linux-kernel - Re: [PATCH] objtool: Improve UACCESS coverage

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Message-ID: <CA+icZUVt0QK9SoWHU3G8e8MXOYOJSsh70+PCCWx15buZDvu8nQ@mail.gmail.com>
Date:   Wed, 24 Jul 2019 20:30:55 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        x86@...nel.org, Arnd Bergmann <arnd@...db.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] objtool: Improve UACCESS coverage

On Wed, Jul 24, 2019 at 6:48 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Jul 24, 2019 at 04:10:40PM +0200, Peter Zijlstra wrote:
> > And that most certainly should trigger...
> >
> > Let me gdb that objtool thing.
>
> ---
> Subject: objtool: Improve UACCESS coverage
>
> A clang build reported an (obvious) double CLAC while a GCC build did
> not; it turns out we only re-visit instructions if the first visit was
> with AC=0. If OTOH the first visit was with AC=1, we completely ignore
> any subsequent visit, even when it has AC=0.
>
> Fix this by using a visited mask, instead of boolean and (explicitly)
> mark the AC state.
>
> $ ./objtool check -b --no-fp --retpoline --uaccess ../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o
> ../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool: .altinstr_replacement+0x22: redundant UACCESS disable
> ../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0xea: (alt)
> ../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   .altinstr_replacement+0xffffffffffffffff: (branch)
> ../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0xd9: (alt)
> ../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0xb2: (branch)
> ../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0x39: (branch)
> ../../defconfig-build/drivers/gpu/drm/i915/gem/i915_gem_execbuffer.o: warning: objtool:   eb_copy_relocations.isra.34()+0x0: <=== (func)
>
> Reported-by: Josh Poimboeuf <jpoimboe@...hat.com>
> Reported-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Thanks Peter Z. and Josh P.!

Reported-by: Sedat Dilek <sedat.dilek@...il.com>
Tested-by: Sedat Dilek <sedat.dilek@...il.com>

[1] https://github.com/ClangBuiltLinux/linux/issues/617

> ---
>  tools/objtool/check.c | 7 ++++---
>  tools/objtool/check.h | 3 ++-
>  2 files changed, 6 insertions(+), 4 deletions(-)
>
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 5f26620f13f5..176f2f084060 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -1946,6 +1946,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>         struct alternative *alt;
>         struct instruction *insn, *next_insn;
>         struct section *sec;
> +       u8 visited;
>         int ret;
>
>         insn = first;
> @@ -1972,12 +1973,12 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>                         return 1;
>                 }
>
> +               visited = 1 << state.uaccess;
>                 if (insn->visited) {
>                         if (!insn->hint && !insn_state_match(insn, &state))
>                                 return 1;
>
> -                       /* If we were here with AC=0, but now have AC=1, go again */
> -                       if (insn->state.uaccess || !state.uaccess)
> +                       if (insn->visited & visited)
>                                 return 0;
>                 }
>
> @@ -2024,7 +2025,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
>                 } else
>                         insn->state = state;
>
> -               insn->visited = true;
> +               insn->visited |= visited;
>
>                 if (!insn->ignore_alts) {
>                         bool skip_orig = false;
> diff --git a/tools/objtool/check.h b/tools/objtool/check.h
> index b881fafcf55d..6d875ca6fce0 100644
> --- a/tools/objtool/check.h
> +++ b/tools/objtool/check.h
> @@ -33,8 +33,9 @@ struct instruction {
>         unsigned int len;
>         enum insn_type type;
>         unsigned long immediate;
> -       bool alt_group, visited, dead_end, ignore, hint, save, restore, ignore_alts;
> +       bool alt_group, dead_end, ignore, hint, save, restore, ignore_alts;
>         bool retpoline_safe;
> +       u8 visited;
>         struct symbol *call_dest;
>         struct instruction *jump_dest;
>         struct instruction *first_jump_src;