermediate
+ * jump table. Undo the conversion so objtool can make sense of things.
+ */
+static int fix_cfi_relocs(const struct elf *elf)
+{
+     struct section *sec;
+     struct reloc *reloc;
+
+     list_for_each_entry(sec, &elf->sections, list) {
+             list_for_each_entry(reloc, &sec->reloc_list, list) {
+                     struct reloc *cfi_reloc;
+                     unsigned long offset;
+
+                     if (!reloc->sym->sec->cfi_jt)
+                             continue;
+
+                     if (reloc->sym->type == STT_SECTION)
+                             offset = arch_cfi_section_reloc_offset(reloc);
+                     else
+                             offset = reloc->sym->offset;
+
+                     /*
+                      * The jump table immediately jumps to the actual function,
+                      * so look up the relocation there.
+                      */
+                     offset = arch_cfi_jump_reloc_offset(offset);
+                     cfi_reloc = find_reloc_by_dest(elf, reloc->sym->sec, offset);
+
+                     if (!cfi_reloc || !cfi_reloc->sym) {
+                             WARN("can't find a CFI jump table relocation at %s+0x%lx",
+                                     reloc->sym->sec->name, offset);
+                             return -1;
+                     }
+
+                     reloc->sym = cfi_reloc->sym;
+                     reloc->addend = 0;
+             }
+     }
+
+     return 0;
+}

If that section ever gets modified and we end up running
elf_rebuild_reloc_section() on it, we're in trouble, right?

Do we want a fatal error in elf_rebuild_reloc_section() when ->cfi_jt is
set?

That's a valid point. Since ->cfi_jt is only set for jump table
sections, I think we'll need a different flag for this though.

Sami