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Date: Fri, 11 Dec 2020 12:57:00 -0800
From: Nick Desaulniers <ndesaulniers@...gle.com>
To: Josh Poimboeuf <jpoimboe@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Arnd Bergmann <arnd@...nel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: objtool crashes with some clang produced .o files
On Fri, Dec 11, 2020 at 9:46 AM Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> On Fri, Dec 11, 2020 at 05:49:15PM +0100, Peter Zijlstra wrote:
> > Do we want to capture all that gunk in something like
> > elf_reloc_to_insn(reloc, insn) instead of duplicating the magic?
>
> Yup, here's an actual patch
>
> From: Josh Poimboeuf <jpoimboe@...hat.com>
> Subject: [PATCH] objtool: Support Clang non-section symbols in static call generation
>
> The Clang assembler likes to strip section symbols, which means you
> can't reference some text code by its section. This confuses objtool
> greatly, causing it to seg fault.
>
> The fix is similar to what was done before, for ORC reloc generation:
>
> e81e07244325 ("objtool: Support Clang non-section symbols in ORC generation")
>
> Factor out that code into a common helper and use it for static call
> reloc generation as well.
>
> Reported-by: Arnd Bergmann <arnd@...nel.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
Thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1207
> ---
> tools/objtool/check.c | 11 +++++++++--
> tools/objtool/elf.c | 26 ++++++++++++++++++++++++++
> tools/objtool/elf.h | 2 ++
> tools/objtool/orc_gen.c | 29 +++++------------------------
> 4 files changed, 42 insertions(+), 26 deletions(-)
>
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index c6ab44543c92..5f8d3eed78a1 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -467,13 +467,20 @@ static int create_static_call_sections(struct objtool_file *file)
>
> /* populate reloc for 'addr' */
> reloc = malloc(sizeof(*reloc));
> +
> if (!reloc) {
> perror("malloc");
> return -1;
> }
> memset(reloc, 0, sizeof(*reloc));
> - reloc->sym = insn->sec->sym;
> - reloc->addend = insn->offset;
> +
> + insn_to_reloc_sym_addend(insn->sec, insn->offset, reloc);
> + if (!reloc->sym) {
> + WARN_FUNC("static call tramp: missing containing symbol",
> + insn->sec, insn->offset);
> + return -1;
> + }
> +
> reloc->type = R_X86_64_PC32;
> reloc->offset = idx * sizeof(struct static_call_site);
> reloc->sec = reloc_sec;
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index 4e1d7460574b..be89c741ba9a 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -262,6 +262,32 @@ struct reloc *find_reloc_by_dest(const struct elf *elf, struct section *sec, uns
> return find_reloc_by_dest_range(elf, sec, offset, 1);
> }
>
> +void insn_to_reloc_sym_addend(struct section *sec, unsigned long offset,
> + struct reloc *reloc)
> +{
> + if (sec->sym) {
> + reloc->sym = sec->sym;
> + reloc->addend = offset;
> + return;
> + }
> +
> + /*
> + * The Clang assembler strips section symbols, so we have to reference
> + * the function symbol instead:
> + */
> + reloc->sym = find_symbol_containing(sec, offset);
> + if (!reloc->sym) {
> + /*
> + * Hack alert. This happens when we need to reference the NOP
> + * pad insn immediately after the function.
> + */
> + reloc->sym = find_symbol_containing(sec, offset - 1);
> + }
> +
> + if (reloc->sym)
> + reloc->addend = offset - reloc->sym->offset;
> +}
> +
> static int read_sections(struct elf *elf)
> {
> Elf_Scn *s = NULL;
> diff --git a/tools/objtool/elf.h b/tools/objtool/elf.h
> index 807f8c670097..e6890cc70a25 100644
> --- a/tools/objtool/elf.h
> +++ b/tools/objtool/elf.h
> @@ -140,6 +140,8 @@ struct reloc *find_reloc_by_dest(const struct elf *elf, struct section *sec, uns
> struct reloc *find_reloc_by_dest_range(const struct elf *elf, struct section *sec,
> unsigned long offset, unsigned int len);
> struct symbol *find_func_containing(struct section *sec, unsigned long offset);
> +void insn_to_reloc_sym_addend(struct section *sec, unsigned long offset,
> + struct reloc *reloc);
> int elf_rebuild_reloc_section(struct elf *elf, struct section *sec);
>
> #define for_each_sec(file, sec) \
> diff --git a/tools/objtool/orc_gen.c b/tools/objtool/orc_gen.c
> index 235663b96adc..9ce68b385a1b 100644
> --- a/tools/objtool/orc_gen.c
> +++ b/tools/objtool/orc_gen.c
> @@ -105,30 +105,11 @@ static int create_orc_entry(struct elf *elf, struct section *u_sec, struct secti
> }
> memset(reloc, 0, sizeof(*reloc));
>
> - if (insn_sec->sym) {
> - reloc->sym = insn_sec->sym;
> - reloc->addend = insn_off;
> - } else {
> - /*
> - * The Clang assembler doesn't produce section symbols, so we
> - * have to reference the function symbol instead:
> - */
> - reloc->sym = find_symbol_containing(insn_sec, insn_off);
> - if (!reloc->sym) {
> - /*
> - * Hack alert. This happens when we need to reference
> - * the NOP pad insn immediately after the function.
> - */
> - reloc->sym = find_symbol_containing(insn_sec,
> - insn_off - 1);
> - }
> - if (!reloc->sym) {
> - WARN("missing symbol for insn at offset 0x%lx\n",
> - insn_off);
> - return -1;
> - }
> -
> - reloc->addend = insn_off - reloc->sym->offset;
> + insn_to_reloc_sym_addend(insn_sec, insn_off, reloc);
> + if (!reloc->sym) {
> + WARN("missing symbol for insn at offset 0x%lx",
> + insn_off);
> + return -1;
> }
>
> reloc->type = R_X86_64_PC32;
> --
> 2.25.4
>
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--
Thanks,
~Nick Desaulniers
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