Re: [PATCH v4 01/15] objtool: Add CONFIG_CFI_CLANG support

Nick Desaulniers
2021-09-30T18:40:52Z

On Thu, Sep 30, 2021 at 11:05 AM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> With CONFIG_CFI_CLANG, the compiler replaces function references with
> references to the CFI jump table, which confuses objtool. This change,
> based on Josh's initial patch [1], goes through the list of relocations
> and replaces jump table symbols with the actual function symbols.
>
> [1] https://lore.kernel.org/r/d743f4b36e120c06506567a9f87a062ae03da47f.1611263462.git.jpoimboe@redhat.com/
>
> Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
> Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> ---
> tools/objtool/arch/x86/decode.c | 17 ++++++++++
> tools/objtool/elf.c | 51 ++++++++++++++++++++++++++++
> tools/objtool/include/objtool/arch.h | 3 ++
> tools/objtool/include/objtool/elf.h | 2 +-
> 4 files changed, 72 insertions(+), 1 deletion(-)
>
> diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> index 3172983bf808..9b043220b0af 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 */

Thanks for the added comment. ;)

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> + 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 8676c7598728..05a5f51aad2c 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -18,6 +18,7 @@
> #include <errno.h>
> #include <objtool/builtin.h>
>
> +#include <objtool/arch.h>
> #include <objtool/elf.h>
> #include <objtool/warn.h>
>
> @@ -291,6 +292,10 @@ static int read_sections(struct elf *elf)
> if (sec->sh.sh_flags & SHF_EXECINSTR)
> elf->text_size += sec->len;
>
> + /* Detect -fsanitize=cfi jump table sections */
> + if (!strncmp(sec->name, ".text..L.cfi.jumptable", 22))
> + sec->cfi_jt = true;
> +
> list_add_tail(&sec->list, &elf->sections);
> elf_hash_add(section, &sec->hash, sec->idx);
> elf_hash_add(section_name, &sec->name_hash, str_hash(sec->name));
> @@ -576,6 +581,49 @@ 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;
> +}
> +
> static int read_relocs(struct elf *elf)
> {
> struct section *sec;
> @@ -639,6 +687,9 @@ static int read_relocs(struct elf *elf)
> tot_reloc += nr_reloc;
> }
>
> + if (fix_cfi_relocs(elf))
> + return -1;
> +
> if (stats) {
> printf("max_reloc: %lu\n", max_reloc);
> printf("tot_reloc: %lu\n", tot_reloc);
> diff --git a/tools/objtool/include/objtool/arch.h b/tools/objtool/include/objtool/arch.h
> index 589ff58426ab..93bde8aaf2e3 100644
> --- a/tools/objtool/include/objtool/arch.h
> +++ b/tools/objtool/include/objtool/arch.h
> @@ -81,6 +81,9 @@ unsigned long arch_jump_destination(struct instruction *insn);
>
> unsigned long arch_dest_reloc_offset(int addend);
>
> +unsigned long arch_cfi_section_reloc_offset(struct reloc *reloc);
> +unsigned long arch_cfi_jump_reloc_offset(unsigned long offset);
> +
> const char *arch_nop_insn(int len);
> const char *arch_ret_insn(int len);
>
> diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
> index c3857fadee7a..e8d838217c77 100644
> --- a/tools/objtool/include/objtool/elf.h
> +++ b/tools/objtool/include/objtool/elf.h
> @@ -39,7 +39,7 @@ struct section {
> char *name;
> int idx;
> unsigned int len;
> - bool changed, text, rodata, noinstr;
> + bool changed, text, rodata, noinstr, cfi_jt;
> };
>
> struct symbol {
> --
> 2.33.0.800.g4c38ced690-goog
>

--
Thanks,
~Nick Desaulniers